linux-kernel - Re: [patch 10/11] rseq: Skip fixup when returning from a syscall

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Message-ID: <874iua6nom.ffs@tglx>
Date: Thu, 14 Aug 2025 15:24:41 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Peter Zijlstra <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Michael Jeanson
 <mjeanson@...icios.com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, "Paul E. McKenney" <paulmck@...nel.org>,
 Boqun Feng <boqun.feng@...il.com>, Wei Liu <wei.liu@...nel.org>, Jens
 Axboe <axboe@...nel.dk>
Subject: Re: [patch 10/11] rseq: Skip fixup when returning from a syscall

On Thu, Aug 14 2025 at 10:54, Peter Zijlstra wrote:
> On Wed, Aug 13, 2025 at 06:29:37PM +0200, Thomas Gleixner wrote:
>
>> --- a/kernel/rseq.c
>> +++ b/kernel/rseq.c
>> @@ -408,6 +408,22 @@ static int rseq_ip_fixup(struct pt_regs
>>  	return 0;
>>  }
>>  
>> +static inline bool rseq_ignore_event(bool from_irq, bool ksig)
>> +{
>> +	/*
>> +	 * On architectures which do not select_GENERIC_ENTRY
>> +	 * @from_irq is not usable.
>> +	 */
>> +	if (IS_ENABLED(CONFIG_DEBUG_RSEQ) || !IS_ENABLED(CONFIG_GENERIC_ENTRY))
>> +		return false;
>> +
>> +	/*
>> +	 * Avoid the heavy lifting when this is a return from syscall,
>> +	 * i.e. not from interrupt and not from signal delivery.
>> +	 */
>> +	return !from_irq && !ksig;
>> +}
>> +
>>  /*
>>   * This resume handler must always be executed between any of:
>>   * - preemption,
>
>> @@ -467,6 +484,9 @@ void __rseq_handle_notify_resume(struct
>>  			t->rseq_event_pending = false;
>>  		}
>>  
>> +		if (rseq_ignore_event(from_irq, !!ksig))
>> +			event = false;
>> +
>>  		if (IS_ENABLED(CONFIG_DEBUG_RSEQ) || event) {
>>  			ret = rseq_ip_fixup(regs, event);
>>  			if (unlikely(ret < 0))
>> 
>
> You now have a double check for CONFIG_DEBUG_RSEQ.
>
> Since the value of @event is immaterial when DEBUG_RSEQ, might as well
> remove it from rseq_ignore_event(), right?

Not really. debug wants the event preserved even if it's !from_irq

Yes, it's not pretty, but I wanted to preserve the debug behaviour as
much as it goes.