linux-kernel - Re: objtool warnings on 4.14-stable/gcc-7.3.0

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Date:   Thu, 15 Feb 2018 15:04:27 +0100
From:   Greg KH <greg@...ah.com>
To:     Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: objtool warnings on 4.14-stable/gcc-7.3.0

+Kees

On Wed, Feb 14, 2018 at 04:45:25PM -0600, Josh Poimboeuf wrote:
> On Wed, Feb 14, 2018 at 04:24:12PM -0600, Josh Poimboeuf wrote:
> > On Wed, Feb 14, 2018 at 04:11:15PM +0100, Arnd Bergmann wrote:
> > > Hi Josh,
> > > 
> > > I recently did some randconfig testing with a plain 4.14-stable kernel
> > > and gcc-7.3.0, and came across three distinct objtool warnings:
> > > 
> > > drivers/misc/lkdtm_bugs.o: warning: objtool:
> > > lkdtm_CORRUPT_LIST_ADD()+0x15: return with modified stack frame
> 
> While this is probably an objtool bug, the code is very odd:
> 
> 00000000000001a8 <lkdtm_CORRUPT_LIST_ADD>:
>  1a8:   e8 00 00 00 00          callq  1ad <lkdtm_CORRUPT_LIST_ADD+0x5>
>                         1a9: R_X86_64_PC32      __fentry__-0x4
>  1ad:   55                      push   %rbp
>  1ae:   48 89 e5                mov    %rsp,%rbp
>  1b1:   48 83 e4 f0             and    $0xfffffffffffffff0,%rsp
>  1b5:   48 83 ec 20             sub    $0x20,%rsp
>  1b9:   48 89 ec                mov    %rbp,%rsp
>  1bc:   5d                      pop    %rbp
>  1bd:   c3                      retq
> 
> The function just allocates/aligns its stack space and then returns.  It
> seems like GCC was too smart for its own good here, as the function
> doesn't test what it's supposed to.  Can you share the config for this
> one?

lkdtm is a "interesting" module in that it tries to do bad things to
test for various stuff.  Kees, does the above look like it is doing the
correct thing?

thanks,

greg k-h