linux-kernel - Re: [patch V2 22/37] rseq: Use static branch for syscall exit debug when GENERIC_IRQ

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Message-ID: <e63dd2e6-ade5-4032-b9ce-51e97d1c2a0f@efficios.com>
Date: Tue, 26 Aug 2025 10:28:04 -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 22/37] rseq: Use static branch for syscall exit debug
 when GENERIC_IRQ_ENTRY=y

On 2025-08-23 12:40, Thomas Gleixner wrote:
> Make the syscall exit debug mechanism available via the static branch on
> architectures which utilize the generic entry code.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

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

> ---
>   include/linux/entry-common.h |    2 +-
>   include/linux/rseq_entry.h   |    9 +++++++++
>   kernel/rseq.c                |   19 +++++++++++++------
>   3 files changed, 23 insertions(+), 7 deletions(-)
> 
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -146,7 +146,7 @@ static __always_inline void syscall_exit
>   			local_irq_enable();
>   	}
>   
> -	rseq_syscall(regs);
> +	rseq_debug_syscall_return(regs);
>   
>   	/*
>   	 * Do one-time syscall specific work. If these work items are
> --- a/include/linux/rseq_entry.h
> +++ b/include/linux/rseq_entry.h
> @@ -286,9 +286,18 @@ static __always_inline void rseq_exit_to
>   	ev->events = 0;
>   }
>   
> +void __rseq_debug_syscall_return(struct pt_regs *regs);
> +
> +static inline void rseq_debug_syscall_return(struct pt_regs *regs)
> +{
> +	if (static_branch_unlikely(&rseq_debug_enabled))
> +		__rseq_debug_syscall_return(regs);
> +}
> +
>   #else /* CONFIG_RSEQ */
>   static inline void rseq_note_user_irq_entry(void) { }
>   static inline void rseq_exit_to_user_mode(void) { }
> +static inline void rseq_debug_syscall_return(struct pt_regs *regs) { }
>   #endif /* !CONFIG_RSEQ */
>   
>   #endif /* _LINUX_RSEQ_ENTRY_H */
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -470,12 +470,7 @@ void __rseq_handle_notify_resume(struct
>   	force_sigsegv(sig);
>   }
>   
> -#ifdef CONFIG_DEBUG_RSEQ
> -/*
> - * Terminate the process if a syscall is issued within a restartable
> - * sequence.
> - */
> -void rseq_syscall(struct pt_regs *regs)
> +void __rseq_debug_syscall_return(struct pt_regs *regs)
>   {
>   	struct task_struct *t = current;
>   	u64 csaddr;
> @@ -493,6 +488,18 @@ void rseq_syscall(struct pt_regs *regs)
>   fail:
>   	force_sig(SIGSEGV);
>   }
> +
> +#ifdef CONFIG_DEBUG_RSEQ
> +/*
> + * Kept around to keep GENERIC_ENTRY=n architectures supported.
> + *
> + * Terminate the process if a syscall is issued within a restartable
> + * sequence.
> + */
> +void rseq_syscall(struct pt_regs *regs)
> +{
> +	__rseq_debug_syscall_return(regs);
> +}
>   #endif
>   
>   /*
> 


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