Re: [PATCH v3] tools/libs/stat: fix broken build

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.10.2020 06:50, Jürgen Groß wrote:
> On 01.10.20 18:38, Bertrand Marquis wrote:
>> Hi Juergen,
>>
>>> On 14 Sep 2020, at 11:58, Bertrand Marquis <bertrand.marquis@xxxxxxx> wrote:
>>>
>>>
>>>
>>>> On 12 Sep 2020, at 14:08, Juergen Gross <jgross@xxxxxxxx> wrote:
>>>>
>>>> Making getBridge() static triggered a build error with some gcc versions:
>>>>
>>>> error: 'strncpy' output may be truncated copying 15 bytes from a string of
>>>> length 255 [-Werror=stringop-truncation]
>>>>
>>>> Fix that by using a buffer with 256 bytes instead.
>>>>
>>>> Fixes: 6d0ec053907794 ("tools: split libxenstat into new tools/libs/stat 
>>>> directory")
>>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>> Reviewed-by: Bertrand Marquis <bertrand.marquis@xxxxxxx>
>>
>> Sorry i have to come back on this one.
>>
>> I still see an error compiling with Yocto on this one:
>> |     inlined from 'xenstat_collect_networks' at xenstat_linux.c:306:2:
>> | xenstat_linux.c:81:6: error: 'strncpy' output may be truncated copying 255 
>> bytes from a string of length 255 [-Werror=stringop-truncation]
>> |    81 |      strncpy(result, de->d_name, resultLen);
>> |       |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> To solve it, I need to define devBridge[257] as devNoBrideg.
> 
> IMHO this is a real compiler error.
> 
> de->d_name is an array of 256 bytes, so doing strncpy() from that to
> another array of 256 bytes with a length of 256 won't truncate anything.

That's a matter of how you look at it, I think: If the original array
doesn't hold a nul-terminated string, the destination array won't
either, yet the common goal of strncpy() is to yield a properly nul-
terminated string. IOW the warning may be since the standard even has
a specific foot note to point out this possible pitfall.

Jan