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

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

> On 2 Oct 2020, at 12:03, Bertrand Marquis <bertrand.marquis@xxxxxxx> wrote:
> 
> 
> 
>> On 2 Oct 2020, at 11:44, Jürgen Groß <jgross@xxxxxxxx> wrote:
>> 
>> On 02.10.20 12:12, Bertrand Marquis wrote:
>>> Hi,
>>>> On 2 Oct 2020, at 08:25, Jürgen Groß <jgross@xxxxxxxx> wrote:
>>>> 
>>>> On 02.10.20 08:59, Jan Beulich wrote:
>>>>> On 02.10.2020 08:51, Jürgen Groß wrote:
>>>>>> On 02.10.20 08:20, Jan Beulich wrote:
>>>>>>> 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.
>>>>>> 
>>>>>> If the source doesn't hold a nul-terminated string there will still be
>>>>>> 256 bytes copied, so there is no truncation done during strncpy().
>>>>>> 
>>>>>> In fact there is no way to use strncpy() in a safe way on a fixed sized
>>>>>> source array with the above semantics: either the target is larger than
>>>>>> the source and length is at least sizeof(source) + 1, resulting in a
>>>>>> possible read beyond the end of source, or the target is the same length
>>>>>> leading to the error.
>>>>> I agree with all of what you say, but I can also see why said foot note
>>>>> alone may have motivated the emission of the warning.
>>>> 
>>>> The motivation can be explained, yes, but it is wrong. strncpy() is not
>>>> limited to source arrays of unknown length. So this warning is making
>>>> strncpy() unusable for fixed sized source strings and -Werror. And that
>>>> is nothing a compiler should be allowed to do, hence a compiler bug.
>>> I do agree that in this case the compiler is doing to much.
>> 
>> It is plain wrong here. Rendering a Posix defined function unusable for
>> a completely legal use case is in no way a matter of taste or of "doing
>> to much". It is a bug.
> 
> Agree.
> 
>> 
>>> We could also choose to turn off the warning either using pragma (which
>>> i really do not like) or by adding a cflag for this specific file (but this 
>>> might
>>> hit us later in other places).
>>> All in all this currently makes Xen master and staging not possible to
>>> compile with Yocto so we need to find a solution as this will also
>>> come in any distribution using a new compiler,
>> 
>> A variant you didn't mention would be open coding of strncpy() (or
>> having a related inline function in a common header). This route would
>> be the one I'd prefer in case the compiler guys insist on the behavior
>> being fine.
> 
> True this possible, even though I do not like to modify the code that deeply
> for one specific compiler.
> 
>> 
>> You didn't tell us which compiler is being used and whether it really is
>> up to date. A workaround might be to set EXTRA_CFLAGS_XEN_TOOLS to
>> "-Wno-stringop-truncation" for the build.
> 
> That’s what i meant by “adding a cflag”, as you suggest we could also
> make it global (and not limit it to this file).
> 
> The compiler I am using is the one from Yocto Gatesgarth (release coming in
> october): gcc version 10.2.0 (released in july 2020).

After some discussion, I will propose a patch to solve the issue using memcpy
instead of strncpy as in this specific case we know that we can truncate the
string.
This will also allow to revert the big buffers in stack.

Cheers
Bertrand