Re: [patch V4 15/36] rseq: Record interrupt from user space

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-09-08 17:31, Thomas Gleixner wrote:
> For RSEQ the only relevant reason to inspect and eventually fixup (abort)
> user space critical sections is when user space was interrupted and the
> task was scheduled out.
> 
> If the user to kernel entry was from a syscall no fixup is required. If
> user space invokes a syscall from a critical section it can keep the
> pieces as documented.
> 
> This is only supported on architectures which utilize the generic entry
> code. If your architecture does not use it, bad luck.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> 
> ---
>   include/linux/irq-entry-common.h |    3 ++-
>   include/linux/rseq.h             |   16 +++++++++++-----
>   include/linux/rseq_entry.h       |   18 ++++++++++++++++++
>   include/linux/rseq_types.h       |    2 ++
>   4 files changed, 33 insertions(+), 6 deletions(-)
> 
> --- a/include/linux/irq-entry-common.h
> +++ b/include/linux/irq-entry-common.h
> @@ -4,7 +4,7 @@
>   
>   #include <linux/context_tracking.h>
>   #include <linux/kmsan.h>
> -#include <linux/rseq.h>
> +#include <linux/rseq_entry.h>
>   #include <linux/static_call_types.h>
>   #include <linux/syscalls.h>
>   #include <linux/tick.h>
> @@ -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();
>   }
>   
>   /**
> --- a/include/linux/rseq.h
> +++ b/include/linux/rseq.h
> @@ -31,11 +31,17 @@ static inline void rseq_sched_switch_eve
>   
>   static __always_inline void rseq_exit_to_user_mode(void)
>   {
> -	if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) {
> -		if (WARN_ON_ONCE(current->rseq.event.has_rseq &&
> -				 current->rseq.event.events))
> -			current->rseq.event.events = 0;
> -	}
> +	struct rseq_event *ev = &current->rseq.event;
> +
> +	if (IS_ENABLED(CONFIG_DEBUG_RSEQ))
> +		WARN_ON_ONCE(ev->sched_switch);

OK. Now I'm confused.

It is perfectly legal to issue a system call from userspace as long
as it's not from within an rseq critical section.

That system call can be scheduled out, and can set the ev->sched_switch.

This would cause the rseq_exit_to_user_mode from system call to
hit this.

What is disallowed is only issuing a system call from a rseq critical
section. The other parts of rseq (updates of cpu id and mm cid) still
have to happen when returning from a system call.

What am I missing ?

Thanks,

Mathieu

> +
> +	/*
> +	 * Ensure that event (especially user_irq) is cleared when the
> +	 * interrupt did not result in a schedule and therefore the
> +	 * rseq processing did not clear it.
> +	 */
> +	ev->events = 0;
>   }
>   
>   /*
> --- /dev/null
> +++ b/include/linux/rseq_entry.h
> @@ -0,0 +1,18 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _LINUX_RSEQ_ENTRY_H
> +#define _LINUX_RSEQ_ENTRY_H
> +
> +#ifdef CONFIG_RSEQ
> +#include <linux/rseq.h>
> +
> +static __always_inline void rseq_note_user_irq_entry(void)
> +{
> +	if (IS_ENABLED(CONFIG_GENERIC_IRQ_ENTRY))
> +		current->rseq.event.user_irq = true;
> +}
> +
> +#else /* CONFIG_RSEQ */
> +static inline void rseq_note_user_irq_entry(void) { }
> +#endif /* !CONFIG_RSEQ */
> +
> +#endif /* _LINUX_RSEQ_ENTRY_H */
> --- a/include/linux/rseq_types.h
> +++ b/include/linux/rseq_types.h
> @@ -12,6 +12,7 @@ struct rseq;
>    * @all:		Compound to initialize and clear the data efficiently
>    * @events:		Compound to access events with a single load/store
>    * @sched_switch:	True if the task was scheduled out
> + * @user_irq:		True on interrupt entry from user mode
>    * @has_rseq:		True if the task has a rseq pointer installed
>    */
>   struct rseq_event {
> @@ -22,6 +23,7 @@ struct rseq_event {
>   				u16		events;
>   				struct {
>   					u8	sched_switch;
> +					u8	user_irq;
>   				};
>   			};
>   
> 


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