linux-kernel - Re: [patch V6 15/31] rseq: Record interrupt from user space

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Message-ID: <d142d345-6a6c-4a90-a773-ec3fa11144fb@efficios.com>
Date: Tue, 28 Oct 2025 13:53:54 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Michael Jeanson <mjeanson@...icios.com>, Jens Axboe <axboe@...nel.dk>,
 Peter Zijlstra <peterz@...radead.org>, "Paul E. McKenney"
 <paulmck@...nel.org>, x86@...nel.org, Sean Christopherson
 <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>
Subject: Re: [patch V6 15/31] rseq: Record interrupt from user space

On 2025-10-28 13:02, Thomas Gleixner wrote:
> On Tue, Oct 28 2025 at 11:26, Mathieu Desnoyers wrote:
>> On 2025-10-27 04:44, Thomas Gleixner wrote:
>> [...]
>>> @@ -281,6 +281,7 @@ static __always_inline void exit_to_user
>>>    static __always_inline void irqentry_enter_from_user_mode(struct pt_regs *regs)
>>>    {
>>>    	enter_from_user_mode(regs);
>>> +	rseq_note_user_irq_entry();
>>>    }
>>>    
>> Looking at x86, both exc_debug_user() and exc_int3() invoke
>> irqentry_enter_from_user_mode(), but there are various
>> other traps that can come from userspace (e.g. math_error,
>> exc_general_protection, ...). Some of those traps don't
>> necessarily end with a signal delivery to the offending
>> process. And some of those traps enable interrupts.
> 
> They all go through irqentry_enter_from_user_mode(). See
> DEFINE_IDTENTRY*() macros. They invoke:
> 
>     irqentry_enter()
>       if (user_mode())
>          irqentry_enter_from_user_mode();
> 
> If that wouldn't be the case then all the RCU/NOHZ magic would not work
> either. So any exception, trap, interrupt must go through this to
> establish state correctly. Whether that's explicit as it's required for
> int3 and debug_user or implicit through the IDTENTRY magic.

That's what I missed, thanks for the explanation.

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

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