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

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

On Wed, 9 Aug 2023 18:17:47 +0200
Florent Revest <revest@chromium.org> wrote:

> On Wed, Aug 9, 2023 at 6:09 PM Florent Revest <revest@chromium.org> wrote:
> >
> > On Wed, Aug 9, 2023 at 4:10 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > >
> > > 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)
> >
> > No, I mean that fprobe still registers its ftrace ops with the
> > FTRACE_OPS_FL_SAVE_REGS flag:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/fprobe.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n185
> >
> > Which means that __register_ftrace_function will return -EINVAL on
> > builds !WITH_REGS:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/kernel/trace/ftrace.c?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n338
> >
> > As documented here:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/include/linux/ftrace.h?h=topic/fprobe-ftrace-regs&id=2ca022b2753ae0d2a2513c95f7ed5b5b727fb2c4#n188
> >
> > There are two parts to using sparse pt_regs. One is "static": having
> > WITH_ARGS in the config, the second one is "dynamic": a ftrace ops
> > needs to specify that it doesn't want to go through the ftrace
> > trampoline that saves a full pt_regs, by not giving
> > FTRACE_OPS_FL_SAVE_REGS. If we want fprobe to work on builds
> > !WITH_REGS then we should both remove Kconfig dependencies to
> > WITH_REGS (like you've done) but also stop passing this ftrace ops
> > flag.
> 
> Said in a different way: there are arches that support both WITH_ARGS
> and WITH_REGS (like x86 actually). They have two ftrace trampolines
> compiled in: ftrace_caller and ftrace_regs_caller, one for each
> usecase. If you register to ftrace with the FTRACE_OPS_FL_SAVE_REGS
> flag you are telling it that what you want is a pt_regs. If you are
> trying to move away from pt_regs and support ftrace_regs in the more
> general case (meaning, in the case where it can contain a sparse
> pt_regs) then you should stop passing that flag so you go through the
> lighter, faster trampoline and test your code in the circumstances
> where ftrace_regs isn't just a regular pt_regs but an actually sparse
> or light data structure.
> 
> I hope that makes my thoughts clearer? It's a hairy topic ahah

Ah, I see your point.

static void fprobe_init(struct fprobe *fp)
{
        fp->nmissed = 0;
        if (fprobe_shared_with_kprobes(fp))
                fp->ops.func = fprobe_kprobe_handler;
        else
                fp->ops.func = fprobe_handler;
        fp->ops.flags |= FTRACE_OPS_FL_SAVE_REGS; <---- This flag!
}

So it should be FTRACE_OPS_FL_ARGS. Let me fix that.

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