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

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Date:   Mon, 7 Jun 2021 09:56:48 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fāng-ruì Sòng <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,
        Radosław Biernacki <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 Sat, Jun 05, 2021 at 06:58:39PM -0700, Fāng-ruì Sòng wrote:
> On Sat, Jun 5, 2021 at 3:39 AM Peter Zijlstra <peterz@...radead.org> wrote:

> > 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.

Right, that's about as far as I got, but never could find details on how
the extension worked in detail, and I clearly muddled it :/

> 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 specific location within a section, but that the
> > section header index for that section is too large to be represented
> > directly in the symbol table entry. The actual section header index
> > is found in the associated SHT_SYMTAB_SHNDX section. The entries in
> > that section correspond one to one with the entries in the symbol
> > table. Only those entries in SHT_SYMTAB_SHNDX that correspond to
> > symbol table entries with SHN_XINDEX will hold valid section header
> > 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.

Excellent, lemme go read up and attempt to fix this.