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Date: Mon, 16 Sep 2002 10:42:04 -0400
From: Daniel Jacobowitz <dan@debian.org>
To: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@transmeta.com>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Fix for ptrace breakage
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On Mon, Sep 16, 2002 at 11:28:57PM +0900, OGAWA Hirofumi wrote:
> Daniel Jacobowitz <dan@debian.org> writes:
> 
> > On Mon, Sep 16, 2002 at 09:44:34PM +0900, OGAWA Hirofumi wrote:
> > > OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> writes:
> > > 
> > > > Grr, sorry. This patch is bad version.
> > > > 
> > > >  	list_for_each(_p, &father->ptrace_children) {
> > > > 
> > > > of course, this should
> > > > 
> > > >  	list_for_each_safe(_p, _n, &father->ptrace_children) {
> > > 
> > > This is a patch which fixed the above. Please apply.
> > 
> > Some comments.  First of all, you said you fixed a race on
> > current->ptrace and some other bugs - would you mind saying where they
> > were?  It's definitely cleaner after your patch but I'd like to
> > understand where you found bugs, since I think you're introducing more.
> 
> It's the following
> 
> 		task_t *trace_task = p->parent;
> 		int ptrace_flag = p->ptrace;
> 		BUG_ON (ptrace_flag == 0);
> 
> 		__ptrace_unlink(p);
> 		p->ptrace = ptrace_flag;
> 		__ptrace_link(p, trace_task);

We have the tasklist lock.  How can there be a race here?  The parent
can't detach while we're holding the tasklist lock.  If there is a race
with PTRACE_SETOPTIONS, then PTRACE_SETOPTIONS should take the lock.

> > If this is unnecessary, perhaps a BUG_ON()?  This function is called
> > from daemonize at which point the ptrace flag should be clear, and
> > that's it.
> 
> You aren't looking the source.
> 
> void reparent_to_init(void)
> {
> 	write_lock_irq(&tasklist_lock);
> 
> 	ptrace_unlink(current);

You're right on this one, I missed ptrace_unlink.  Thanks.

> > > @@ -443,7 +442,7 @@ void exit_mm(struct task_struct *tsk)
> > >  static inline void forget_original_parent(struct task_struct * father)
> > >  {
> > >  	struct task_struct *p, *reaper = father;
> > > -	struct list_head *_p;
> > > +	struct list_head *_p, *_n;
> > >  
> > >  	reaper = father->group_leader;
> > >  	if (reaper == father)
> > > @@ -462,52 +461,37 @@ static inline void forget_original_paren
> > >  		if (father == p->real_parent)
> > >  			reparent_thread(p, reaper, child_reaper);
> > >  	}
> > > -	list_for_each(_p, &father->ptrace_children) {
> > > +	list_for_each_safe(_p, _n, &father->ptrace_children) {
> > >  		p = list_entry(_p,struct task_struct,ptrace_list);
> > > +		list_del_init(&p->ptrace_list);
> > >  		reparent_thread(p, reaper, child_reaper);
> > > +		if (p->parent != p->real_parent)
> > > +			list_add(&p->ptrace_list, &p->real_parent->ptrace_children);
> > >  	}
> > >  }
> > 
> > So you reparent children on the ptrace_list right here.  But they still
> > need to go through zap_thread!  You're right, the do_notify_parent in
> > zap_thread isn't necessary; it'll be taken care of in sys_wait4.  The
> > orphaned pgrp check is still relevant though.
> 
> ??? You forget tasklist_lock?

Huh?

The problem I am describing is when a child - which will become an
orphaned pgrp when ``father'' dies - is being ptraced at the moment of
``father''s death.  With your patch it will be moved to
reaper->ptrace_children (or child_reaper->ptrace_children) but never
orphaned properly.  It'll miss a signal.

> > If you're going to remove the if, you need to maintain its effect! 
> > See:
> > > -		if (p->parent != father) {
> > > -			BUG_ON(p->parent != p->real_parent);
> > > -			return;
> > > -		}
> > 
> > This is the case where we were tracing something.  The ptrace_unlink
> > returned it to its original parent.  It doesn't need the
> > remove_parent/add_parent (though they are harmless); it does need to
> > avoid the orphaned pgrp check.  It may need the do_notify_parent check,
> > which was a bug in the previous code.
> 
> What is the basis which you think it is bug?

The death of a tracing process should not have any effect on the traced
process except to untrace it.  It should not go through the orphaning
checks.  The orphaning checks assume that the exiting process is the
real parent, and will orphan the pgrp if it is not in the same
session... as its tracer!  That's a bug.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer
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