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

From: Oleg Nesterov
Date: Wed Sep 10 2008 - 12:15:35 EST

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.


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