linux-kernel - Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

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Date:   Sat, 5 Jun 2021 12:38:51 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>, lma@...ihalf.com,
        Guenter Roeck <groeck@...gle.com>,
        Juergen Gross <jgross@...e.com>, lb@...ihalf.com,
        LKML <linux-kernel@...r.kernel.org>, mbenes@...e.com,
        rad@...ihalf.com, upstream@...ihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

On Fri, Jun 04, 2021 at 04:50:46PM -0700, Fangrui Song wrote:
> On 2021-06-04, 'Nick Desaulniers' via Clang Built Linux wrote:

> > is producing the linker error:
> > 
> > ld.lld: error: drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o:
> > SHT_SYMTAB_SHNDX has 79581 entries, but the symbol table associated
> > has 79582
> > 
> > Readelf having issues with the output:
> > $ readelf -s amdgpu.lto.o.orig
> > <works fine>
> > $ readelf -s amdgpu.lto.o
> > readelf: Error: Reading 73014451695 bytes extends past end of file for
> > string table
> > $ llvm-readelf -s amdgpu.lto.o
> > llvm-readelf: error: 'amdgpu.lto.o': unable to continue dumping, the
> > file is corrupt: section table goes past the end of file
> > 

> tools/objtool/elf.c:elf_add_symbol may not update .symtab_shndx .
> Speaking of llvm-objcopy, it finalizes the content of .symtab_shndx when .symtab
> is finalized. objtool may want to adopt a similar approach.
> 
> read_symbols searches for the section ".symtab_shndx". It'd be better to
> use the section type SHT_SYMTAB_SHNDX.

I think you've absolutely nailed it; but would you have more information
or a code reference to what you're speaking about? My complete ELF
and libelf knowledge is very limited and as demonstrated here, I'm not
at all sure how all that extended index stuff is supposed to work.