[lld] Warn for missing needed sections by --gdb-index

[aury6623]

Issue a warning when lld is passed --gdb-index and some object files have general debug info but not the specific debug sections generated by -ggnu-pubnames or -gsplit-dwarf. Missing those sections on all inputs leads to an empty index, which gdb ignores. Missing those sections on only some sources leads to a partially complete index, which confuses gdb.

Addresses #156732 and #34168

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[llvmbot]

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: None (aury6623)

Changes

Issue a warning when lld is passed --gdb-index and some object files have general debug info but not the specific debug sections generated by -ggnu-pubnames or -gsplit-dwarf. Missing those sections on all inputs leads to an empty index, which gdb ignores. Missing those sections on only some sources leads to a partially complete index, which confuses gdb.

Addresses #156732 and #34168

---

Full diff: https://github.com/llvm/llvm-project/pull/166626.diff

1 Files Affected:

• (modified) lld/ELF/SyntheticSections.cpp (+10)

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp index a4150ebfa1653..77487de805960 100644 --- a/lld/ELF/SyntheticSections.cpp +++ b/lld/ELF/SyntheticSections.cpp @@ -3566,6 +3566,16 @@ std::unique_ptr<GdbIndexSection> GdbIndexSection::create(Ctx &ctx) { DWARFContext dwarf(std::make_unique<LLDDwarfObj<ELFT>>(file)); auto &dobj = static_cast<const LLDDwarfObj<ELFT> &>(dwarf.getDWARFObj()); + if (dobj.getGnuPubnamesSection().sec == nullptr && + dobj.getGnuPubtypesSection().sec == nullptr) { + Warn(ctx) + << files[i] + << ": file contains debug info but is missing .debug_gnu_pubnames or " + ".debug_gnu_pubtypes sections required by --gdb-index. Compile " + "the file using -gsplit-dwarf or -ggnu-pubnames to generate these " + "sections."; + } + // If the are multiple compile units .debug_info (very rare ld -r --unique), // this only picks the last one. Other address ranges are lost. chunks[i].sec = dobj.getInfoSection(); 

[aury6623]

This edit is a solution for Issue #156732 I posted a few months ago. Having this warning would have saved me a day or two of debugging! In particular, when only some files are compiled with -ggnu-pubnames, but --gdb-index is still used, gdb can act in strange ways. E.g. in one case, gdb would only acknowledge the existence of a global type within certain scopes of the same function!

Now knowing what the issue is, I found this forum post where someone is running into a similar issue:
https://groups.google.com/g/llvm-dev/c/hxnPll-6de0
and this feature request asking for --gdb-index to work without -ggnu-pubnames:
#34168

which suggests that other people may find this warning to be helpful too.

---

This is my first time contributing to llvm-project. I know adding new tests for a change is normally expected, but I'm not sure what test changes (if any) are expected for only an addition of a warning message, or where that test should be added. Please advise. Thanks!

[lenary]

Please can you add a test?

[aury6623]

@lenary

Please can you add a test?

Sure, happy to! This is my first contribution to llvm-project, so I'm not quite familiar with the testing infrastructure. I've read this page:
https://llvm.org/docs/TestingGuide.html
But I can't tell which category of test this edit fits into. Based on the guide, and that this issue is best tested by compiling a whole program (it requires submitting multiple .o files to lld), it seems like the external test-suite repo is best suited. However, I don't see anywhere in that repo dedicated to lld tests.

If you could let me know where a test for an edit like this should go, that would be helpful! Thanks!

[lenary]

Yeah, that guide is good at the higher level, but I can see why you need more guidance :)

I think what we'd want is an assembly + linker test. This would consist of 1 (maybe 2) assembly files, which are assembled with llvm-mc, and then linked. These can be written all in the same file using the split-file utility, and the approach documented in "Extra Files" in that document.

The core assertion behind the test would be to invoke LLD, and then run FileCheck on its output. FileCheck would then check for the warnings in LLD's output. This tends to be done something like:

# REQUIRES: <target you chose> # RUN: rm -rf %t && split-file %s %t && cd %t # RUN: llvm-mc -filetype=obj -triple=<target you choose> input.s -o input.o # RUN: ld.lld input.o -o output.exe 2>&1 | FileCheck %s --check-prefix=WARNING --implicit-check-not=warning: # WARNING: <the text from your warning> #--- input.s <contents of your assembly file> 

It's fine to take a simple C example, and compile it on your own using clang -S -g file.c -o - to get some assembly to base your input on - I know assembling dwarf by hand isn't fun/easy.