linux-kernel - Re: gcc-8 objtool warnings

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Date:   Tue, 19 Sep 2017 14:52:26 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: gcc-8 objtool warnings

On Mon, Sep 11, 2017 at 04:34:52PM +0200, Arnd Bergmann wrote:
> On Thu, Aug 24, 2017 at 9:19 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Thu, Aug 24, 2017 at 12:14:27PM +0200, Arnd Bergmann wrote:
> >> On Wed, Aug 23, 2017 at 6:01 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >> > On Wed, Aug 23, 2017 at 03:38:02PM +0200, Arnd Bergmann wrote:
> >> >> On Wed, Aug 23, 2017 at 2:48 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> >> >> > On Wed, Aug 23, 2017 at 02:22:34PM +0200, Arnd Bergmann wrote:
> >> >> >> ...
> >> >> >>
> >> >> >> 0000000000000000 <put_cred_rcu.cold.1>:
> >> >> >>    0:   e8 00 00 00 00          callq  5 <put_cred_rcu.cold.1+0x5>
> >> >> >>                         1: R_X86_64_PC32        __sanitizer_cov_trace_pc-0x4
> >> >> >>    5:   44 8b 8b 64 ff ff ff    mov    -0x9c(%rbx),%r9d
> >> >> >>    c:   48 8b 8b 68 ff ff ff    mov    -0x98(%rbx),%rcx
> >> >> >>   13:   44 89 e2                mov    %r12d,%edx
> >> >> >>   16:   44 8b 83 60 ff ff ff    mov    -0xa0(%rbx),%r8d
> >> >> >>   1d:   4c 89 ee                mov    %r13,%rsi
> >> >> >>   20:   48 c7 c7 00 00 00 00    mov    $0x0,%rdi
> >> >> >>                         23: R_X86_64_32S        .rodata.str1.8+0x28
> >> >> >>   27:   e8 00 00 00 00          callq  2c <__kstrtab_creds_are_invalid+0x3>
> >> >> >>                         28: R_X86_64_PC32       panic-0x4
> >> >> >
> >> >> > Thanks.  Can you send me one of the .o files?
> >> >>
> >> >> Attached here now.
> >> >
> >> > Ok, looks like I'll need to add support for this new pattern (jumping to
> >> > a .cold section in .text.unlikely).
> >> >
> >> > I'm also about to start work on fixing that other issue you found with
> >> > GCC's inefficient update of the stack pointer.
> >> >
> >> > I really appreciate your finding all these warnings (and getting advance
> >> > GCC 8 testing).  Thanks again!
> >>
> >> No worries. I've disabled the four warnings in objtool that triggered now
> >> and almost all are gone, but I still get a few warnings after doing additional
> >> randconfig builds.
> >
> > Ok, I've got a fix for *most* of them below.
> >
> > Still need to fix the gc.o warning, which at first glance looks like a
> > new switch statement pattern.
> 
> I now got around to testing your patch, and ran into a few remaining
> problems:
> 
> 1. one configuration causing tons of warnings, on most compiler
>   versions (4.9 and newer), and even a couple of "unreachable
>   instruction warnings on gcc-4.3.
> 
> arch/x86/mm/pageattr.o: warning: objtool: set_memory_x()+0x3a: call
> without frame pointer save/setup
> security/keys/keyring.o: warning: objtool: keyring_read()+0x70: leave
> instruction with modified stack frame
> arch/x86/events/intel/pt.o: warning: objtool:
> pt_event_addr_filters_sync uses BP as a scratch register

I downloaded the tarball, and I see configs/logs for the other problems,
but not for this one.  Did you forget to attach it, or did I miss it?

-- 
Josh