Re: [PATCH -tip v8 11/13] x86/unwind: Recover kretprobe trampoline entry

[Peter Zijlstra]

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

> 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.

> 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.. :-)

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.