[PATCH v3 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.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() on 64bit arches.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: "Paul E. McKenney" <paul...@kernel.org>
Cc: Boqun Feng <boqun.f...@gmail.com>
Cc: Arjun Roy <arjun...@google.com>
Cc: Ingo Molnar <mi...@kernel.org>
---
 kernel/rseq.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/rseq.c b/kernel/rseq.c
index 
cfe01ab5253c1c424c0e8b25acbb6a8e1b41a5b6..35f7bd0fced0e2dd8aed819e054dac03f024388a
 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(0UL, &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