From: Peter Zijlstra
> Sent: 13 April 2021 10:10
>
> On Tue, Apr 13, 2021 at 12:36:57AM -0700, Eric Dumazet wrote:
> > 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
> > 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.
Does that work for 32bit BE ?
David
> 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;
>
-
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