[llvm-dev] Adding sections in a binary

Joseph via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 13 22:44:17 PDT 2020

Many thanks for the great info. What about removing a section? Is there an
implementation for that already in the codebase?

On Thu, 13 Aug 2020, 20:14 Fangrui Song, <maskray at google.com> wrote:

> 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
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