From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Sun, 16 Oct 2016 06:16:25 +0000 (+0200)
Subject: Restartable sequences: clear rseq_cs even if non-nested
X-Git-Url: http://git.efficios.com/?p=deliverable%2Flinux.git;a=commitdiff_plain;h=88833b353c257ba7b6bcb1bcc5a9f0ae388fa25b

Restartable sequences: clear rseq_cs even if non-nested

We can speed up the user-space fast path by removing the final
"rseq_cs=NULL" assignment at the end of the critical section, since we
check for both upper and lower bounds anyway in the fixup code.

Take care of setting rseq_cs back to NULL even if we are not nested over
an actual rseq critical section.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/kernel/rseq.c b/kernel/rseq.c
index a102fcc9994e..a24da12e6b08 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -160,6 +160,17 @@ static bool rseq_get_rseq_cs(struct task_struct *t,
 	urseq_cs = (struct rseq_cs __user *)ptr;
 	if (copy_from_user(&rseq_cs, urseq_cs, sizeof(rseq_cs)))
 		return false;
+	/*
+	 * We need to clear rseq_cs upon entry into a signal handler
+	 * nested on top of a rseq assembly block, so the signal handler
+	 * will not be fixed up if itself interrupted by a nested signal
+	 * handler or preempted.  We also need to clear rseq_cs if we
+	 * preempt or deliver a signal on top of code outside of the
+	 * rseq assembly block, to ensure that a following preemption or
+	 * signal delivery will not try to perform a fixup needlessly.
+	 */
+	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
+		return false;
 	*start_ip = (void __user *)rseq_cs.start_ip;
 	*post_commit_ip = (void __user *)rseq_cs.post_commit_ip;
 	*abort_ip = (void __user *)rseq_cs.abort_ip;
@@ -186,15 +197,6 @@ static bool rseq_ip_fixup(struct pt_regs *regs)
 			(void __user *)instruction_pointer(regs) < start_ip)
 		return true;
 
-	/*
-	 * We need to clear rseq_cs upon entry into a signal
-	 * handler nested on top of a rseq assembly block, so
-	 * the signal handler will not be fixed up if itself
-	 * interrupted by a nested signal handler or preempted.
-	 */
-	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
-		return false;
-
 	/*
 	 * We set this after potentially failing in
 	 * clear_user so that the signal arrives at the