Re: Help with debugging


Andrew Fish
 



On Jun 11, 2021, at 4:29 PM, Ethin Probst <harlydavidsen@...> wrote:

Your suggestion of adding 0x240 worked. I'm able to successfully step
through the code now. Thank you!


OK that makes sense. The address in the add-symbol-file command is not the load address of the image, but the start address of the text section. So that is why you had to add 0x240. 

Sorry I had to work backwards from how it works, but maybe that info will be helpful for others?

Thanks,

Andrew Fish

On 6/11/21, Andrew Fish <afish@...> wrote:


On Jun 11, 2021, at 2:29 PM, Ethin Probst <harlydavidsen@...>
wrote:

Initial connection and loading symbols:
Remote debugging using :1234
0x000000007e4b9517 in ?? ()
add symbol table from file "Build/MdeModule/DEBUG_GCC5/X64/UsbAudio.debug"
at
.text_addr = 0x7e4b8000
Reading symbols from Build/MdeModule/DEBUG_GCC5/X64/UsbAudio.debug...
Expanding full symbols from
Build/MdeModule/DEBUG_GCC5/X64/UsbAudio.debug...
Backtrace:
#0  0x000000007e4b9517 in UefiMain (st=0x7f9ee018,
imageHandle=0x7e4f7518) at
/home/ethin/source/edk/edk2/MdeModulePkg/Application/UsbAudio/UsbAudio.c:72
#1  ProcessModuleEntryPointList (SystemTable=0x7f9ee018,
ImageHandle=0x7e4f7518) at
/home/ethin/source/edk/edk2/Build/MdeModule/DEBUG_GCC5/X64/MdeModulePkg/Application/UsbAudio/UsbAudio/DEBUG/AutoGen.c:300
#2  _ModuleEntryPoint (ImageHandle=0x7e4f7518, SystemTable=0x7f9ee018)
at
/home/ethin/source/edk/edk2/MdePkg/Library/UefiApplicationEntryPoint/ApplicationEntryPoint.c:59
#3  0x000000007fead316 in ?? ()
#4  0x000000007e4f7518 in ?? ()
#5  0x000000007feab5c7 in ?? ()
#6  0x000000007fea3520 in ?? ()
#7  0x0000000101000000 in ?? ()
#8  0x0000000000000030 in ?? ()
#9  0x000000007e4f6018 in ?? ()
#10 0x000000007e60a918 in ?? ()
#11 0x000000000000011d in ?? ()
#12 0x000000007fea3528 in ?? ()
#13 0x000000007e4f7818 in ?? ()
#14 0x000000007e4f7c98 in ?? ()
#15 0x000000007fea3538 in ?? ()
#16 0x000000007e3abfca in ?? ()
#17 0x000000007e4f7418 in ?? ()
#18 0x000000007fea3528 in ?? ()
#19 0x0000000000000000 in ?? ()
Source-code listing:
1 /** @file
2   GCC inline implementation of BaseLib processor specific functions.
3
4   Copyright (c) 2006 - 2020, Intel Corporation. All rights
reserved.<BR>
5   Portions copyright (c) 2008 - 2009, Apple Inc. All rights
reserved.<BR>
6   SPDX-License-Identifier: BSD-2-Clause-Patent
7
8 **/
9
10
Attempt to use "next":
72 } else if (interfaceDescriptor.InterfaceClass == 0x01 &&
interfaceDescriptor.InterfaceSubClass == 0x03) {
(This is my code but it continuously prints this same line over and
over every time "next" is used.)
Attempt to use "print Index":
No symbol "Index" in current context.
info local:
UsbIo = 0x0
interfaceDescriptor = {Length = 0 '\000', DescriptorType = 8 '\b',
InterfaceNumber = 1 '\001', AlternateSetting = 0 '\000', NumEndpoints
= 0 '\000', InterfaceClass = 0 '\000', InterfaceSubClass = 0 '\000',
InterfaceProtocol = 0 '\000',
Interface = 0 '\000'}
i = 2118887920
numHandles = 264
handles = 0x4
status = <optimized out>
info symbol 0x0007E4B9440:
_ModuleEntryPoint + 576 in section .text of
/home/ethin/source/edk/edk2/Build/MdeModule/DEBUG_GCC5/X64/UsbAudio.debug

OK that is interesting…. +576 -> 0x240 witch is about the size of the
PE/COFF header.

For mach-O (macOS executables) we have to link at 0x240 to make space for
the PE/COFF header in memory….

So the PE/COFF header starts at 0x7e4b8000 it is likely the text section
starts at 0x7e4b8240? So try adding 0x240 to the load address on the
add-symbol-file command. If that does not work trip subtracting 0x240 from
the load address.

We would need to dump out the UsbAudio.efi file to figure out exactly what
is going on. What distro are you on? Do you have the readpe utility? I’m not
sure what you can dump with objcopy?

Can you mail me a copy of UsbAudio.efi off list? I can take a quick look.

Thanks,

Andrew Fish

The extra weird thing about this is that CpuDeadLoop() is at the start
of the UefiMain function, its not on line 72. The program doesn't even
start there -- it starts by attempting to get the list of
EFI_USB_IO_PROTOCOL handles available. And GDB is making it look like
its skipping all of that.

On 6/11/21, Andrew Fish <afish@... <mailto:afish@...>> wrote:


On Jun 11, 2021, at 1:48 PM, Ethin Probst <harlydavidsen@...>
wrote:

Okay, so I just tried exactly what you told me to do -- use
CpuDeadLoop() and then just modify index to get out of it. Here's what
I do in GDB:
- Load the EFI application and connect via target remote :1234
- type `add-symbol-file Build/MdeModule/DEBUG_GCC5/X64/UsbAudio.debug
0x0007E4B8000` and answer yes when it prompts me to do so.
(0x0007E4B8000 is the image base, the entry point is at
0x0007E4B9440.)
- When I try to print the Index symbol, GDB tells me that it isn't in
the current context.
I feel like I'm missing something. I'm also not the best with GDB
myself.
:)

What do you get from the following gdb commands?
bt
info local
info symbol 0x0007E4B9440

What exactly is gdb showing you?

Thanks,

Andrew Fish


On 6/11/21, Andrew Fish <afish@... <mailto:afish@...>
<mailto:afish@... <mailto:afish@...>>> wrote:


On Jun 11, 2021, at 11:39 AM, Ethin Probst <harlydavidsen@...
<mailto:harlydavidsen@...>>
wrote:

Hi Andrew,
How do you debug the EFI binary with LLDB? Can LLDB use GDB stubs or
does that work differently?


Ethin,

Lldb is the command line debugger that comes with Xcode on Mac. There
is
no
gdb with Xcode, so I have to use lldb for my day job.

Lldb can speak the gdb remote serial protocol: lldb -o “gdb-remote
9000”
That assumes you passed `-gdb tcp::9000`to QEMU.

Thanks,

Andrew Fish

On 6/11/21, Andrew Fish <afish@... <mailto:afish@...>
<mailto:afish@... <mailto:afish@...>>
<mailto:afish@... <mailto:afish@...>
<mailto:afish@... <mailto:afish@...>>>> wrote:


On Jun 11, 2021, at 10:06 AM, Ethin Probst <harlydavidsen@...
<mailto:harlydavidsen@...>
<mailto:harlydavidsen@... <mailto:harlydavidsen@...>>>
wrote:

Hey all,

So Leif and I have discussed this at length but I thought I'd reach
out to all of you for more help.

I'm having a lot of trouble debugging my UEFI app. Here's how I do
things:

- I load the app using uefi-run
(https://github.com/Richard-W/uefi-run
<https://github.com/Richard-W/uefi-run>
<https://github.com/Richard-W/uefi-run
<https://github.com/Richard-W/uefi-run>>) like this (from the main
EDK
II directory): uefi-run -b Build/OvmfX64/DEBUG_GCC5/FV/OVMF.fd
Build/OvmfX64/DEBUG_GCC5/X64/Shell.efi -- -M q35 -m 24G -usb
-device
qemu-xhci -device usb-audio,audiodev=audio -audiodev alsa,id=audio
-s
-debugcon file:../debug.log -global isa-debugcon.iobase=0x402
-nographic
Or:
uefi-run -b Build/OvmfX64/DEBUG_GCC5/FV/OVMF.fd
Build/OvmfX64/DEBUG_GCC5/X64/Shell.efi -- -M q35 -m 24G -usb
-device
qemu-xhci -device usb-audio,audiodev=audio -audiodev alsa,id=audio
-s
-debugcon stdio -global isa-debugcon.iobase=0x402
- I connect to the remote GDB stub (localhost:1234) and wait until
OVMF gives me the image base. Then I use:
add-symbol-file UsbAudio.debug <image base>
Here's where everything breaks down. One of two things happens at
this
point:
1. Either I get the wrong debug information (I get source code but
the
image isn't loaded anymore), and resetting the system and placing a
breakpoint (either software or hardware) has no effect; or
2. If I use CpuBreakpoint(), the firmware gives me the registers
and
the image base and entry point addresses, and then appears to just
sit
there waiting for something. Once I load the symbols using the
image
base it gives me, I can't actually do anything in the debugger; I
can't list code because I get "1 in <artificial>", I can't jump
into
my code without triggering a general protection exception or not
actually causing anything to happen... You get the idea.

So I'm really, really confused on what's going wrong. Do you guys
have
any advice?

Ethin,

Caveat emptor as I use lldb for my daily driver debugger so I might
be
a
little off on gdb specifics…. Also my terminology may be lldb
centric.

Easy one 1st. When you run on top of a debugger using
CpuBreakpoint()
works
great as the debugger hides its self from you. On x86
CpuBreakpoint()
is
an
INT 3h instruction (0xCC) and it causes an exception 3. If you don’t
have
a
debugger hooked in underneath  the exception 3 is going to get
handled
in
the unexpected exception handler, and that is probably in the CPUD
DXE
driver or DXE Core or some such. So you are going to end up with the
PC/IP/RIP in the wrong driver. A lot of times for hardware debuggers
it
works better to use CpuDeadLoop(). The gdb-remote stub from QEMU
acts
a
lot
more like a JTAG hardware debugger than a pure software debugger.
Also
note
that CpuDeadLoop() is an infinite loop, so you can modify the loop
variable
with the debugger to continue.

I’d suggest a work flow of run your App/Driver, hit the
CpuDeadLoop(),
attach gdb. Now after you have the target established load the
symbols.
The
reason for me suggesting this flow is the debugger has a flexible
concept
of
what the target is. If you load symbols that will create a target
for
a
stock x86-64 image. When you connect to the QEMU gdb-remote there is
a
handshake that describes the target and what registers are
available.
I
seem
to remember QEMU exports some of the system registers, like the
control
registers, so it is an extended version of the x86-64 target. So
this
changing the target definition might confuse the debugger. To be
safe
I
always connect 1st and then load symbols.

The EFI images are PE/COFF relocatable executables that are linked
around
zero. They get loaded into memory and relocated, so that is why you
need
to
specify the load address to get the symbols to resolve. One trick I
use
is
to load the ELF (or PE/COFF) build output directly into the
debugger.
This
lets you poke around the image at the linked address. You can
disassemble
the functions to see what they look like, obviously you can read any
variables. This can be useful if you get the unhandled exception and
it
prints out the load address and offset (you can use the offset
directly).
It
is also a good way to debug why your symbols are not quite loaded at
the
correct address, as you can see what bytes/instructions should be at
a
given
address.

Thanks,

Andrew Fish


--
Signed,
Ethin D. Probst









--
Signed,
Ethin D. Probst







--
Signed,
Ethin D. Probst







--
Signed,
Ethin D. Probst




-- 
Signed,
Ethin D. Probst



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