linux-kernel - Re: include/linux/string.h:246:9: warning: '__builtin_strncpy' output truncated before terminating nul copying 4 bytes from a string of the same length

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Date:   Tue, 5 Jun 2018 10:30:03 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     kbuild test robot <lkp@...el.com>,
        Michal Januszewski <spock@...too.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        kbuild-all@...org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: include/linux/string.h:246:9: warning: '__builtin_strncpy'
 output truncated before terminating nul copying 4 bytes from a string of the
 same length

On Tue, Jun 05, 2018 at 06:19:07PM +0300, Andy Shevchenko wrote:
> On Tue, May 29, 2018 at 4:35 AM, kbuild test robot <lkp@...el.com> wrote:
> > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > head:   786b71f5b754273ccef6d9462e52062b3e1f9877
> > commit: 854e55ad289ef8888e7991f0ada85d5846f5afb9 objtool, perf: Fix GCC 8 -Wrestrict error
> > date:   2 months ago
> > config: x86_64-randconfig-s4-05290856 (attached as .config)
> > compiler: gcc-8 (Debian 8.1.0-3) 8.1.0
> > reproduce:
> >         git checkout 854e55ad289ef8888e7991f0ada85d5846f5afb9
> >         # save the attached .config to linux build tree
> >         make ARCH=x86_64
> >
> 
> I guess it's easy to fix by
> 
> --- a/drivers/video/fbdev/uvesafb.c
> +++ b/drivers/video/fbdev/uvesafb.c
> @@ -422,7 +422,7 @@ static int uvesafb_vbe_getinfo(struct uvesafb_ktask *task,
>        task->t.flags = TF_VBEIB;
>        task->t.buf_len = sizeof(struct vbe_ib);
>        task->buf = &par->vbe_ib;
> -       strncpy(par->vbe_ib.vbe_signature, "VBE2", 4);
> +       snprintf(par->vbe_ib.vbe_signature,
> sizeof(par->vbe_ib.vbe_signature), "VBE2");
> 
> The question is do we want this to just shut up a compiler? It's
> obviously false positive.

What about just changing it to a memcpy?  Seems like that would be
cleaner anyway, since the whole point of strncpy is to add the
terminating NULL, which isn't need here.

> 
> > All warnings (new ones prefixed by >>):
> >
> >    In file included from include/linux/bitmap.h:9,
> >                     from include/linux/cpumask.h:12,
> >                     from arch/x86/include/asm/cpumask.h:5,
> >                     from arch/x86/include/asm/msr.h:11,
> >                     from arch/x86/include/asm/processor.h:21,
> >                     from arch/x86/include/asm/cpufeature.h:5,
> >                     from arch/x86/include/asm/thread_info.h:53,
> >                     from include/linux/thread_info.h:38,
> >                     from arch/x86/include/asm/preempt.h:7,
> >                     from include/linux/preempt.h:81,
> >                     from include/linux/spinlock.h:51,
> >                     from include/linux/seqlock.h:36,
> >                     from include/linux/time.h:6,
> >                     from include/linux/stat.h:19,
> >                     from include/linux/module.h:10,
> >                     from drivers/video/fbdev/uvesafb.c:12:
> >    In function 'strncpy',
> >        inlined from 'uvesafb_vbe_getinfo' at drivers/video/fbdev/uvesafb.c:425:2:
> >>> include/linux/string.h:246:9: warning: '__builtin_strncpy' output truncated before terminating nul copying 4 bytes from a string of the same length [-Wstringop-truncation]
> >      return __builtin_strncpy(p, q, size);
> >             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > vim +/__builtin_strncpy +246 include/linux/string.h
> >
> > 6974f0c4 Daniel Micay 2017-07-12  237
> > 6974f0c4 Daniel Micay 2017-07-12  238  #if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) && defined(CONFIG_FORTIFY_SOURCE)
> > 6974f0c4 Daniel Micay 2017-07-12  239  __FORTIFY_INLINE char *strncpy(char *p, const char *q, __kernel_size_t size)
> > 6974f0c4 Daniel Micay 2017-07-12  240  {
> > 6974f0c4 Daniel Micay 2017-07-12  241   size_t p_size = __builtin_object_size(p, 0);
> > 6974f0c4 Daniel Micay 2017-07-12  242   if (__builtin_constant_p(size) && p_size < size)
> > 6974f0c4 Daniel Micay 2017-07-12  243           __write_overflow();
> > 6974f0c4 Daniel Micay 2017-07-12  244   if (p_size < size)
> > 6974f0c4 Daniel Micay 2017-07-12  245           fortify_panic(__func__);
> > 6974f0c4 Daniel Micay 2017-07-12 @246   return __builtin_strncpy(p, q, size);
> > 6974f0c4 Daniel Micay 2017-07-12  247  }
> > 6974f0c4 Daniel Micay 2017-07-12  248
> >
> > :::::: The code at line 246 was first introduced by commit
> > :::::: 6974f0c4555e285ab217cee58b6e874f776ff409 include/linux/string.h: add the option of fortified string.h functions
> >
> > :::::: TO: Daniel Micay <danielmicay@...il.com>
> > :::::: CC: Linus Torvalds <torvalds@...ux-foundation.org>
> >
> > ---
> > 0-DAY kernel test infrastructure                Open Source Technology Center
> > https://lists.01.org/pipermail/kbuild-all                   Intel Corporation
> 
> 
> 
> -- 
> With Best Regards,
> Andy Shevchenko

-- 
Josh