Re: [PATCHv4 02/14] uprobe: Add support for session consumer

Oleg Nesterov
2024-09-17T12:03:44Z

I don't see anything wrong after a quick glance, but I don't
really understand the UPROBE_HANDLER_IGNORE logic, see below.

On 09/17, Jiri Olsa wrote:
>
> + * UPROBE_HANDLER_IWANTMYCOOKIE
> + * - Store cookie and pass it to ret_handler (if defined).

Cough ;) yes it was me who used this name in the previous discussion, but maybe

UPROBE_HANDLER_COOKIE

will look a bit better? Feel free to ignore.

> static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
...
> + if (!uc->ret_handler || rc == UPROBE_HANDLER_REMOVE)
> + continue;
> +
> + /*
> + * If alloc_return_instance and push_consumer fail, the return probe
> + * won't be prepared, but we'll finish to execute all entry handlers.
> + *
> + * We need to store handler's return value in case the return uprobe
> + * gets installed and contains consumers that need to be ignored.
> + */
> + if (!ri)
> + ri = alloc_return_instance();
> +
> + if (rc == UPROBE_HANDLER_IWANTMYCOOKIE || rc == UPROBE_HANDLER_IGNORE)
> + ri = push_consumer(ri, push_idx++, uc->id, cookie, rc);

So this code allocates ri (which implies prepare_uretprobe!) and calls push_consumer()
even if rc == UPROBE_HANDLER_IGNORE.

Why? The comment in uprobes.h says:

UPROBE_HANDLER_IGNORE
- Ignore ret_handler callback for this consumer

but the ret_handler callback won't be ignored?

To me this code should do:

if (!uc->ret_handler || UPROBE_HANDLER_REMOVE || UPROBE_HANDLER_IGNORE)
continue;

if (!ri)
ri = alloc_return_instance();

if (rc == UPROBE_HANDLER_IWANTMYCOOKIE)
ri = push_consumer(...);

And,

> handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
...
> list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
> srcu_read_lock_held(&uprobes_srcu)) {
> + ric = return_consumer_find(ri, &ric_idx, uc->id);
> + if (ric && ric->rc == UPROBE_HANDLER_IGNORE)
> + continue;
> if (uc->ret_handler)
> - uc->ret_handler(uc, ri->func, regs);
> + uc->ret_handler(uc, ri->func, regs, ric ? &ric->cookie : NULL);
> }

the UPROBE_HANDLER_IGNORE check above and the new ric->rc member should die,

if (!uc->ret_handler)
continue;

ric = return_consumer_find(...);
uc->ret_handler(..., ric ? &ric->cookie : NULL);

as we have already discussed, the session ret_handler(data) can simply do

// my ->handler() wasn't called or it didn't return
// UPROBE_HANDLER_IWANTMYCOOKIE
if (!data)
return;

at the start.

Could you explain why this can't work?

Oleg.