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

[Jiri Olsa]

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