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

[Matt Wu]

On 2021/7/7 PM9:29, Masami Hiramatsu wrote:
> On Wed, 7 Jul 2021 19:45:30 +0900
> Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>
> > On Wed, 7 Jul 2021 12:20:57 +0200
> > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > > 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.
>
> Yeah, maybe it is not hard to update.
>
> > > > 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.)
> >
> > > 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.

Should use "struct kretprobe_holder *rph", since "struct kretprobe" belongs
to 3rd-party module (which might be unloaded any time).

User's own pool might not work if the module can be unloaded. Better manage
the pool in kretprobe_holder, which needs no changes from user side.