linux-kernel - Re: [PATCH] objtool: Fix GCC 8 cold subfunction detection for aliased functions

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Date:   Thu, 31 May 2018 11:35:09 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] objtool: Fix GCC 8 cold subfunction detection for
 aliased functions

On Wed, May 30, 2018 at 11:16:39AM -0500, Josh Poimboeuf wrote:
> The kbuild test robot reported the following issue:
> 
>   kernel/time/posix-stubs.o: warning: objtool: sys_ni_posix_timers.cold.1()+0x0: unreachable instruction
> 
> This file creates symbol aliases for the sys_ni_posix_timers() function.
> So there are multiple ELF function symbols for the same function:
> 
>   23: 0000000000000150     26 FUNC    GLOBAL DEFAULT        1 __x64_sys_timer_create
>   24: 0000000000000150     26 FUNC    GLOBAL DEFAULT        1 sys_ni_posix_timers
>   25: 0000000000000150     26 FUNC    GLOBAL DEFAULT        1 __ia32_sys_timer_create
>   26: 0000000000000150     26 FUNC    GLOBAL DEFAULT        1 __x64_sys_timer_gettime
> 
> Here's the corresponding cold subfunction:
> 
>   11: 0000000000000000     45 FUNC    LOCAL  DEFAULT        6 sys_ni_posix_timers.cold.1
> 
> When analyzing overlapping functions, objtool only looks at the first
> one in the symbol list.  The rest of the functions are basically ignored
> because they point to instructions which have already been analyzed.
> 
> So in this case it analyzes the __x64_sys_timer_create() function, but
> then it fails to recognize that its cold subfunction is
> sys_ni_posix_timers.cold.1(), because the names are different.
> 
> Make the subfunction detection a little smarter by associating each
> subfunction with the first function which jumps to it, since that's the
> one which will be analyzed.
> 
> Reported-by: kbuild test robot <lkp@...el.com>
> Fixes: 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

NACK.  This adds another warning:

arch/x86/xen/enlighten.o: warning: objtool: xen_pin_vcpu()+0x4b: can't find switch jump table

-- 
Josh