On 2026/1/27 17:44, Kevin Brodsky wrote:
> On 26/01/2026 20:02, Will Deacon wrote:
>> On Mon, Dec 22, 2025 at 07:47:26PM +0800, Jinjie Ruan wrote:
>>> [...]
>>>
>>> To make it more reasonable and in preparation for moving arm64 over to
>>> the generic entry code, move rseq_syscall() ahead before
>>> audit_syscall_exit().
>> I've been struggling a bit to see how this helps to align with the
>> generic code.
>
> rseq_syscall(), or rather rseq_debug_syscall_return() since eaa9088d568c
> ("rseq: Use static branch for syscall exit debug when
> GENERIC_IRQ_ENTRY=y"), is called first in the generic
> syscall_exit_to_user_mode_work(), so the aim of that patch is to align
> the order of calls with generic entry.
>
>> I'm also concerned that rseq_debug_update_user_cs()
>> operates on instruction_pointer(regs) which is something that can be
>> chaned by ptrace.
>
> Isn't that true regardless of where rseq_syscall() is called on the
> syscall exit path, though?
My understanding is that if instruction_pointer(regs) is hijacked and
modified via ptrace at the syscall exit (ptrace_report_syscall_exit()),
this modification will not be observed by rseq. Specifically, in the
generic entry syscall exit path, rseq_syscall() is unable to detect such
a PC modification.
Regards,
Jinjie
>
>> So, I'm not saying this is wrong, but it feels like a user-visible
>> change that needs better justification.
>
> This seems to hang on whether the force_sig(SIGSEGV) that rseq_syscall()
> might issue interacts in any way with the tracing calls. My feeling is
> that it doesn't, but I haven't confirmed it. Worth noting this is only
> relevant if rseq debugging is enabled, so any potential user-visible
> effect is limited.
>
> - Kevin
>
