llvm-strip utility strips debuglink sections by default whereas GNU strip does not

[kraj]

I ran into a problem where debug info was not being loaded by gdb. In yocto we split the debug info and keep them separate.
so it turns out to be that when we use llvm-strip for providing STRIP it ends up eating .gnu_debuglink section as well which contains the link to separated out debug info file and hence the debugger can not find the link any more. This is seen with clang-14/clang-15 and trunk too.

Here is a testcase

set -x
#CROSS=x86_64-yoe-linux-
#TC=llvm-
rm -rf a.out*
clang -g -xc /dev/null -ffreestanding -shared -o a.out
${CROSS}${TC}objcopy --version
${CROSS}${TC}objcopy --only-keep-debug a.out a.out.dbg
${CROSS}${TC}objcopy --add-gnu-debuglink=a.out.dbg a.out
#${CROSS}${TC}strip --remove-section=.note -o a.out.stripped a.out
${CROSS}${TC}strip -o a.out.stripped a.out
#${CROSS}${TC}objcopy --strip-debug a.out a.out.stripped
${CROSS}readelf -wk a.out.stripped

bad

TC=llvm- sh build.sh

good

sh build.sh

[kraj]

@MaskRay FYI

[llvmbot]

@llvm/issue-subscribers-tools-llvm-objcopy-strip

[kraj]

workaround for now is to add --keep-section=.gnu_debuglink explicitly to llvm-strip cmdline. But I think it would be good if it behaved like GNU strip here.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

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2. Fix the issue locally.
3. Run the test suite locally.
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4. Create a git commit
5. Run git clang-format HEAD~1 to format your changes.
6. Submit the patch to Phabricator.
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[felixkellenbenz]

Hey is anyone working on this issue? if not could you please assign it to me ?

[felixkellenbenz]

Hey, my approach would be to add the .gnu_debuglink file to KeepSection of the CommonConfig by default. Maybe someone could tell me if that's viable or if I should think of another approach. Thanks!

[jh7370]

Hey, my approach would be to add the .gnu_debuglink file to KeepSection of the CommonConfig by default. Maybe someone could tell me if that's viable or if I should think of another approach. Thanks!

I suspect this functionality wants to be specific to the ELF file format, so adding it to CommonConfig wouldn't be the right approach. Some questions to answer:

1. Should this impact llvm-objcopy as well as llvm-strip (I suspect it should)?
2. Is the section considered a "debug" section? In other words, if you used the --strip-debug option, would it be removed by GNU strip or objcopy?

If the answer to both questions is yes, then it probably makes sense to add the section name to the list of section names specified in isDebugSection in ELFObjcopy.cpp (see

llvm-project/llvm/lib/ObjCopy/ELF/ELFObjcopy.cpp

Line 55 in 295edaa

return StringRef(Sec.Name).startswith(".debug") || Sec.Name == ".gdb_index";

).

[felixkellenbenz]

Thanks for your answer, I will look into this as soon as possible.

[felixkellenbenz]

Hey,

the issue was fixed by #78919, could someone close it ?