nptl: Use SA_RESTART for SIGCANCEL handler

The usage of signals to implementation pthread cancellation is an
implementation detail and should not be visible through cancellation
entrypoints.

However now that pthread_cancel always send the SIGCANCEL, some
entrypoint might be interruptable and return EINTR to the caller
(for instance on sem_wait).

Using SA_RESTART hides this, since the cancellation handler should
either act uppon cancellation (if asynchronous cancellation is enable)
or ignore the cancellation internal signal.

Checked on x86_64-linux-gnu and i686-linux-gnu.

2 files changed, 13 insertions, 1 deletions

diff --git a/NEWS b/NEWS
index d64be288da..48d77d706f 100644
--- a/NEWS
+++ b/NEWS
@@ -64,6 +64,14 @@ Deprecated and removed features, and other changes affecting compatibility:
 * The function pthread_yield has been deprecated; programs should use
   the equivalent standard function sched_yield instead.
 
+* The pthread cancellation handler is now installed with SA_RESTART and
+  pthread_cancel will always send the internal SIGCANCEL on a cancellation
+  request.  It should not be visible to application since the cancellation
+  handler should either act upon cancellation (if asynchronous cancellation
+  is enabled) or ignore the cancellation internal signal.  However there are
+  buggy kernel interfaces (for instance some CIFS version) that could still
+  see spurious EINTR error when cancellation interrupts a blocking syscall.
+
 Changes to build and runtime requirements:
 
 * On Linux, the shm_open, sem_open, and related functions now expect the
diff --git a/nptl/pthread_cancel.c b/nptl/pthread_cancel.c
index 0698cd2046..cc25ff21f3 100644
--- a/nptl/pthread_cancel.c
+++ b/nptl/pthread_cancel.c
@@ -72,7 +72,11 @@ __pthread_cancel (pthread_t th)
     {
       struct sigaction sa;
       sa.sa_sigaction = sigcancel_handler;
-      sa.sa_flags = SA_SIGINFO;
+      /* The signal handle should be non-interruptible to avoid the risk of
+	 spurious EINTR caused by SIGCANCEL sent to process or if
+	 pthread_cancel() is called while cancellation is disabled in the
+	 target thread.  */
+      sa.sa_flags = SA_SIGINFO | SA_RESTART;
       __sigemptyset (&sa.sa_mask);
       __libc_sigaction (SIGCANCEL, &sa, NULL);
       atomic_store_relaxed (&init_sigcancel, 1);