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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 13, 2021 at 12:36:57AM -0700, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@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
> 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.

Argh, what a mess :/ afaict only nios32 lacks put_user_8, but get_user_8
is missing in a fair number of archs.

That said; 32bit archs never have to actually set the top bits in that
word, so they _could_ only set the low 32 bits. That works provided
userspace itself keeps the high bits clear.

So I suppose that if we're going to #ifdef this, we might as well do the
whole thing.

Mathieu; did I forget a reason why this cannot work?

diff --git a/kernel/rseq.c b/kernel/rseq.c
index a4f86a9d6937..94006190b8eb 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -115,20 +115,25 @@ static int rseq_reset_rseq_cpu_id(struct task_struct *t)
 static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
 {
 	struct rseq_cs __user *urseq_cs;
-	u64 ptr;
+	unsigned long ptr;
 	u32 __user *usig;
 	u32 sig;
 	int ret;
 
-	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
+#ifdef CONFIG_64BIT
+	if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
 		return -EFAULT;
+#else
+	if (get_user(ptr, &t->rseq->rseq_cs.ptr32))
+		return -EFAULT;
+#endif
 	if (!ptr) {
 		memset(rseq_cs, 0, sizeof(*rseq_cs));
 		return 0;
 	}
 	if (ptr >= TASK_SIZE)
 		return -EINVAL;
-	urseq_cs = (struct rseq_cs __user *)(unsigned long)ptr;
+	urseq_cs = (struct rseq_cs __user *)ptr;
 	if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
 		return -EFAULT;