nptl: pthread_kill, pthread_cancel should not fail after exit (bug 19193)

This closes one remaining race condition related to bug 12889: if
the thread already exited on the kernel side, returning ESRCH
is not correct because that error is reserved for the thread IDs
(pthread_t values) whose lifetime has ended.  In case of a
kernel-side exit and a valid thread ID, no signal needs to be sent
and cancellation does not have an effect, so just return 0.

sysdeps/pthread/tst-kill4.c triggers undefined behavior and is
removed with this commit.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

2 files changed, 11 insertions, 5 deletions

diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 2bb523c0ec..a8aa3b3d15 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -61,10 +61,11 @@ __pthread_cancel (pthread_t th)
 {
   volatile struct pthread *pd = (volatile struct pthread *) th;
 
-  /* Make sure the descriptor is valid.  */
-  if (INVALID_TD_P (pd))
-    /* Not a valid thread handle.  */
-    return ESRCH;
+  if (pd->tid == 0)
+    /* The thread has already exited on the kernel side.  Its outcome
+       (regular exit, other cancelation) has already been
+       determined.  */
+    return 0;
 
   static int init_sigcancel = 0;
   if (atomic_load_relaxed (&init_sigcancel) == 0)
diff --git a/nptl/pthread_kill.c b/nptl/pthread_kill.c
index f79a2b26fc..5d4c86f920 100644
--- a/nptl/pthread_kill.c
+++ b/nptl/pthread_kill.c
@@ -46,7 +46,12 @@ __pthread_kill_internal (pthread_t threadid, int signo)
 	    ? INTERNAL_SYSCALL_ERRNO (val) : 0);
     }
   else
-    val = ESRCH;
+    /* The kernel reports that the thread has exited.  POSIX specifies
+       the ESRCH error only for the case when the lifetime of a thread
+       ID has ended, but calling pthread_kill on such a thread ID is
+       undefined in glibc.  Therefore, do not treat kernel thread exit
+       as an error.  */
+    val = 0;
 
   return val;
 }