linux-kernel - Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list locklessly under SRCU protection

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Message-ID: <20240830202050.GA7440@redhat.com>
Date: Fri, 30 Aug 2024 22:20:50 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Jiri Olsa <olsajiri@...il.com>, Andrii Nakryiko <andrii@...nel.org>,
	linux-trace-kernel@...r.kernel.org, peterz@...radead.org,
	rostedt@...dmis.org, mhiramat@...nel.org, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, paulmck@...nel.org,
	willy@...radead.org, surenb@...gle.com, akpm@...ux-foundation.org,
	linux-mm@...ck.org
Subject: Re: [PATCH v4 4/8] uprobes: travers uprobe's consumer list
 locklessly under SRCU protection

On 08/30, Andrii Nakryiko wrote:
>

Andrii, let me reply to your email "out of order". First of all:

> Can we please let me land these patches first? It's been a while. I
> don't think anything is really broken with the logic.

OK, agreed.

I'll probably write another email (too late for me today), but I agree
that "avoid register_rwsem in handler_chain" is obviously a good goal,
lets discuss the possible cleanups or even fixlets later, when this
series is already applied.

> On Fri, Aug 30, 2024 at 7:33 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > No, I think you found a problem. UPROBE_HANDLER_REMOVE can be lost if
> > uc->filter == NULL of if it returns true. See another reply I sent a
> > minute ago.
> >
>
> For better or worse, but I think there is (or has to be) and implicit
> contract that if uprobe (or uretprobe for that matter as well, but
> that's a separate issue) handler can return UPROBE_HANDLER_REMOVE,
> then it *has to* also provide filter.

IOW, uc->handler and uc->filter must be consistent. But the current API
doesn't require this contract, so this patch adds a difference which I
didn't notice when I reviewed this change.

(In fact I noticed the difference, but I thought that it should be fine).

> If it doesn't provide filter
> callback, it doesn't care about PID filtering and thus can't and
> shouldn't cause unregistration.

At first glance I disagree, but see above.

> > I think the fix is simple, plus we need to cleanup this logic anyway,
> > I'll try to send some code on Monday.

Damn I am stupid. Nothing new ;) The "simple" fix I had in mind can't work.
But we can do other things which we can discuss later.

Oleg.