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

Jiri Olsa
2024-06-06T16:46:31Z

On Wed, Jun 05, 2024 at 10:50:11PM +0200, Jiri Olsa wrote:
> 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' ?

ok, hash table is probably too big for this.. I guess some solution that
would iterate consumers and cookies made sure it matches would be fine

jirka

>
> 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