On Wed, 7 Jul 2021 19:45:30 +0900
Masami Hiramatsu [off-list ref] wrote:

On Wed, 7 Jul 2021 12:20:57 +0200
Peter Zijlstra [off-list ref] wrote:

quoted

On Wed, Jul 07, 2021 at 07:15:10PM +0900, Masami Hiramatsu wrote:

quoted

I actually don't want to keep this feature because no one use it.
(only systemtap needs it?)

Yeah, you mentioned systemtap, but since that's out-of-tree I don't
care. Their problem.

Yeah, maybe it is not hard to update.

quoted

quoted

Anyway, if we keep the idea-level compatibility (not code level),
what we need is 'void *data' in the struct kretprobe_instance.
User who needs it can allocate their own instance data for their
kretprobes when initialising it and sets in their entry handler.

Then we can have a simple kretprobe_instance.

When would you do the alloc? When installing the retprobe, but that
might be inside the allocator, which means you can't call the allocator
etc.. :-)

Yes, so the user may need to allocate a pool right before register_kretprobe().
(whether per-kretprobe or per-task or global pool, that is user's choice.)

quoted

If we look at struct ftrace_ret_stack, it has a few fixed function
fields. The calltime one is all that is needed for the kretprobe
example code.

kretprobe consumes 3 fields, a pointer to 'struct kretprobe' (which
stores callee function address in 'kretprobe::kp.addr'), a return
address and a frame pointer (*).

Oops, I forgot to add "void *data" for storing user data.

Thank you,

-- 
Masami Hiramatsu [off-list ref]