Messages in this thread | | | Date | Mon, 21 Jan 2013 18:21:51 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 2/4] ptrace: ensure arch_ptrace/ptrace_request can never race with SIGKILL |
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On 01/20, Linus Torvalds wrote: > > On Sun, Jan 20, 2013 at 11:25 AM, Oleg Nesterov <oleg@redhat.com> wrote: > > + > > +static void ptrace_unfreeze_traced(struct task_struct *task) > > +{ > > + if (task->state != __TASK_TRACED) > > + return; > > + > > + if (WARN_ON(!task->ptrace || task->parent != current)) > > + return; > > This WARN_ON() is bogus. > > Because you added the warning, you then need to make the callers check > for the whole PTRACE_UNATTACH. > > So I think you should just remove the WARN_ON(), and then just call > ptrace_unfreeze_traced() unconditionally after you've successfully > done a ptrace_check_attach(). Just to keep the code simpler.
This is what initial patch did. But, assuming that ptrace_unfreeze_traced() is called unconditionally, we need a locking or barriers, otherwise
// another debugger attached after we did PTRACE_DETACH ? if (!task->ptrace || task->parent != current) return;
is racy. Suppose we trace the natural child, then do PTRACE_DETACH, then another tracer comes. We can see ->ptrace and __TASK_TRACED, but see the old task->parent == current.
Of course, this is only theoretical, and probably we can add a barrier before this check, but I am not sure this will make the code simpler. If nothing else, this needs a comment.
If PTRACE_DETACH doesn't do _unfreeze_, we know that the task is either traced by us or it is exiting/exited, so we can always trust the "state == __TASK_TRACED" check.
So I'd prefer to keep this code, but I won't insist if you still disagree.
> Also, in your corrected version, you had > > + if (!wait_task_inactive(child, __TASK_TRACED)) { > + /* This can only happen if may_ptrace_stop() fails */ > + WARN_ON(child->state == __TASK_TRACED); > + ret = -ESRCH; > > where I actually think that the comment is not really helpful. It > doesn't really explain what he child can do to get to ptrace_stop() in > the first place, and what that does to the child state...
OK. Agreed. This comment reflects the fact that the first version removed may_ptrace_stop() to ensure wait_task_inactive() can't fail.
I'll update the comment and resend.
Oleg.
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