linux-kernel - RE: [PATCH 3/3] rseq: optimise for 64bit arches

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Message-ID: <fbf1a4449b0148b5b9c3baa32088c32a@AcuMS.aculab.com>
Date:   Tue, 13 Apr 2021 10:36:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Eric Dumazet <eric.dumazet@...il.com>
CC:     Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Arjun Roy <arjunroy@...gle.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: RE: [PATCH 3/3] rseq: optimise for 64bit arches

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 <edumazet@...gle.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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