ArmVirt and Self-Updating Code


Bret Barkelew <bret.barkelew@...>
 

Expanding audience to the full dev list…

See below…

 

- Bret

 

From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code

 

+ Sami

 

From: Bret Barkelew <Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham <thomas.abraham@...>; Ard Biesheuvel (TianoCore) <ardb+tianocore@...>; Lindholm, Leif <leif@...>; Laszlo Ersek <lersek@...>; Marvin Häuser <mhaeuser@...>
Subject: ArmVirt and Self-Updating Code

 

All,

 

Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…

 

“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.

 

“Now, StandaloneMmPkg has similar (self-)relocation code too: https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above. The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158

 

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”

 

Hoping that one of you could get me closer to an answer for him. Also happy to take this to the greater mailing list, but thought I’d avoid churn.

 

Thanks in advance!

- Bret

 

 


Ard Biesheuvel
 

On Thu, 22 Jul 2021 at 16:54, Bret Barkelew <Bret.Barkelew@...> wrote:

Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew <Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham <thomas.abraham@...>; Ard Biesheuvel (TianoCore) <ardb+tianocore@...>; Lindholm, Leif <leif@...>; Laszlo Ersek <lersek@...>; Marvin Häuser <mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.



“Now, StandaloneMmPkg has similar (self-)relocation code too: https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158



“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.


4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.



Hoping that one of you could get me closer to an answer for him. Also happy to take this to the greater mailing list, but thought I’d avoid churn.



Thanks in advance!

- Bret




Marvin Häuser
 

On 22.07.21 17:14, Ard Biesheuvel wrote:
On Thu, 22 Jul 2021 at 16:54, Bret Barkelew<Bret.Barkelew@...> wrote:
Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew<Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham<thomas.abraham@...>; Ard Biesheuvel (TianoCore)<ardb+tianocore@...>; Lindholm, Leif<leif@...>; Laszlo Ersek<lersek@...>; Marvin Häuser<mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.
And I understood it right that the idea is to use "-fpie" to
1) have all control flow instructions be position-independent (i.e. jumps, calls, etc; ARM docs don't spill it out, but vaguely imply this always is possible?), and
2) emit a GOT, which ends up being converted to PE/COFF Relocations (-> self-relocation), for global data that cannot be referenced relatively? Is there any way to know/force that no symbol in GOT is accessed up until the end of the self-relocation process?

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here. Is the Standalone MM core of *type* SecCore in the FFS (without *being* SecCore)? This confused me the most.

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!

Best regards,
Marvin

Hoping that one of you could get me closer to an answer for him. Also happy to take this to the greater mailing list, but thought I’d avoid churn.



Thanks in advance!

- Bret




Ard Biesheuvel
 

On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:

On 22.07.21 17:14, Ard Biesheuvel wrote:
On Thu, 22 Jul 2021 at 16:54, Bret Barkelew<Bret.Barkelew@...> wrote:
Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew<Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham<thomas.abraham@...>; Ard Biesheuvel (TianoCore)<ardb+tianocore@...>; Lindholm, Leif<leif@...>; Laszlo Ersek<lersek@...>; Marvin Häuser<mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.
And I understood it right that the idea is to use "-fpie" to
1) have all control flow instructions be position-independent (i.e.
jumps, calls, etc; ARM docs don't spill it out, but vaguely imply this
always is possible?), and
The primary reason to use -fpie and PIE linking is to ensure that the
resulting ELF executable contains a RELA section that describes every
location in the binary where a memory address is stored that needs to
be updated according to the actual placement in memory. The side
effect of -fpie is that position independent global references are
emitted (i.e., ADRP/ADD instructions which are relative to the program
counter). However, the AArch64 compiler uses those by default anyway,
so for this it is not strictly needed.

2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Marvin Häuser
 

On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
On 22.07.21 17:14, Ard Biesheuvel wrote:
On Thu, 22 Jul 2021 at 16:54, Bret Barkelew<Bret.Barkelew@...> wrote:
Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew<Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham<thomas.abraham@...>; Ard Biesheuvel (TianoCore)<ardb+tianocore@...>; Lindholm, Leif<leif@...>; Laszlo Ersek<lersek@...>; Marvin Häuser<mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.
And I understood it right that the idea is to use "-fpie" to
1) have all control flow instructions be position-independent (i.e.
jumps, calls, etc; ARM docs don't spill it out, but vaguely imply this
always is possible?), and
The primary reason to use -fpie and PIE linking is to ensure that the
resulting ELF executable contains a RELA section that describes every
location in the binary where a memory address is stored that needs to
be updated according to the actual placement in memory. The side
effect of -fpie is that position independent global references are
emitted (i.e., ADRP/ADD instructions which are relative to the program
counter). However, the AArch64 compiler uses those by default anyway,
so for this it is not strictly needed.

2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections for instructions that embed and variables that hold an absolute address (I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in 1), because all absolute addresses are indirected by GOT (just GOT references are relocated)

If I understood the process right, but the term (GOT) is wrong, sorry, that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my understanding above?

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not necessarily "lazy", for ARM there simply seems to not be any reason to distinguish if the environments are fully separate), just wanted to make sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Ard Biesheuvel
 

On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:

On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
On 22.07.21 17:14, Ard Biesheuvel wrote:
On Thu, 22 Jul 2021 at 16:54, Bret Barkelew<Bret.Barkelew@...> wrote:
Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew<Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham<thomas.abraham@...>; Ard Biesheuvel (TianoCore)<ardb+tianocore@...>; Lindholm, Leif<leif@...>; Laszlo Ersek<lersek@...>; Marvin Häuser<mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.
And I understood it right that the idea is to use "-fpie" to
1) have all control flow instructions be position-independent (i.e.
jumps, calls, etc; ARM docs don't spill it out, but vaguely imply this
always is possible?), and
The primary reason to use -fpie and PIE linking is to ensure that the
resulting ELF executable contains a RELA section that describes every
location in the binary where a memory address is stored that needs to
be updated according to the actual placement in memory. The side
effect of -fpie is that position independent global references are
emitted (i.e., ADRP/ADD instructions which are relative to the program
counter). However, the AArch64 compiler uses those by default anyway,
so for this it is not strictly needed.

2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took
it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections
for instructions that embed and variables that hold an absolute address
(I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in
1), because all absolute addresses are indirected by GOT (just GOT
references are relocated)
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.

However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

If I understood the process right, but the term (GOT) is wrong, sorry,
that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my
understanding above?
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not
necessarily "lazy", for ARM there simply seems to not be any reason to
distinguish if the environments are fully separate), just wanted to make
sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Marvin Häuser
 

On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
On 22.07.21 17:14, Ard Biesheuvel wrote:
On Thu, 22 Jul 2021 at 16:54, Bret Barkelew<Bret.Barkelew@...> wrote:
Expanding audience to the full dev list…

See below…



- Bret



From: Thomas Abraham
Sent: Wednesday, July 7, 2021 11:07 PM
To: Bret Barkelew; Ard Biesheuvel (TianoCore); Lindholm, Leif; Laszlo Ersek; Marvin Häuser; Sami Mujawar
Cc: nd
Subject: [EXTERNAL] RE: ArmVirt and Self-Updating Code



+ Sami



From: Bret Barkelew<Bret.Barkelew@...>
Sent: Thursday, July 8, 2021 11:05 AM
To: Thomas Abraham<thomas.abraham@...>; Ard Biesheuvel (TianoCore)<ardb+tianocore@...>; Lindholm, Leif<leif@...>; Laszlo Ersek<lersek@...>; Marvin Häuser<mhaeuser@...>
Subject: ArmVirt and Self-Updating Code



All,



Marvin asked me a question on the UEFI Talkbox Discord that’s a little beyond my ken…



“There is self-relocating code in ArmVirtPkg:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/ArmVirtPkg/PrePi/PrePi.c#L133-L165

According to comments in the ASM, it seems like this is for Linux-based RAM boot (I saw further stuff for KVM, so it makes sense I guess?). It seems unfortunate it cannot be mapped into a known address range so that self-relocation is not necessary, but that's out of my scope to understand.
"Mapping" implies that the MMU is on, but this code boots with the MMU
off. Unlike x86, ARM does not define any physical address ranges that
are guaranteed to be backed by DRAM, so a portable image either needs
to be fully position independent, or carry the metadata it needs to
relocate itself as it is invoked.
And I understood it right that the idea is to use "-fpie" to
1) have all control flow instructions be position-independent (i.e.
jumps, calls, etc; ARM docs don't spill it out, but vaguely imply this
always is possible?), and
The primary reason to use -fpie and PIE linking is to ensure that the
resulting ELF executable contains a RELA section that describes every
location in the binary where a memory address is stored that needs to
be updated according to the actual placement in memory. The side
effect of -fpie is that position independent global references are
emitted (i.e., ADRP/ADD instructions which are relative to the program
counter). However, the AArch64 compiler uses those by default anyway,
so for this it is not strictly needed.

2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
Do you maybe have one final comment regarding that second question, please? :)
Let's drop "GOT" and make it "any instruction that requires prior relocation to function correctly".

It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took
it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections
for instructions that embed and variables that hold an absolute address
(I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in
1), because all absolute addresses are indirected by GOT (just GOT
references are relocated)
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.

However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

If I understood the process right, but the term (GOT) is wrong, sorry,
that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my
understanding above?
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!

Best regards,
Marvin

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not
necessarily "lazy", for ARM there simply seems to not be any reason to
distinguish if the environments are fully separate), just wanted to make
sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Ard Biesheuvel
 

On Fri, 23 Jul 2021 at 16:27, Marvin Häuser <mhaeuser@...> wrote:



On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
...
2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
Do you maybe have one final comment regarding that second question,
please? :)
The RELA section is not converted into PE/COFF relocations. This would
not achieve a lot, given that no prior PE/COFF loader exists to
process them. There is a snippet of asm code in the startup code that
processes the R_AARCH64_RELATIVE relocation entries before calling
into C code.

This also gives us the guarantee that no GOT indirections are
dereferenced, given that our asm code simply does not do that.

Let's drop "GOT" and make it "any instruction that requires prior
relocation to function correctly".
The thing to keep in mind here is that R_AARCH64_RELATIVE relocations
never target instructions, but only memory locations that carry
absolute addresses. This could be locations in .rodata or .data
(global vars carrying pointer values), or GOT entries.

It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took
it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections
for instructions that embed and variables that hold an absolute address
(I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in
1), because all absolute addresses are indirected by GOT (just GOT
references are relocated)
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.

However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

If I understood the process right, but the term (GOT) is wrong, sorry,
that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my
understanding above?
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!

Best regards,
Marvin

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not
necessarily "lazy", for ARM there simply seems to not be any reason to
distinguish if the environments are fully separate), just wanted to make
sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Marvin Häuser
 

On 23.07.21 16:34, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 16:27, Marvin Häuser <mhaeuser@...> wrote:


On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
...
2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
Do you maybe have one final comment regarding that second question,
please? :)
The RELA section is not converted into PE/COFF relocations. This would
not achieve a lot, given that no prior PE/COFF loader exists to
process them. There is a snippet of asm code in the startup code that
processes the R_AARCH64_RELATIVE relocation entries before calling
into C code.
I searched for said ASM code till my fingers fell asleep and at last found this: https://github.com/tianocore/edk2/commit/b16fd231f6d8124fa05a0f086840934b8709faf9#diff-3d563cc4775c7720900f4895bf619eed06291044aaa277fcc57eddc7618351a1L12-R148

If I understand the commit message correctly, it is basically "pray the C code does not use globals at all", which is fair enough, so maybe I should document this in my proposed new library? I trust that this is enough of a constraint for both ARM and AArch64, because I do not know them at all.

What worries me is that StandaloneMmCore has no such ASM entry point at all and instead it's just executing C directly. Also, it is not passed the "-fno-jump-tables" flag that is commented to be important in the commit linked above.

Best regards,
Marvin

This also gives us the guarantee that no GOT indirections are
dereferenced, given that our asm code simply does not do that.

Let's drop "GOT" and make it "any instruction that requires prior
relocation to function correctly".
The thing to keep in mind here is that R_AARCH64_RELATIVE relocations
never target instructions, but only memory locations that carry
absolute addresses. This could be locations in .rodata or .data
(global vars carrying pointer values), or GOT entries.

It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took
it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections
for instructions that embed and variables that hold an absolute address
(I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in
1), because all absolute addresses are indirected by GOT (just GOT
references are relocated)
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.

However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

If I understood the process right, but the term (GOT) is wrong, sorry,
that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my
understanding above?
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!

Best regards,
Marvin

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not
necessarily "lazy", for ARM there simply seems to not be any reason to
distinguish if the environments are fully separate), just wanted to make
sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Ard Biesheuvel
 

On Sat, 31 Jul 2021 at 21:08, Marvin Häuser <mhaeuser@...> wrote:

On 23.07.21 16:34, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 16:27, Marvin Häuser <mhaeuser@...> wrote:


On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 12:13, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 11:55, Marvin Häuser <mhaeuser@...> wrote:
...
2) emit a GOT, which ends up being converted to PE/COFF Relocations (->
self-relocation), for global data that cannot be referenced relatively?
Is there any way to know/force that no symbol in GOT is accessed up
until the end of the self-relocation process?
Do you maybe have one final comment regarding that second question,
please? :)
The RELA section is not converted into PE/COFF relocations. This would
not achieve a lot, given that no prior PE/COFF loader exists to
process them. There is a snippet of asm code in the startup code that
processes the R_AARCH64_RELATIVE relocation entries before calling
into C code.
I searched for said ASM code till my fingers fell asleep and at last
found this:
https://github.com/tianocore/edk2/commit/b16fd231f6d8124fa05a0f086840934b8709faf9#diff-3d563cc4775c7720900f4895bf619eed06291044aaa277fcc57eddc7618351a1L12-R148

If I understand the commit message correctly, it is basically "pray the
C code does not use globals at all", which is fair enough, so maybe I
should document this in my proposed new library? I trust that this is
enough of a constraint for both ARM and AArch64, because I do not know
them at all.
The C code can use globals, but not global pointer variables. But you
are right, this is not very robust at all.


What worries me is that StandaloneMmCore has no such ASM entry point at
all and instead it's just executing C directly. Also, it is not passed
the "-fno-jump-tables" flag that is commented to be important in the
commit linked above.
This is because the StandaloneMmCore is built with -fpie, which
already implies -fno-jump-tables, although I suppose this may not
offer complete coverage for BASE libraries that are pulled into the
link.


Best regards,
Marvin

This also gives us the guarantee that no GOT indirections are
dereferenced, given that our asm code simply does not do that.

Let's drop "GOT" and make it "any instruction that requires prior
relocation to function correctly".
The thing to keep in mind here is that R_AARCH64_RELATIVE relocations
never target instructions, but only memory locations that carry
absolute addresses. This could be locations in .rodata or .data
(global vars carrying pointer values), or GOT entries.

It is not really a GOT. Actually, a GOT is undesirable, as it forces
global variables to be referenced via an absolute address, even when a
relative reference could be used.
Hmm, the GCC docs say a GOT is used for "all constant addresses" (I took
it as "absolute"?), it is kind of vague. I understood it this way:
1) no-pie emits relocations that can target the .text and .data sections
for instructions that embed and variables that hold an absolute address
(I thought this was RELA?)
2) pie emits a GOT such that there are no relocations as described in
1), because all absolute addresses are indirected by GOT (just GOT
references are relocated)
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.

However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

If I understood the process right, but the term (GOT) is wrong, sorry,
that is what I gathered from the docs. :)
I have a x86 + PE background, so ARM + ELF is a bit of a learning curve...
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

For instance, a statically initialized pointer always carries an
absolute address, and so it always needs an entry in the RELA table

E.g.,

int foo = 10; // external linkage
static int *bar = &foo;

In this case, there is no way to use relative addressing because the
address of foo is taken at build time.

However, if bar would be something like

static int *bar() { return &foo; }

the address is only taken at runtime, and the compiler can use a
relative reference instead, and no RELA entry is needed. With a GOT,
we force the compiler to allocate a variable that holds the absolute
address, which we would prefer to avoid.
And this is not forced by whatever table -fpie uses, as per my
understanding above?
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!

Best regards,
Marvin

“Now, StandaloneMmPkg has similar (self-)relocation code too:https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/StandaloneMmCoreEntryPoint.c#L379-L386

Because I cannot find such elsewhere, I assume it must be for the same ARM virtualised environment as above.
No.

The binary it applies the Relocations to is documented to be the Standalone MM core, but in fact SecCore is located:

https://github.com/tianocore/edk2/blob/17143c4837393d42c484b42d1789b85b2cff1aaf/StandaloneMmPkg/Library/StandaloneMmCoreEntryPoint/AArch64/SetPermissions.c#L131-L158
As per your comments below, I think SecCore should not be located here.
Is the Standalone MM core of *type* SecCore in the FFS (without *being*
SecCore)? This confused me the most.
If the FFS SecCore section type is used here, it does not mean that
the image is a SEC image in the strict PI sense.

Perhaps we were just too lazy to add a new type to the FFS spec?
That is what I meant to imply with the middle question (well, not
necessarily "lazy", for ARM there simply seems to not be any reason to
distinguish if the environments are fully separate), just wanted to make
sure I understand what the code does before modifying it.

Thank you again!

Best regards,
Marvin

“This yields the following questions to me:

1) What even invokes Standalone MM on ARM? It is documented it is spawned during SEC, but I could not find any actual invocation.
It is not spawned by the normal world code that runs UEFI. It is a
secure world component that runs in a completely different execution
context (TrustZone). The code does run with the MMU enabled from the
start, but running from an a priori fixed offset was considered to be
a security hazard, so we added self relocation support.

The alternative would have been to add metadata to the StMmCore
component that can be interpreted by the secure world component that
loads it, but this would go beyond any existing specs, and make
portability more problematic.

2) Why does Standalone MM (self-)relocation locate SecCore? Should it not already have been relocated with the code from ArmPlatformPkg? Is Standalone MM embedded into ARM SecCore?
No and no. Standalone MM has nothing to do with the code that runs as
part of UEFI itself. ArmPlatformPkg is completely separate from
StandaloneMmPkg.

3) Why is SecCore the only module relocated? Are all others guaranteed to be "properly" loaded?
SecCore contains a PE/COFF loader, so all subsequent modules are
loaded normally. This is similar to the ArmVirtQemuKernel
self-relocating SEC module, which only relocates itself in this
manner, and relies on standard PE/COFF metadata for loading other
modules.
Interesting... this definitely is vastly different from the x86 side of
things. I think most things became very clear. Thanks a lot!

4) Is there maybe some high-level documented about the ARM boot flow? It seems to be significantly different from the x86 routes quite vastly.”
trustedfirmware.org may have some useful documentation.
I'll check it some time, hopefully this weekend. Thanks!
My pleasure.


Marvin Häuser
 

01.08.2021 18:33:47 Ard Biesheuvel <ardb@...>:

On Sat, 31 Jul 2021 at 21:08, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 16:34, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 16:27, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
...
Do you maybe have one final comment regarding that second question,
please? :)
The RELA section is not converted into PE/COFF relocations. This would
not achieve a lot, given that no prior PE/COFF loader exists to
process them. There is a snippet of asm code in the startup code that
processes the R_AARCH64_RELATIVE relocation entries before calling
into C code.
I searched for said ASM code till my fingers fell asleep and at last
found this:
https://github.com/tianocore/edk2/commit/b16fd231f6d8124fa05a0f086840934b8709faf9#diff-3d563cc4775c7720900f4895bf619eed06291044aaa277fcc57eddc7618351a1L12-R148
If I understand the commit message correctly, it is basically "pray the
C code does not use globals at all", which is fair enough, so maybe I
should document this in my proposed new library? I trust that this is
enough of a constraint for both ARM and AArch64, because I do not know
them at all.
The C code can use globals, but not global pointer variables. But you
are right, this is not very robust at all.
Right... Will document for my PE library.

What worries me is that StandaloneMmCore has no such ASM entry point at
all and instead it's just executing C directly. Also, it is not passed
the "-fno-jump-tables" flag that is commented to be important in the
commit linked above.
This is because the StandaloneMmCore is built with -fpie, which
already implies -fno-jump-tables, although I suppose this may not
offer complete coverage for BASE libraries that are pulled into the
link.
Ah okay, thanks. Out of curiosity of how ARM implements PIE, and how StMmCore self-relocation can work *after* the PE/COFF section permissions have been applied with .got merged into .text (i.e. read-only), I checked the GCC5 "DLL" with readelf and found many relocations into the .text section. I have no idea how any of this works, and no idea where to find out, but as it apparently does, I might just update the PE calls and call it a day. I cannot test anything either because there is no QEMU code for StMmCore I can find. :(

Thanks for your tireless replies!

Best regards,
Marvin


Best regards,
Marvin

This also gives us the guarantee that no GOT indirections are
dereferenced, given that our asm code simply does not do that.

Let's drop "GOT" and make it "any instruction that requires prior
relocation to function correctly".
The thing to keep in mind here is that R_AARCH64_RELATIVE relocations
never target instructions, but only memory locations that carry
absolute addresses. This could be locations in .rodata or .data
(global vars carrying pointer values), or GOT entries.

Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.
However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,

The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.

The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!
Best regards,
Marvin


Andrew Fish
 

On Aug 1, 2021, at 2:40 PM, Marvin Häuser <mhaeuser@...> wrote:

01.08.2021 18:33:47 Ard Biesheuvel <ardb@...>:

On Sat, 31 Jul 2021 at 21:08, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 16:34, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 16:27, Marvin Häuser <mhaeuser@...> wrote:
On 23.07.21 16:09, Ard Biesheuvel wrote:
On Fri, 23 Jul 2021 at 12:47, Marvin Häuser <mhaeuser@...> wrote:
...
Do you maybe have one final comment regarding that second question,
please? :)
The RELA section is not converted into PE/COFF relocations. This would
not achieve a lot, given that no prior PE/COFF loader exists to
process them. There is a snippet of asm code in the startup code that
processes the R_AARCH64_RELATIVE relocation entries before calling
into C code.
I searched for said ASM code till my fingers fell asleep and at last
found this:
https://github.com/tianocore/edk2/commit/b16fd231f6d8124fa05a0f086840934b8709faf9#diff-3d563cc4775c7720900f4895bf619eed06291044aaa277fcc57eddc7618351a1L12-R148
If I understand the commit message correctly, it is basically "pray the
C code does not use globals at all", which is fair enough, so maybe I
should document this in my proposed new library? I trust that this is
enough of a constraint for both ARM and AArch64, because I do not know
them at all.
The C code can use globals, but not global pointer variables. But you
are right, this is not very robust at all.
Right... Will document for my PE library.

What worries me is that StandaloneMmCore has no such ASM entry point at
all and instead it's just executing C directly. Also, it is not passed
the "-fno-jump-tables" flag that is commented to be important in the
commit linked above.
This is because the StandaloneMmCore is built with -fpie, which
already implies -fno-jump-tables, although I suppose this may not
offer complete coverage for BASE libraries that are pulled into the
link.
Ah okay, thanks. Out of curiosity of how ARM implements PIE, and how StMmCore self-relocation can work *after* the PE/COFF section permissions have been applied with .got merged into .text (i.e. read-only), I checked the GCC5 "DLL" with readelf and found many relocations into the .text section. I have no idea how any of this works, and no idea where to find out, but as it apparently does, I might just update the PE calls and call it a day. I cannot test anything either because there is no QEMU code for StMmCore I can find. :(
Marvin,

It is useful to remember that there are object file (resolved by the linker), dynamic loading (resolved when the DLL is bound at runtime), and image relocations. In the EFI PE/COFF we only end up with the image relocations that need to be processed when an image is loaded into memory. I seem to remember seeing the other classes of relocations still being present in the ELF files, but they end up being a no-opt for EFI. You can look at the EFI PE/COFF relocations to see the things EFI cares about.

Side note… The Xcode/clang toolchain requires the TEXT section to not contain relocations for X64, and the linker will fail if there is code that requires a relocation in the text section. This generally is not a problem, but hand coded assembler can trigger a link failure that is specific to Xcode.

Thanks,

Andrew Fish

Thanks for your tireless replies!

Best regards,
Marvin

Best regards,
Marvin
This also gives us the guarantee that no GOT indirections are
dereferenced, given that our asm code simply does not do that.
Let's drop "GOT" and make it "any instruction that requires prior
relocation to function correctly".
The thing to keep in mind here is that R_AARCH64_RELATIVE relocations
never target instructions, but only memory locations that carry
absolute addresses. This could be locations in .rodata or .data
(global vars carrying pointer values), or GOT entries.
Correct. And this works really well for shared libraries, where all
text and data sections can be shared between processes, as they will
not be modified by the loader. All locations targeted by relocations
will be nicely lumped together in the GOT.
However, for bare metal style programs, there is no sharing, and there
is no advantage to lumping anything together. It is much better to use
relative references where possible, and simply apply relocations
wherever needed across the text and data sections,
The GOT is a special data structure used for implicit variable
accesses, i.e., global vars used in the code. Statically initialized
pointer variables are the other category, which are not code, and for
which the same considerations do not apply, given that the right value
simply needs to be stored in the variable before the program starts.
The selection of 'code model' as it is called is controlled by GCC's
-mcmodel= argument, which defaults to 'small' on AArch64, regardless
of whether you use PIC/PIE or not.
Aha, makes sense, thanks!
Best regards,
Marvin