[llvm-dev] Modifying objects with MC

Sean Silva via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 26 20:33:51 PDT 2015

On Wed, Aug 26, 2015 at 7:07 PM, Jim Grosbach via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> A mutable interface to object files would be great to have but doesn't
> exist in any meaningful sense in LLVM today. David's hack and similar
> tricks are what's necessary right now.
> I'd love to fix that as its a question that comes up not infrequently.

Unfortunately this is a pretty tough problem to do in any generality.
Anything that isn't simply overwriting the contents of sections in the file
can effectively require rewriting the entire file. One of my realizations
when writing `yaml2obj -format=elf` was how intricately tied all the
offsets are; if you search for every use of
ContiguousBlobAccumulator::getOSAndAlignedOffset, essentially any or all of
those will need to be edited when making any nontrivial modification of an
ELF object file (and this isn't even considering e.g. PHDRS covering
sections etc.).

 (there's a reason that most of this functionality is usually in a program
called objCOPY; basically objcopy is a program that copies an object file
in a semantic way (e.g. logically section by section or whatever); it then
has some if's and for's sprinkled in at various key places to handle the
various different options it has).

-- Sean Silva

> Sent from my iPhone
> > On Aug 25, 2015, at 2:44 AM, David Chisnall via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >
> >> On 25 Aug 2015, at 10:30, Simon Cook via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >>
> >> I've been trying to write a tool that reads in an ELF file and modifies
> >> one section within that ELF file based on the data contained within
> >> another section. Using llvm-objdump as a template of how to read an
> >> object file, I've been able to read the latter section that tells me
> >> what to edit, but I haven't found a way to edit the former, as
> >> everything in the MCSection/ELFObjectFile/SectionRef classes seem to be
> >> read-only as far as section contents are concerned.
> >>
> >> The changes I would want to make don't change the size of any section,
> >> all I need to do is for example zero the first n bytes of .text whilst
> >> keeping the rest of the object the same. What would be the recommended
> >> approach to achieving this goal?
> >
> > I have a similar use case and would also be interested in an answer.  In
> the meantime, I have a horrible hack that may work for you.
> >
> > My current hack works because the returned StringRefs for the
> SymbolRef::getContents are within the range of the MemoryBuffer and so the
> offset relative to the memory buffer corresponds to the file offset and
> this offset can be used to write into the file directly.
> >
> > It’s a horrible hack though, and it would be nice to simply have a
> mutable memory buffer that can be written back to the file (or to a
> different file).
> >
> > David
> >
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