[PATCH 3/3] rseq: optimise for 64bit arches

[Eric Dumazet]

From: Eric Dumazet <edumazet@xxxxxxxxxx>

Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
update includes") added regressions for our servers.

Using copy_from_user() and clear_user() for 64bit values
on 64bit arches is suboptimal.

We might revisit this patch once all 32bit arches support
get_user() and/or put_user() for 8 bytes values.

Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: Arjun Roy <arjunroy@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
---
 kernel/rseq.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 57344f9abb43905c7dd2b6081205ff508d963e1e..18a75a804008d2f564d1f7789f09216f1a8760bd 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -127,8 +127,13 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 	u32 sig;
 	int ret;
 
+#ifdef CONFIG_64BIT
+	if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
+		return -EFAULT;
+#else
 	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
 		return -EFAULT;
+#endif
 	if (!ptr) {
 		memset(rseq_cs, 0, sizeof(*rseq_cs));
 		return 0;
@@ -211,9 +216,13 @@ static int clear_rseq_cs(struct task_struct *t)
 	 *
 	 * Set rseq_cs to NULL.
 	 */
+#ifdef CONFIG_64BIT
+	return put_user(0ULL, &t->rseq->rseq_cs.ptr64);
+#else
 	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
 		return -EFAULT;
 	return 0;
+#endif
 }
 
 /*
-- 
2.31.1.295.g9ea45b61b8-goog