linux-kernel - Re: [PATCH 3/3] uprobes: make uprobe

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Message-ID: <20240711182606.GA29154@redhat.com>
Date: Thu, 11 Jul 2024 20:26:06 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: andrii@...nel.org, mhiramat@...nel.org, peterz@...radead.org,
	clm@...a.com, jolsa@...nel.org, mingo@...nel.org,
	paulmck@...nel.org, rostedt@...dmis.org,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] uprobes: make uprobe_register() return struct uprobe
 *

On 07/11, Andrii Nakryiko wrote:
>
> On Thu, Jul 11, 2024 at 2:28 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > On 07/10, Oleg Nesterov wrote:
> > >
> > > -void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consumer *uc)
> > > +void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
> > >  {
> > > -     struct uprobe *uprobe;
> > > -
> > > -     uprobe = find_uprobe(inode, offset);
> > > -     if (WARN_ON(!uprobe))
> > > -             return;
> > > -
> > >       down_write(&uprobe->register_rwsem);
> > >       __uprobe_unregister(uprobe, uc);
> > >       up_write(&uprobe->register_rwsem);
> > > -     put_uprobe(uprobe);
> >
> > OK, this is obviously wrong, needs get_uprobe/put_uprobe. __uprobe_unregister()
> > can free this uprobe, so up_write(&uprobe->register_rwsem) is not safe.
>
> uprobe_register(), given it returns an uprobe instance to the caller
> should keep refcount on it (it belongs to uprobe_consumer).

Of course. And again, this patch doesn't change the curent behaviour.

> That's
> what I did for my patches, are you going to do that as well?
>
> We basically do the same thing, just interfaces look a bit different.

Not sure. Well I do not really know, I didn't read your series to the
end, sorry ;) The same for V1/V2 from Peter so far.

But let me say this just in case... With or without this change,
currently uprobe_consumer doesn't have an "individual" ref to uprobe.
The fact that uprobe->consumers != NULL adds a reference.

Lets not discuss if this is good or bad right now, this cleanup is
only cleanup.

------------------------------------------------------------------------
Now, let me add another "just in case" note to explain what I am going
to do in V2.

So. this patch should turn uprobe_unregister() into something like

	void uprobe_unregister(struct uprobe *uprobe, struct uprobe_consumer *uc)
	{
		// Ugly !!!! please kill me!!!
		get_uprobe(uprobe);
		down_write(&uprobe->register_rwsem);
		__uprobe_unregister(uprobe, uc);
		up_write(&uprobe->register_rwsem);
		put_uprobe(uprobe);
	}

to simplify this change. And the next (simple) patch will kill these
get_uprobe + put_uprobe, we just need to shift the (possibly) final
put_uprobe() from delete_uprobe() to unregister().

But of course, I will recheck before I send V2.

Oleg.