linux-user: avoid string truncation warnings in uname field copying

In file included from /usr/include/string.h:494,
                 from include/qemu/osdep.h:101,
                 from linux-user/uname.c:20:
In function ‘strncpy’,
    inlined from ‘sys_uname’ at linux-user/uname.c:94:3:
/usr/include/bits/string_fortified.h:106:10: warning: ‘__builtin_strncpy’ output may be truncated copying 64 bytes from a string of length 64 [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We don't care where the NUL terminator in the original uname
field was. It suffices to copy the entire original field and
simply force a NUL terminator at the end of the new field.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190501144646.4851-1-berrange@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

linux-user/uname.c

@@ -72,9 +72,8 @@ const char *cpu_to_uname_machine(void *cpu_env)
 
 #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'; \
+      memcpy((dest), (src), MIN(sizeof(src), sizeof(dest))); \
+      (dest)[sizeof(dest) - 1] = '\0'; \
   } while (0)
 
 int sys_uname(struct new_utsname *buf)