linux-kernel - Re: [patch V2 05/37] rseq: Simplify registration

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Message-ID: <bb93b82d-d125-46c4-bac4-dafe4d9fc146@efficios.com>
Date: Mon, 25 Aug 2025 11:44:38 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>,
 Dexuan Cui <decui@...rosoft.com>, x86@...nel.org,
 Arnd Bergmann <arnd@...db.de>, Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, Huacai Chen <chenhuacai@...nel.org>,
 Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 05/37] rseq: Simplify registration

On 2025-08-23 12:39, Thomas Gleixner wrote:
> There is no point to read the critical section element in the newly
> registered user space RSEQ struct first in order to clear it.
> 
> Just clear it and be done with it.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>

> ---
>   kernel/rseq.c |   10 +++-------
>   1 file changed, 3 insertions(+), 7 deletions(-)
> 
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -492,11 +492,9 @@ void rseq_syscall(struct pt_regs *regs)
>   /*
>    * sys_rseq - setup restartable sequences for caller thread.
>    */
> -SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len,
> -		int, flags, u32, sig)
> +SYSCALL_DEFINE4(rseq, struct rseq __user *, rseq, u32, rseq_len, int, flags, u32, sig)
>   {
>   	int ret;
> -	u64 rseq_cs;
>   
>   	if (flags & RSEQ_FLAG_UNREGISTER) {
>   		if (flags & ~RSEQ_FLAG_UNREGISTER)
> @@ -557,11 +555,9 @@ SYSCALL_DEFINE4(rseq, struct rseq __user
>   	 * avoid a potential segfault on return to user-space. The proper thing
>   	 * to do would have been to fail the registration but this would break
>   	 * older libcs that reuse the rseq area for new threads without
> -	 * clearing the fields.
> +	 * clearing the fields. Don't bother reading it, just reset it.
>   	 */
> -	if (rseq_get_rseq_cs_ptr_val(rseq, &rseq_cs))
> -	        return -EFAULT;
> -	if (rseq_cs && clear_rseq_cs(rseq))
> +	if (put_user_masked_u64(0UL, &rseq->rseq_cs))
>   		return -EFAULT;
>   
>   #ifdef CONFIG_DEBUG_RSEQ
> 

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