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Date:   Wed, 19 Jul 2023 08:26:56 +0300
From:   Andy Shevchenko <andriy.shevchenko@linux.intel.com>
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 1/1] seq_file: Replace strncpy()+nul by strscpy()
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On Tue, Jul 18, 2023 at 10:00:24PM -0700, Kees Cook wrote:
> On Mon, Jul 17, 2023 at 04:09:23PM -0700, Kees Cook wrote:
> > On Mon, 17 Jul 2023 12:33:32 +0300, Andy Shevchenko wrote:
> > > Privided seq_show_option_n() macro breaks build with -Werror
> > > and W=1, e.g.:
> > > 
> > > In function ‘strncpy’,
> > >     inlined from ‘ocfs2_show_options’ at fs/ocfs2/super.c:1520:3:
> > > include/linux/fortify-string.h:68:33: error: ‘__builtin_strncpy’ output may be truncated copying 4 bytes from a string of length 4 [-Werror=stringop-truncation]
> > >    68 | #define __underlying_strncpy    __builtin_strncpy
> > >       |                                 ^
> > > include/linux/fortify-string.h:151:16: note: in expansion of macro ‘__underlying_strncpy’
> > >   151 |         return __underlying_strncpy(p, q, size);
> > >       |                ^~~~~~~~~~~~~~~~~~~~
> > > cc1: all warnings being treated as errors
> > > 
> > > [...]
> > 
> > Applied, thanks!
> > 
> > [1/1] seq_file: Replace strncpy()+nul by strscpy()
> >       https://git.kernel.org/kees/c/c30417b20f49
> 
> Gah, I dropped this from my tree since it was actually wrong[1]. This is an
> ugly corner case with strscpy vs strncpy: the cast be32 from hfs/hfsplus[2]
> looks unterminated to strscpy, so it would return -E2BIG, but really
> FORTIFY noticed the over-read (strscpy is correctly checking the 5th
> byte for NUL).
> 
> So... I think we need to fix seq_show_option_n() using memcpy+NUL, drop
> the ocfs2 usage, and clarify that the seq_show_option_n() docs mean
> "n means _exactly_ n bytes"...

Sounds like a plan!

Please go ahead with that. My intention is to make eventually build kernel with
`make W=1` when CONFIG_WERROR=y.

> [1] https://lore.kernel.org/lkml/0000000000000a88cb0600ccef54@google.com/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/hfsplus/options.c?h=v6.4#n221

-- 
With Best Regards,
Andy Shevchenko