linux-kernel - Re: [PATCH v11 03/14] arm64: ptrace: Move rseq_syscall() before audit_syscall

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Message-ID: <c70d3fd6-0b62-464b-8e99-e74f07c20846@arm.com>
Date: Thu, 29 Jan 2026 13:06:29 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Jinjie Ruan <ruanjinjie@...wei.com>, catalin.marinas@....com,
 will@...nel.org, oleg@...hat.com, tglx@...utronix.de, peterz@...radead.org,
 luto@...nel.org, shuah@...nel.org, kees@...nel.org, wad@...omium.org,
 deller@....de, akpm@...ux-foundation.org, charlie@...osinc.com,
 mark.rutland@....com, anshuman.khandual@....com, song@...nel.org,
 ryan.roberts@....com, thuth@...hat.com, ada.coupriediaz@....com,
 broonie@...nel.org, pengcan@...inos.cn, liqiang01@...inos.cn, kmal@...k.li,
 dvyukov@...gle.com, reddybalavignesh9979@...il.com,
 richard.weiyang@...il.com, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v11 03/14] arm64: ptrace: Move rseq_syscall() before
 audit_syscall_exit()

On 28/01/2026 04:19, Jinjie Ruan wrote:
> commit a9f3a74a29af ("entry: Provide generic syscall exit function")
> introduce generic syscall exit function and call rseq_syscall()
> before audit_syscall_exit() and arch_syscall_exit_tracehook().
>
> And commit b74406f37737 ("arm: Add syscall detection for restartable
> sequences") add rseq support for arm32, which also call rseq_syscall()
> before audit_syscall_exit() and tracehook_report_syscall().
>
> However, commit 409d5db49867c ("arm64: rseq: Implement backend rseq
> calls and select HAVE_RSEQ") implement arm64 rseq and call
> rseq_syscall() after audit_syscall_exit() and tracehook_report_syscall().
> So compared to the generic entry and arm32 code, arm64 calls
> rseq_syscall() a bit later.
>
> But as commit b74406f37737 ("arm: Add syscall detection for restartable
> sequences") said, syscalls are not allowed inside restartable sequences,
> so should call rseq_syscall() at the very beginning of system call
> exiting path for CONFIG_DEBUG_RSEQ=y kernel. This could help us to detect
> whether there is a syscall issued inside restartable sequences.
>
> As for the impact of raising SIGSEGV via rseq_syscall(), it makes no
> practical difference to signal delivery because signals are processed
> in arm64_exit_to_user_mode() at the very end.
>
> As for the "regs", rseq_syscall() only checks and update
> instruction_pointer(regs), ptrace can not modify the "pc" on syscall exit
> path but 'only changes the return value', so calling rseq_syscall()
> before or after ptrace_report_syscall_exit() makes no difference.

Let's update this as discussed on v10 - PC can be modified when
ptrace_report_syscall_exit() is called.

> And audit_syscall_exit() only checks the return value (x0 for arm64),
> so calling rseq_syscall() before or after audit syscall exit makes
> no difference. trace_sys_exit() only uses syscallno and the return value,
> so calling rseq_syscall() before or after trace_sys_exit() also makes
> no difference.
>
> In preparation for moving arm64 over to the generic entry code, move
> rseq_syscall() ahead before audit_syscall_exit().
>
> No functional changes.

And naturally this is not the case.

- Kevin

> Reviewed-by: Kevin Brodsky <kevin.brodsky@....com>
> Signed-off-by: Jinjie Ruan <ruanjinjie@...wei.com>
> ---
>  arch/arm64/kernel/ptrace.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 9f9aa3087c09..785280c76317 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -2443,6 +2443,8 @@ int syscall_trace_enter(struct pt_regs *regs, unsigned long flags)
>  
>  void syscall_trace_exit(struct pt_regs *regs, unsigned long flags)
>  {
> +	rseq_syscall(regs);
> +
>  	audit_syscall_exit(regs);
>  
>  	if (flags & _TIF_SYSCALL_TRACEPOINT)
> @@ -2450,8 +2452,6 @@ void syscall_trace_exit(struct pt_regs *regs, unsigned long flags)
>  
>  	if (flags & (_TIF_SYSCALL_TRACE | _TIF_SINGLESTEP))
>  		report_syscall_exit(regs);
> -
> -	rseq_syscall(regs);
>  }
>  
>  /*