linux-kernel - Re: [PATCH v2 00/20] objtool: vmlinux.o and CLANG LTO support

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Message-ID: <CABCJKucsYknH2z8ypGoBEpNwhdy+LpT_8N2jw+14ToLJ5ps+HQ@mail.gmail.com>
Date:   Mon, 25 Jan 2021 14:43:30 -0800
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Clang-Built-Linux ML <clang-built-linux@...glegroups.com>,
        Miroslav Benes <mbenes@...e.cz>,
        Fangrui Song <maskray@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>
Subject: Re: [PATCH v2 00/20] objtool: vmlinux.o and CLANG LTO support

On Fri, Jan 22, 2021 at 6:26 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Fri, Jan 22, 2021 at 05:32:43PM -0800, Nick Desaulniers wrote:
> > > In this specific case, find_func_by_offset returns NULL for
> > > .text..L.cfi.jumptable.43 at addend 0x8, because Clang doesn't emit
> > > jump table symbols for static functions:
> > >
> > > 0000000000000000 <__typeid__ZTSFjmiE_global_addr>:
> > >    0:   e9 00 00 00 00          jmpq   5 <__typeid__ZTSFjmiE_global_addr+0x5>
> > >                         1: R_X86_64_PLT32       io_serial_in-0x4
> > >    5:   cc                      int3
> > >    6:   cc                      int3
> > >    7:   cc                      int3
> > >    8:   e9 00 00 00 00          jmpq   d <__typeid__ZTSFjmiE_global_addr+0xd>
> > >                         9: R_X86_64_PLT32       mem32_serial_in-0x4
> > >    d:   cc                      int3
> > >    e:   cc                      int3
> > >    f:   cc                      int3
> > >
> > > Nick, do you remember if there were plans to change this?
> >
> > Do you have a link to any previous discussion to help jog my mind; I
> > don't really remember this one.
> >
> > Is it that `__typeid__ZTSFjmiE_global_addr` is the synthesized jump
> > table, and yet there is no `__typeid__ZTSFjmiE_global_addr` entry in
> > the symbol table?
>
> I think he means there's not a 'mem32_serial_in.cfi_jt' symbol at
> '__typeid__ZTSFjmiE_global_addr+8'.  Probably more aggressive symbol
> pruning from the assembler.

Correct, the compiler is not emitting mem32_serial_in.cfi_jt here. I
seem to recall that the missing jump table symbols also made stack
traces harder to follow (__typeid__ZTSFjmiE_global_addr+8 is not very
readable), so ideally they shouldn't be pruned.

> It's fine though.  I just need to rewrite the CFI support a bit.
>
> But that can come later.  For now I'll just drop the two CFI-related
> patches from this set so I can merge the others next week.

Sure, sounds good.

Sami