Re: [RFC] [Patch 1/1] [Self Ptrace] System call notification with self_ptrace

Pierre Morel
2008-08-28T13:29:32Z

Oleg Nesterov wrote:
> On 08/28, Pierre Morel wrote:
>
>> Oleg Nesterov wrote:
>>
>>> On 08/27, Pierre Morel wrote:
>>>
>>>
>>>> Oleg Nesterov wrote:
>>>>
>>>>
>>>>
>>>>> On s390 the patch changes handle_signal(), this is not clear to me too.
>>>>>
>>>>>
>>>>>
>>>> The patch clears the trace flags before delivering the signal so
>>>> that the signal handler can use system call without bouncing again.
>>>>
>>>>
>>> Yes I see. But the signal handler for SIGSYS can fisrt do
>>> sys_ptrace(PTRACE_SELF_OFF) (which is filtered out), and then use any
>>> other syscall.
>>>
>>>
>> It is right but brings the overhead of a syscall.
>>
>
> Well, this overhead is very small compared to the signal delivery.
>
May be, but very high compared to the operation to just
clear a flag in the task struct.
This operation must be done anyway.
>
>>> With this patch PT_SELF is cleared on any signal. This doesn't look
>>> right. Let's suppose that another signal comes in parallel with SIGSYS.
>>> It is very possible that the handler for that another signal will be
>>> called first, this handler can do some syscall which will be "missed".
>>>
>>>
>> If the tracing application catches all signals before delivering
>> them to the instrumented original handler there is no problem,
>> the catching code can reset PTRACE_SELF_ON before calling the
>> instrumented application's original handler.
>> The instrumented code will then bounce as expected.
>>
>
> Sorry, can't understand the text above :(
>
> OK, let's suppose the application does
>
> ptrace(PTRACE_SELF_ON);
> ...
> syscall();
>
> This "syscall()" above should trigger the handler for SIGSYS.
> But what if another signal (with handler) comes in between?
> In that case handle_signal() clears PT_SELF/TIF_SYSCALL_TRACE,
> this syscall() (or any other) doesn't send SIGSYS.
>
ok, please read "set PTRACE_SELF_ON"
where I wrote "reset PTRACE_SELF_ON" above.

Now, suppose the application does the following:

sigsys_handler(sig)
{
....
ptrace(PTRACE_SELF_ON);
}

sigx_handler(sig)
{
....
ptrace(PTRACE_SELF_ON);
if (sig_has_a_handler)
call_the_handler()
ptrace(PTRACE_SELF_OFF);
...
ptrace(PTRACE_SELF_ON);
}

main(){
...
signal(SIGSYS, sigsys_handler);
for(i=0; i<MAXSIGS; i++)
signal(i, sigx_handler);
ptrace(PTRACE_SELF_ON);
jump_into_instrumented_code;
...
}

If the instrumented code ever does a call to
signal(2), the call will be received by sig_sys() handler,
where the call the application signal handler can be performed with
PTRACE_SELF_ON.
This implies appropriate instrumentation of signal(2) and sigaction(2).

Pierre