Re: [RFC PATCH v2 1/6] fprobe: Use fprobe_regs in fprobe entry handler

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

Hi Florent,

On Wed, 9 Aug 2023 12:28:38 +0200
Florent Revest <revest@chromium.org> wrote:

> On Mon, Aug 7, 2023 at 8:48 AM Masami Hiramatsu (Google)
> <mhiramat@kernel.org> wrote:
> >
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > This allows fprobes to be available with CONFIG_DYNAMIC_FTRACE_WITH_ARGS
> > instead of CONFIG_DYNAMIC_FTRACE_WITH_REGS, then we can enable fprobe
> > on arm64.
> 
> This patch lets fprobe code build on configs WITH_ARGS and !WITH_REGS
> but fprobe wouldn't run on these builds because fprobe still registers
> to ftrace with FTRACE_OPS_FL_SAVE_REGS, which would fail on
> !WITH_REGS. Shouldn't we also let the fprobe_init callers decide
> whether they want REGS or not ?

Ah, I think you meant FPROBE_EVENTS? Yes I forgot to add the dependency
on it. But fprobe itself can work because fprobe just pass the ftrace_regs
to the handlers. (Note that exit callback may not work until next patch)

> 
> >  static int
> >  kprobe_multi_link_handler(struct fprobe *fp, unsigned long fentry_ip,
> > -                         unsigned long ret_ip, struct pt_regs *regs,
> > +                         unsigned long ret_ip, struct ftrace_regs *fregs,
> >                           void *data)
> >  {
> >         struct bpf_kprobe_multi_link *link;
> > +       struct pt_regs *regs = ftrace_get_regs(fregs);
> > +
> > +       if (!regs)
> > +               return 0;
> 
> (with the above comment addressed) this means that BPF multi_kprobe
> would successfully attach on builds WITH_ARGS but the programs would
> never actually run because here regs would be 0. This is a confusing
> failure mode for the user. I think that if multi_kprobe won't work
> (because we don't have a pt_regs conversion path yet), the user should
> be notified at attachment time that they won't be getting any events.

Yes, so I changed it will not be compiled in that case.

@@ -2460,7 +2460,7 @@ static int __init bpf_event_init(void)
 fs_initcall(bpf_event_init);
 #endif /* CONFIG_MODULES */
 
-#ifdef CONFIG_FPROBE
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
 struct bpf_kprobe_multi_link {
 	struct bpf_link link;
 	struct fprobe fp;

> That's why I think kprobe_multi should inform fprobe_init that it
> wants FTRACE_OPS_FL_SAVE_REGS and fail if that's not possible (no
> trampoline for it for example)

Yeah, that's another possibility, but in the previous thread we
discussed and agreed to introduce the ftrace_partial_regs() which
will copy the partial registers from ftrace_regs to pt_regs.


Thank you,


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>