Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()

[Mathieu Desnoyers]

----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@xxxxxxxxx wrote:

> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> 
> 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.

Ah, now I remember the reason why reading and clearing the entire 64-bit
is important: it's because we don't want to allow user-space processes to
use this change in behavior to figure out whether they are running on a
32-bit or in a 32-bit compat mode on a 64-bit kernel.

So although I'm fine with making 64-bit kernels faster, we'll want to keep
updating the entire 64-bit ptr field on 32-bit kernels as well.

Thanks,

Mathieu

> 
> Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Arjun Roy <arjunroy@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> ---
> 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com