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

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Date:   Sat, 23 Jan 2021 03:50:21 +0100
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Sami Tolvanen <samitolvanen@...gle.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 Sat, Jan 23, 2021 at 3:46 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Sat, Jan 23, 2021 at 03:31:20AM +0100, Sedat Dilek wrote:
> > On Sat, Jan 23, 2021 at 3:26 AM 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.
> > >
> > > 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.
> > >
> >
> > Two CFI-related patches?
> >
> > What's the other than "objtool: Add CONFIG_CFI_CLANG support"?
>
> I was referring to patches 10 and 11:
>
>   objtool: Move unsuffixed symbol conversion to a helper function
>   objtool: Add CONFIG_CFI_CLANG support
>

OK, thanks.

I will test with a revert of these two patches in another scenario
where I have problems with Clang's IAS but not with GNU AS.

- Sedat -

> You can just drop those patches from your testing (and don't test with
> CFI).
>
> > Do you plan (or offer) a v3 of objtool-vmlinux?
>
> Not unless there are any more comments.
>
> --
> Josh
>
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