linux-kernel - [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq

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Date:   Tue, 13 Apr 2021 09:22:39 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

From: Eric Dumazet <edumazet@...gle.com>

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
is suboptimal.

We can use faster put_user() and get_user().

32bit arches can be changed to use the ptr32 field,
since the padding field must always be zero.

v2: added ideas from Peter and Mathieu about making this
    generic, since my initial patch was only dealing with
    64bit arches.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: Arjun Roy <arjunroy@...gle.com>
Cc: Ingo Molnar <mingo@...nel.org>
---
 kernel/rseq.c | 41 +++++++++++++++++++++++++++++++++--------
 1 file changed, 33 insertions(+), 8 deletions(-)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index cfe01ab5253c1c424c0e8b25acbb6a8e1b41a5b6..f2eee3f7f5d330688c81cb2e57d47ca6b843873e 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -119,23 +119,46 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t)
 	return 0;
 }
 
+#ifdef CONFIG_64BIT
+static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp,
+			   const struct rseq __user *rseq)
+{
+	u64 ptr;
+
+	if (get_user(ptr, &rseq->rseq_cs.ptr64))
+		return -EFAULT;
+	*uptrp = (struct rseq_cs __user *)ptr;
+	return 0;
+}
+#else
+static int rseq_get_cs_ptr(struct rseq_cs __user **uptrp,
+			   const struct rseq __user *rseq)
+{
+	u32 ptr;
+
+	if (get_user(ptr, &rseq->rseq_cs.ptr.ptr32))
+		return -EFAULT;
+	*uptrp = (struct rseq_cs __user *)ptr;
+	return 0;
+}
+#endif
+
 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 {
 	struct rseq_cs __user *urseq_cs;
-	u64 ptr;
 	u32 __user *usig;
 	u32 sig;
 	int ret;
 
-	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
+	if (rseq_get_cs_ptr(&urseq_cs, t->rseq))
 		return -EFAULT;
-	if (!ptr) {
+	if (!urseq_cs) {
 		memset(rseq_cs, 0, sizeof(*rseq_cs));
 		return 0;
 	}
-	if (ptr >= TASK_SIZE)
+	if ((unsigned long)urseq_cs >= TASK_SIZE)
 		return -EINVAL;
-	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
+
 	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
 		return -EFAULT;
 
@@ -211,9 +234,11 @@ static int clear_rseq_cs(struct task_struct *t)
 	 *
 	 * Set rseq_cs to NULL.
 	 */
-	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
-		return -EFAULT;
-	return 0;
+#ifdef CONFIG_64BIT
+	return put_user(0UL, &t->rseq->rseq_cs.ptr64);
+#else
+	return put_user(0UL, &t->rseq->rseq_cs.ptr.ptr32);
+#endif
 }
 
 /*
-- 
2.31.1.295.g9ea45b61b8-goog