commit 849151dd5481bc8acb1d287a299b5d6a4ca9f1c3
author Thomas Gleixner <tglx@linutronix.de> Sat Sep 06 12:18:07 2014 +0200
committer Thomas Gleixner <tglx@linutronix.de> Sat Sep 06 12:58:18 2014 +0200
tree e14c9ddf3bda4da1668d4c4c52a77d5e74aea293
parent 40bea039593dfc7f3f9814dab844f6db43ae580b

compat: nanosleep: Clarify error handling

The error handling in compat_sys_nanosleep() is correct, but
completely non obvious. Document it and restrict it to the
-ERESTART_RESTARTBLOCK return value for clarity.

Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/compat.c

diff --git a/kernel/compat.c b/kernel/compat.c
index 633394f..ebb3c36 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -226,7 +226,7 @@
 	ret = hrtimer_nanosleep_restart(restart);
 	set_fs(oldfs);
 
-	if (ret) {
+	if (ret == -ERESTART_RESTARTBLOCK) {
 		rmtp = restart->nanosleep.compat_rmtp;
 
 		if (rmtp && compat_put_timespec(&rmt, rmtp))
@@ -256,7 +256,26 @@
 				HRTIMER_MODE_REL, CLOCK_MONOTONIC);
 	set_fs(oldfs);
 
-	if (ret) {
+	/*
+	 * hrtimer_nanosleep() can only return 0 or
+	 * -ERESTART_RESTARTBLOCK here because:
+	 *
+	 * - we call it with HRTIMER_MODE_REL and therefor exclude the
+	 *   -ERESTARTNOHAND return path.
+	 *
+	 * - we supply the rmtp argument from the task stack (due to
+	 *   the necessary compat conversion. So the update cannot
+	 *   fail, which excludes the -EFAULT return path as well. If
+	 *   it fails nevertheless we have a bigger problem and wont
+	 *   reach this place anymore.
+	 *
+	 * - if the return value is 0, we do not have to update rmtp
+	 *    because there is no remaining time.
+	 *
+	 * We check for -ERESTART_RESTARTBLOCK nevertheless if the
+	 * core implementation decides to return random nonsense.
+	 */
+	if (ret == -ERESTART_RESTARTBLOCK) {
 		struct restart_block *restart
 			= &current_thread_info()->restart_block;
 
@@ -266,7 +285,6 @@
 		if (rmtp && compat_put_timespec(&rmt, rmtp))
 			return -EFAULT;
 	}
-
 	return ret;
 }