[Bug c/81117] Improve buffer overflow checking in strncpy

["bernd.edlinger at hotmail dot de" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81117

Bernd Edlinger <bernd.edlinger at hotmail dot de> changed:

           What    |Removed                     |Added
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                 CC|                            |bernd.edlinger at hotmail dot de

--- Comment #20 from Bernd Edlinger <bernd.edlinger at hotmail dot de> ---
this breaks glibc-2.26:

../sysdeps/unix/sysv/linux/if_index.c: In function '__if_nametoindex':
../sysdeps/unix/sysv/linux/if_index.c:46:3: error: 'strncpy' specified bound 16
equals destination size [-Werror=stringop-truncation]
   strncpy (ifr.ifr_name, ifname, sizeof (ifr.ifr_name));
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [/home/ed/gnu/glibc-build/inet/if_index.o] Error 1

but in this case the code is correct:

  struct ifreq ifr;
  [...]
  strncpy (ifr.ifr_name, ifname, sizeof (ifr.ifr_name));
  if (__ioctl (fd, SIOCGIFINDEX, &ifr) < 0)

Because ifr_name does not need zero-termination.

And the following would not be correct, but does not warn:

  strncpy (ifr.ifr_name, ifname, sizeof (ifr.ifr_name)-1);