linux-kernel - Re: [RFC bpf-next 01/10] uprobe: Add session callbacks to uprobe

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Date: Wed, 5 Jun 2024 22:50:08 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...omium.org>,
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC bpf-next 01/10] uprobe: Add session callbacks to
 uprobe_consumer

On Wed, Jun 05, 2024 at 07:56:19PM +0200, Oleg Nesterov wrote:
> On 06/05, Andrii Nakryiko wrote:
> >
> > so any such
> > limitations will cause problems, issue reports, investigation, etc.
> 
> Agreed...
> 
> > As one possible solution, what if we do
> >
> > struct return_instance {
> >     ...
> >     u64 session_cookies[];
> > };
> >
> > and allocate sizeof(struct return_instance) + 8 *
> > <num-of-session-consumers> and then at runtime pass
> > &session_cookies[i] as data pointer to session-aware callbacks?
> 
> I too thought about this, but I guess it is not that simple.
> 
> Just for example. Suppose we have 2 session-consumers C1 and C2.
> What if uprobe_unregister(C1) comes before the probed function
> returns?
> 
> We need something like map_cookie_to_consumer().

I guess we could have hash table in return_instance that gets 'consumer -> cookie' ?

return instance is freed after the consumers' return handlers are executed,
so there's no leak if some consumer gets unregistered before that

> 
> > > +       /* The handler_session callback return value controls execution of
> > > +        * the return uprobe and ret_handler_session callback.
> > > +        *  0 on success
> > > +        *  1 on failure, DO NOT install/execute the return uprobe
> > > +        *    console warning for anything else
> > > +        */
> > > +       int (*handler_session)(struct uprobe_consumer *self, struct pt_regs *regs,
> > > +                              unsigned long *data);
> > > +       int (*ret_handler_session)(struct uprobe_consumer *self, unsigned long func,
> > > +                                  struct pt_regs *regs, unsigned long *data);
> > > +
> >
> > We should try to avoid an alternative set of callbacks, IMO. Let's
> > extend existing ones with `unsigned long *data`,
> 
> Oh yes, agreed.
> 
> And the comment about the return value looks confusing too. I mean, the
> logic doesn't differ from the ret-code from ->handler().
> 
> "DO NOT install/execute the return uprobe" is not true if another
> non-session-consumer returns 0.

well they are meant to be exclusive, so there'd be no other non-session-consumer

jirka