[lld] r361830 - Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded

[Stephan Bergmann via llvm-commits]

With bisecting, I found that the below commit breaks the debuginfo in my 
Linux LibreOffice builds that use lld with --gdb-index.  I wasn't able 
to reduce the test case in any meaningful way, but what I observe is 
that a gdb backtrace will print source information for some of the 
frames, but for others will only print the .so pathname.  I could not 
reproduce with a small test executable, but the shared libraries 
involved in LibreOffice can be rather large, so maybe the issue is 
related to size.  The backtraces look fine when I rebuild LibreOffice 
without using --gdb-index (though I appear to have some issue with 
"step" commands in gdb not stepping into function calls even then, but 
didn't investigate that further).

(I see that this commit has yesterday been reverted in r362739 and then 
re-reverted in r362743, so I assume the situation on trunk hasn't 
effectively changed from what I observed with my build from earlier this 
week.)

On 28/05/2019 16:34, Fangrui Song via llvm-commits wrote:
> Author: maskray
> Date: Tue May 28 07:34:28 2019
> New Revision: 361830
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=361830&view=rev
> Log:
> Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded
> 
> This is implemented by creating Undefined (instead of Defined) for such
> local STT_SECTION symbols. It allows us to catch errors when there are
> relocations to such discarded sections (e.g. in PR41693, ld.bfd and gold
> error but we don't). Updated comdat-discarded-error.s checks we emit
> friendly error message.
> 
> For relocatable-eh-frame.s, ld.lld -r a.o a.o will now error
> "STT_SECTION symbol should be defined" because the section .eh_frame
> refers to is now an Undefined instead of a Defined.
> So I have to change `error()` to `warn()` to retain the output.
> 
> rLLD361144 inadvertently enabled the error for --gdb-index
> (in LLDDwarfObj<ELFT>::findAux()).
> 
> Relocations from .debug_info (not in comdat) to .text.* (in comdat) for
> DW_AT_low_pc are common. If an .text.* was discarded, rLLD361144 would error,
> which was unexpected. (Note, if we don't error as this patch does,
> InputSection::relocateNonAlloc() will resolve such relocations).
> 
> Added:
>      lld/trunk/test/ELF/comdat-discarded-gdb-index.s
> Modified:
>      lld/trunk/ELF/DWARF.cpp
>      lld/trunk/ELF/InputFiles.cpp
>      lld/trunk/ELF/InputSection.cpp
>      lld/trunk/ELF/Relocations.cpp
>      lld/trunk/test/ELF/comdat-discarded-error.s
>      lld/trunk/test/ELF/comdat-discarded-reloc.s
>      lld/trunk/test/ELF/comdat.s
>      lld/trunk/test/ELF/invalid-undef-section-symbol.test
>      lld/trunk/test/ELF/relocatable-eh-frame.s