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From:   =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Date:   Sat, 5 Jun 2021 18:58:39 -0700
Message-ID: <CAFP8O3+ggR8N-ffsaYSMPX7s2XgrzzTQQjOgCwUe9smyos-waA@mail.gmail.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>, lma@semihalf.com,
        Guenter Roeck <groeck@google.com>,
        Juergen Gross <jgross@suse.com>, lb@semihalf.com,
        LKML <linux-kernel@vger.kernel.org>, mbenes@suse.com,
        =?UTF-8?Q?Rados=C5=82aw_Biernacki?= <rad@semihalf.com>,
        upstream@semihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>
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On Sat, Jun 5, 2021 at 3:39 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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 fo=
r
> > > 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 whe=
n .symtab
> > is finalized. objtool may want to adopt a similar approach.
> >
> > read_symbols searches for the section ".symtab_shndx". It'd be better t=
o
> > 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.

The section index field of an Elf{32,64}_Sym (st_shndx) is 16-bit, so
it cannot represent a section index greater than 0xffff.
ELF actually reserves values in 0xff00~0xff00 for other purposes, so
st_shndx cannot represent a section whose index is greater or equal to
0xff00.
To overcome the 16-bit section index limitation, .symtab_shndx was designed=
.

http://www.sco.com/developers/gabi/latest/ch4.symtab.html says

> SHN_XINDEX
> This value is an escape value. It indicates that the symbol refers to a s=
pecific location within a section, but that the section header index for th=
at section is too large to be represented directly in the symbol table entr=
y. The actual section header index is found in the associated SHT_SYMTAB_SH=
NDX section. The entries in that section correspond one to one with the ent=
ries in the symbol table. Only those entries in SHT_SYMTAB_SHNDX that corre=
spond to symbol table entries with SHN_XINDEX will hold valid section heade=
r indexes; all other entries will have value 0.

You may use https://github.com/llvm/llvm-project/blob/main/llvm/tools/llvm-=
objcopy/ELF/Object.cpp#L843
as a reference.