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

[Jinjie Ruan]

在 2026/10/27 2:32, Catalin Marinas 写道:
> 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 agree, our original intent was to avoid the return value contaminating
orig_x0 at syscall exit; apart from that case, it should be kept in sync.

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

-- 
Best regards,
Jinjie