rseq: Optimize rseq_update_cpu_id()

Two put_user() in rseq_update_cpu_id() are replaced
by a pair of unsafe_put_user() with appropriate surroundings.

This removes one stac/clac pair on x86 in fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lkml.kernel.org/r/20210413203352.71350-2-eric.dumazet@gmail.com

diff --git a/kernel/rseq.c b/kernel/rseq.c
index a4f86a9..f020f18 100644 (file)
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -84,13 +84,20 @@
 static int rseq_update_cpu_id(struct task_struct *t)
 {
        u32 cpu_id = raw_smp_processor_id();
+       struct rseq __user *rseq = t->rseq;
 
-       if (put_user(cpu_id, &t->rseq->cpu_id_start))
-               return -EFAULT;
-       if (put_user(cpu_id, &t->rseq->cpu_id))
-               return -EFAULT;
+       if (!user_write_access_begin(rseq, sizeof(*rseq)))
+               goto efault;
+       unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
+       unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
+       user_write_access_end();
        trace_rseq_update(t);
        return 0;
+
+efault_end:
+       user_write_access_end();
+efault:
+       return -EFAULT;
 }
 
 static int rseq_reset_rseq_cpu_id(struct task_struct *t)