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

Jiri Olsa
2024-06-05T20:50:12Z

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