On 2026/1/28 22:53, Kevin Brodsky wrote:
> On 28/01/2026 02:09, Jinjie Ruan wrote:
>>
>> On 2026/1/27 23:06, Kevin Brodsky wrote:
>>> On 27/01/2026 12:34, Jinjie Ruan wrote:
>>>>> [...]
>>>>>
>>>>>> 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.
>>> Good point. So concretely that means that currently on arm64, one could
>>> make the rseq debug check pass/fail by using the syscall exit trap to
>>> modify PC. OTOH this is impossible with generic entry because the rseq
>>> check is performed first. I'm not sure this is a feature anyone has even
>>> noticed, but it is a user-visible change indeed.
>> After digging into the ptrace code, I found that ptrace does not modify
>> instruction_pointer(regs) on the syscall exit path; it only changes the
>> return value as below.
>> Therefore, if my understanding is correct, Will's concern does not apply
>> here.
>>
>> ptrace_set_syscall_info()
>> -> ptrace_set_syscall_info_exit()
>> -> syscall_set_return_value(child, regs, 0, rval)
>
> I'm not following, how is that related to the call to
> ptrace_report_syscall_exit()? That eventually results in a call to
> ptrace_stop() (via ptrace_notify()), which synchronously causes the
> tracee to sleep and allows the tracer to issue ptrace commands, e.g.
> setting PC.
I realize I had a misunderstanding — PTRACE_SET_SYSCALL_INFO is only one
possible ptrace command. What I actually tried was to modify regs->pc on
the syscall return path using PTRACE_SETREGSET, and the result shows
that I can indeed change regs->pc to make the tracee segment fault.
>
> - Kevin
>
