Re: [PATCH v2 2/7] BaseTools-GenFw:Add new x86_64 Elf relocation types for PIC/PIE code


Shi, Steven <steven.shi@...>
 


CoffAddFixup() must be used for absolute symbol references only. These
instructions contain relative symbol references, which are
recalculated in WriteSections64().

The only absolute symbol reference is the GOT entry for 'n', and your
code (in WriteRelocations64()) calculates the address of the GOT entry
(which is always in .text BTW) and adds a fixup for it, i.e.,

+ CoffAddFixup(
+ (UINT32)(UINTN)((UINT64)
mCoffSectionsOffset[RelShdr->sh_info] + GoTPcRelPtrOffset),
+ EFI_IMAGE_REL_BASED_DIR64);

This code adds a fixup to the PE/COFF .reloc section for the GOT entry
containing the address of 'n', and the instructions perform a IP
relative load of the contents of the GOT entry to retrieve the address
of 'n'.

By adding two fixups, the PE/COFF loader will apply the load offset
twice, resulting in an incorrect value.
OK, I get your point now. Yes, the current patch could generate multiple fixups for the same GOT relocation entry. How about we introduce a simple IsDuplicatedCoffFixup() to check whether a converting fixup offset is duplicated before we use CoffAddFixup() to really add it? If it is new, we add it, otherwise just skip it.

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