[PATCH v2] arm64: ptrace: don't warn on missing siginfo in a group-stop

[Catalin Marinas]

On Fri, Oct 23, 2026 at 07:07:10PM +0800, Zhao Gongyi wrote:
> PTRACE_ATTACH to a group-stopped child can enter do_jobctl_trap(),
> which calls ptrace_stop() without siginfo. If the tracer writes
> NT_ARM_SYSTEM_CALL, system_call_set() can change syscallno from
> NO_SYSCALL before calling update_syscall_orig_x0_after_ptrace(). The
> WARN_ON_ONCE(!info) then fires, although this is a valid ptrace stop.
> 
> Remove the warning and guard the syscall-exit-stop check against a
> missing siginfo. Keep recognizing both SIGTRAP and SIGTRAP | 0x80:
> syscall-exit-stops use either value depending on whether the tracer
> enabled PTRACE_O_TRACESYSGOOD. With no siginfo, allow the final
> assignment to synchronize orig_x0 from x0.
> 
> Fixes: 88b839ce497c ("arm64: ptrace: Keep 'orig_x0' in-sync with x0 on syscall entry")
> Cc: stable at vger.kernel.org
> Signed-off-by: Zhao Gongyi <zhaogongyi at bytedance.com>
> ---
> Changes since v1:
> - Remove the warning and early return, as suggested by Oleg.
> - Keep recognizing syscall-exit-stops with or without PTRACE_O_TRACESYSGOOD.
> 
> v1: https://lore.kernel.org/r/20261016024536.18938-1-zhaogongyi@bytedance.com
> 
> Tested on arm64 KVM at af32da41b032 with this v2 applied.
> At a group-stop, PTRACE_GETSIGINFO returned -EINVAL and an
> NT_ARM_SYSTEM_CALL write succeeded. Syscall-entry and seccomp
> x0 rewrites worked. At syscall-exit-stops, NT_PRSTATUS writes succeeded
> with and without PTRACE_O_TRACESYSGOOD (si_code 133 and 5,
> respectively). No kernel warnings occurred.
> 
>  arch/arm64/kernel/ptrace.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index f743cbec1..22fb9eeed 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -573,10 +573,6 @@ static void update_syscall_orig_x0_after_ptrace(struct task_struct *target)
>  	if (regs->syscallno == NO_SYSCALL)
>  		return;
>  
> -	/* We should only be called when target is in a ptrace stop */
> -	if (WARN_ON_ONCE(!info))
> -		return;

That's a slight semantic change since this function may now update
orig_x0 even when !info. However, I think it makes more sense as the aim
of commit 88b839ce497c ("arm64: ptrace: Keep 'orig_x0' in-sync with x0
on syscall entry") was to update orig_x0 in all cases other than
syscall-exit-stop. We can't have the latter with !info.

I'll wait for a couple of days to see if Will has any comments and queue
it.

Thanks.

-- 
Catalin