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

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Date:   Fri, 22 Jan 2021 20:26:09 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Sami Tolvanen <samitolvanen@...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 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.

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.

-- 
Josh