Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 1/5] util/qemu-sockets: Fix GCC 9 build warnings

[Eric Blake]

On 4/30/19 3:25 PM, Eric Blake wrote:
> On 4/30/19 3:08 PM, Alistair Francis wrote:
>> Fix this warning when building with GCC9 on Fedora 30:
>> In function ‘strncpy’,
>>     inlined from ‘unix_connect_saddr.isra.0’ at util/qemu-sockets.c:925:5:
>> /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
>> specified bound 108 equals destination size [-Werror=stringop-truncation]
>>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
>> (__dest));
>>       |          
>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> In function ‘strncpy’,
>>     inlined from ‘unix_listen_saddr.isra.0’ at util/qemu-sockets.c:880:5:
>>

>> -    strncpy(un.sun_path, path, sizeof(un.sun_path));
>> +    strncpy(un.sun_path, path, sizeof(un.sun_path) - 1);
> 
> NACK. Linux allows you to use the full width of un.sun_path (a NUL
> terminator is required if you copy less than that, but not if you use
> the full width). Rather, we may need to mark path as a potential
> nonstring to silence the warning, or use memcpy instead of strncpy, or
> some other workaround.  (Sadly, this is one of those odd places where
> strncpy is actually the right function to use, but there are so many
> other places where strncpy is used incorrectly that it has turned into a
> battle to use it here)

We don't have control over un (that's from the libc system headers), but
does adding the QEMU_NONSTRING attribute to our declaration of path
serve to silence the warning?

In short, I think most of this series should look at the use of the
QEMU_NONSTRING macro, as that macro goes hand-in-hand with strncpy() for
informing the compiler exactly when we know that we are copying
something that has fixed length and may or may not be NUL-terminated.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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