linux-kernel - Re: [patch 05/12] rseq: Add prctl() to enable time slice extensions

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Message-ID: <16fae242-8eec-473a-b040-5c6d35ddcb4f@amd.com>
Date: Thu, 11 Sep 2025 22:22:18 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Thomas Gleixner
	<tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
CC: Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney"
	<paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>, Jonathan Corbet
	<corbet@....net>, Prakash Sangappa <prakash.sangappa@...cle.com>, Madadi
 Vineeth Reddy <vineethr@...ux.ibm.com>, Steven Rostedt <rostedt@...dmis.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Arnd Bergmann
	<arnd@...db.de>, <linux-arch@...r.kernel.org>
Subject: Re: [patch 05/12] rseq: Add prctl() to enable time slice extensions

Hello Mathieu,

On 9/11/2025 9:20 PM, Mathieu Desnoyers wrote:
>>   +int rseq_slice_extension_prctl(unsigned long arg2, unsigned long arg3)
>> +{
>> +    switch (arg2) {
>> +    case PR_RSEQ_SLICE_EXTENSION_GET:
>> +        if (arg3)
>> +            return -EINVAL;
>> +        return current->rseq.slice.state.enabled ? PR_RSEQ_SLICE_EXT_ENABLE : 0;
>> +
>> +    case PR_RSEQ_SLICE_EXTENSION_SET: {
>> +        u32 rflags, valid = RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
>> +        bool enable = !!(arg3 & PR_RSEQ_SLICE_EXT_ENABLE);
>> +
>> +        if (arg3 & ~PR_RSEQ_SLICE_EXT_ENABLE)
>> +            return -EINVAL;
>> +        if (!rseq_slice_extension_enabled())
>> +            return -ENOTSUPP;
>> +        if (!current->rseq.usrptr)
>> +            return -ENXIO;
>> +
>> +        /* No change? */
>> +        if (enable == !!current->rseq.slice.state.enabled)
>> +            return 0;
>> +
>> +        if (get_user(rflags, &current->rseq.usrptr->flags))
>> +            goto die;
>> +
>> +        if (current->rseq.slice.state.enabled)
>> +            valid |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
>> +
>> +        if ((rflags & valid) != valid)
>> +            goto die;
>> +
>> +        rflags &= ~RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
>> +        rflags |= RSEQ_CS_FLAG_SLICE_EXT_AVAILABLE;
>> +        if (enable)
>> +            rflags |= RSEQ_CS_FLAG_SLICE_EXT_ENABLED;
>> +
>> +        if (put_user(rflags, &current->rseq.usrptr->flags))
>> +            goto die;
>> +
>> +        current->rseq.slice.state.enabled = enable;
> 
> What should happen to this enabled state if rseq is unregistered
> after this prctl ?

Wouldn't rseq_reset() deal with it since it does a:

    memset(&t->rseq, 0, sizeof(t->rseq));

-- 
Thanks and Regards,
Prateek