Re: [Qemu-trivial] [Qemu-devel] [PATCH v1 4/5] linux-user/uname: Fix GCC 9 build warnings

[Laurent Vivier]

Le 30/04/2019 à 22:40, Eric Blake a écrit :
> On 4/30/19 3:09 PM, Alistair Francis wrote:
>> Fix this warning when building with GCC9 on Fedora 30:
>> In function ‘strncpy’,
>>     inlined from ‘sys_uname’ at /home/alistair/qemu/linux-user/uname.c:94:3:
>> /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ 
>> output may be truncated copying 64 bytes from a string of length 64 
>> [-Werror=stringop-truncation]
>>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
>> (__dest));
>>       |          
>> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Signed-off-by: Alistair Francis <address@hidden>
>> ---
>>  linux-user/uname.c | 8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/linux-user/uname.c b/linux-user/uname.c
>> index 313b79dbad..293b2238f2 100644
>> --- a/linux-user/uname.c
>> +++ b/linux-user/uname.c
>> @@ -90,6 +90,11 @@ int sys_uname(struct new_utsname *buf)
>>     * struct linux kernel uses).
>>     */
>>  
>> +#if defined(CONFIG_PRAGMA_DIAGNOSTIC_AVAILABLE) && QEMU_GNUC_PREREQ(9, 0)
>> +#pragma GCC diagnostic push
>> +#pragma GCC diagnostic ignored "-Wstringop-truncation"
>> +#endif
> 
> Why do we need the pragma?
> 
>> +
>>    memset(buf, 0, sizeof(*buf));
> 
> We are prezeroing the entire field, at which point...
> 
>>    COPY_UTSNAME_FIELD(buf->sysname, uts_buf.sysname);
>>    COPY_UTSNAME_FIELD(buf->nodename, uts_buf.nodename);
> 
> ...using strncpy() for a shorter string is wasteful (we're writing the
> tail end twice), and for a long string is warning-prone.  Why not
> rewrite COPY_UTSNAME_FIELD() to use memcpy() for the MIN(strlen(src),
> __NEW_UTS_LEN) and drop the write of the trailing NUL, since it will
> already be NUL from your memset()?
> 

We must modify this very carefully because I think there is some magic
in COPY_UTSNAME_FIELD() we could miss.

#define COPY_UTSNAME_FIELD(dest, src) \
  do { \
      /* __NEW_UTS_LEN doesn't include terminating null */ \
      (void) strncpy((dest), (src), __NEW_UTS_LEN); \
      (dest)[__NEW_UTS_LEN] = '\0'; \
  } while (0)

Thanks,
Laurent

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