Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang afterPTRACE_ATTACH

From: Oleg Nesterov
Date: Mon Feb 28 2011 - 09:45:10 EST

On 02/26, Denys Vlasenko wrote:
> * HOWEVER, this behavior _is_ indeed used by gdb to run small fragments
> of tracee even if it's stopped. Jan's example:
> # gdb -p applicationpid
> (gdb) print getpid()
> (gdb) print show_me_your_internal_debug_dump()
> (gdb) continue
> gdb people want to preserve this feature.

Yes. Jan is looking at this, and probably he will nack this change.

> How we can accomodate this gdb need while fixing this bug?
> Oleg's POV is that gdb should SIGCONT the tracee (at least if it is
> currently in group-stop). This has the advantage of using standard Unix
> tool. The disadvantage is that SIGCONT will wake up *all* threads,

Not necessarily. That is why, btw, I started to like Tejun's suggestion,
the traced task should always stop in TASK_TRACED state. This means
SIGCONT can only wakeup the tracee after PTRACE_CONT from debugger.

Even without enforcing TASK_TRACED from the kernel side, gdb should
do at least one ptrace() call after attach, this makes it TASK_TRACED

> gdb people
> do want here a "secret" backdoor-ish way to make a *thread*
> (not the whole process) running even when the process is in group-stop.

And this is what I disagree with. This was my main motivation to start
this hopeless^W lengthy discussion ;) I simply can't accept the current
behaviour: the task runs while the kernel and parent think the whole
process is stopped.

That is why I also considered another (and imho worse) option. OK, let's
resume the tracee even if it is stopped. But in this case, let's clear
SIGNAL_STOP_STOPPED and notify its parent.

> how to open a backdoor in ptrace API for gdb:

Probably I am wrong, but in the context of this discussion I do not
care much about the new possible requests/improvements in gdb/kernel.

Of course we can do something to make gdb happy, but the problem is
the current/old code. The main objection (and I have to respect it)
is: this change is not compatible.


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