exec: avoid gcc-8 warning for get_task_comm

exec: avoid gcc-8 warning for get_task_comm

gcc-8 warns about using strncpy() with the source size as the limit:

  fs/exec.c:1223:32: error: argument to 'sizeof' in 'strncpy' call is the same expression as the source; did you mean to use the size of the destination? [-Werror=sizeof-pointer-memaccess]

This is indeed slightly suspicious, as it protects us from source
arguments without NUL-termination, but does not guarantee that the
destination is terminated.

This keeps the strncpy() to ensure we have properly padded target
buffer, but ensures that we use the correct length, by passing the
actual length of the destination buffer as well as adding a build-time
check to ensure it is exactly TASK_COMM_LEN.

There are only 23 callsites which I all reviewed to ensure this is
currently the case.  We could get away with doing only the check or
passing the right length, but it doesn't hurt to do both.

Link: http://lkml.kernel.org/r/20171205151724.1764896-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Kees Cook <keescook@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Serge Hallyn <serge@hallyn.com>
Cc: James Morris <james.l.morris@oracle.com>
Cc: Aleksa Sarai <asarai@suse.de>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[sw0312.kim: cherry-pick mainline commit 3756f6401c30 for gcc 9 build]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: Id98efe5de4803db524b4a39a140be13709c9b19f