llvm dev - Aug 2020 - Adding sections in a binary

Hey,

LLVM has logic to parse ELF and PE binaries using
`llvm::object::createBinary`. I tried to search in the codebase to see if
there's a possibility to add/remove sections after parsing a binary and
re-write the binary to another location. Basically, like what llvm-objcopy
does. Can you point me to the right classes to look into, if this is
something that LLVM has?

Many thanks

Joseph
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Sounds like the llvm-objcopy source code (llvm/tools/llvm-objcopy) is
probably a good place to start.

On Thu, Aug 13, 2020 at 8:11 AM Joseph via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hey,
>
> LLVM has logic to parse ELF and PE binaries using
`llvm::object::createBinary`. I tried to search in the codebase to see if
there's a possibility to add/remove sections after parsing a binary and
re-write the binary to another location. Basically, like what llvm-objcopy does.
Can you point me to the right classes to look into, if this is something that
LLVM has?
>
> Many thanks
>
> Joseph
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On 2020-08-13, David Blaikie via llvm-dev wrote:
>Sounds like the llvm-objcopy source code (llvm/tools/llvm-objcopy) is
>probably a good place to start.
>
>On Thu, Aug 13, 2020 at 8:11 AM Joseph via llvm-dev
><llvm-dev at lists.llvm.org> wrote:
>>
>> Hey,
>>
>> LLVM has logic to parse ELF and PE binaries using
`llvm::object::createBinary`. I tried to search in the codebase to see if
there's a possibility to add/remove sections after parsing a binary and
re-write the binary to another location. Basically, like what llvm-objcopy does.
Can you point me to the right classes to look into, if this is something that
LLVM has?
>>
>> Many thanks
>>
>> Joseph
For ELF,

* adding a non-SHF_ALLOC section is simple. A non-SHF_ALLOC section is not part
of
   the memory image and not used by the program (unless for some rare
   introspection use cases)

* adding a SHF_ALLOC section is difficult. You likely need to fix the
   containing PT_LOAD segment. llvm-objcopy only does the base p_offset fix.
   You need to take care p_vaddr/p_paddr/p_filesz/p_memsz by your self.

   + adding a section smaller than the known lowest address (ET_EXEC with a
     non-zero image base) or larger than the known largest address:
     The PT_LOAD fixes are doable.
   + adding a section within the existing address ranges: this is very difficult
     due to many implicit inter-section references. If you have an advanced
     binary rewriting tool, this is still doable, but definitely brittle.

   File offsets (p_offset,sh_offset) can be reconstructed from addresses.
   llvm-objcopy/ELF/Object.cpp layoutSections has some code.
   A more sophisticated implementation is in the linker: lld/ELF/Writer.cpp
assignFileOffsets