linux-kernel - Re: [PATCH] ext4: fix -Wstringop-truncation warnings

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Date:   Tue, 15 Dec 2020 23:20:18 -0500
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Kang Wenlin <wenlin.kang@...driver.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: fix -Wstringop-truncation warnings

On Thu, Nov 12, 2020 at 05:33:24PM +0800, Kang Wenlin wrote:
> From: Wenlin Kang <wenlin.kang@...driver.com>
> 
> The strncpy() function may create a unterminated string,
> use strscpy_pad() instead.
> 
> This fixes the following warning:
> 
> fs/ext4/super.c: In function '__save_error_info':
> fs/ext4/super.c:349:2: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
>   strncpy(es->s_last_error_func, func, sizeof(es->s_last_error_func));
>   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> fs/ext4/super.c:353:3: warning: 'strncpy' specified bound 32 equals destination size [-Wstringop-truncation]
>    strncpy(es->s_first_error_func, func,
>    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>     sizeof(es->s_first_error_func));
>     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

What compiler are you using?  s_last_error_func is defined to not
necessarily be NUL terminated.  So strscpy_pad() is not a proper
replacement for strncpy() in this use case.

>From Documentation/process/deprecated:

   If a caller is using non-NUL-terminated strings, strncpy() can
   still be used, but destinations should be marked with the `__nonstring
   <https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
   attribute to avoid future compiler warnings.

s_{first,last}_error_func is properly annotated with __nonstring in
fs/ext4/ext4.h.

						- Ted