perf record: Fix event fd races

The write call may set errno which is problematic if occurring in a
function also setting errno. Save and restore errno around the write
call.

done_fd may be used after close, clear it as part of the close and check
its validity in the signal handler.

Suggested-by: <[e-mail]>
Reviewed-by: Leo Yan <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Anand K Mistry <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-record.c

···
 static volatile int signr = -1;
 static volatile int child_finished;
 #ifdef HAVE_EVENTFD_SUPPORT
-static int done_fd = -1;
+static volatile int done_fd = -1;
 #endif
 
 static void sig_handler(int sig)
···
 
 	done = 1;
 #ifdef HAVE_EVENTFD_SUPPORT
-{
-	u64 tmp = 1;
-	/*
-	 * It is possible for this signal handler to run after done is checked
-	 * in the main loop, but before the perf counter fds are polled. If this
-	 * happens, the poll() will continue to wait even though done is set,
-	 * and will only break out if either another signal is received, or the
-	 * counters are ready for read. To ensure the poll() doesn't sleep when
-	 * done is set, use an eventfd (done_fd) to wake up the poll().
-	 */
-	if (write(done_fd, &tmp, sizeof(tmp)) < 0)
-		pr_err("failed to signal wakeup fd, error: %m\n");
-}
+	if (done_fd >= 0) {
+		u64 tmp = 1;
+		int orig_errno = errno;
+
+		/*
+		 * It is possible for this signal handler to run after done is
+		 * checked in the main loop, but before the perf counter fds are
+		 * polled. If this happens, the poll() will continue to wait
+		 * even though done is set, and will only break out if either
+		 * another signal is received, or the counters are ready for
+		 * read. To ensure the poll() doesn't sleep when done is set,
+		 * use an eventfd (done_fd) to wake up the poll().
+		 */
+		if (write(done_fd, &tmp, sizeof(tmp)) < 0)
+			pr_err("failed to signal wakeup fd, error: %m\n");
+
+		errno = orig_errno;
+	}
 #endif // HAVE_EVENTFD_SUPPORT
 }
 
···
 
 out_delete_session:
 #ifdef HAVE_EVENTFD_SUPPORT
-	if (done_fd >= 0)
-		close(done_fd);
+	if (done_fd >= 0) {
+		fd = done_fd;
+		done_fd = -1;
+
+		close(fd);
+	}
 #endif
 	zstd_fini(&session->zstd_data);
 	perf_session__delete(session);