Re: [uml-devel] [PATCH 1/1] system call notification with self_ptrace

[Oleg Nesterov <oleg@tv-sign.ru>]

On 09/10, Pierre Morel wrote:
>
> Oleg Nesterov wrote:
> >
> >I still think this patch shouldn't change handle_signal().
> >
> >Once again. The signal handler for SIGSYS can first do
> >sys_ptrace(PTRACE_SELF_OFF) (which is filtered out), and then use any
> >other syscall, so this change is not needed, afaics.
> >
> Yes it can but what if the application forget to do it?
> It is a security so that the application do not bounce for ever.

The (buggy) task can be killed, this has nothing to do with security.

From the security pov, this case doesn't differ from, say,

	void sigh(int sig)
	{
		kill(getpid(), sig);
	}

	void main(void)
	{
		signal(SIGSYS, sigh);
		kill(getpid(), SIGSYS);
	}

Or I missed something?

> >So, PTRACE_SELF_XXX disables the "normal" ptrace. Not sure this is good.
> >
> I think that having two tracing system one over the other may be
> quite difficult to handle.

Yes I see.

But... well, I think we need Roland's opinion. I must admit, I am a bit
sceptical about this patch ;) I mean, I don't really understand why it
is useful. We can do the same with fork() + ptrace(). Yes, in that
case we need an "extra" context switch for any traced syscall. But,
do you have any "real life" example to demonstrate that the user-space
solution sucks? We can even use CLONE_MM to speedup the context switch.

Pierre, don't get me wrong. I never used debuggers for myself, I will
be happy to know I am wrong. I just don't understand.


As for ->instrumentation. If you are going to remove PTS_INSTRUMENTED,
we need only one bit. We could use PF_PTS_SELF, but ->flags is already
"contended". Perhaps you can do something like

	--- include/linux/sched.h
	+++ include/linux/sched.h
	@@ -1088,6 +1088,7 @@ struct task_struct {
		/* ??? */
		unsigned int personality;
		unsigned did_exec:1;
	+	unsigned pts_self:1;
		pid_t pid;
		pid_t tgid;
	 

Both did_exec and pts_self can only be changed by current, so it is
safe to share the same word. This way we don't enlarge task_struct.

Oleg.


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