[PATCH v2 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().

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 <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 | 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