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From:   Florent Revest <revest@chromium.org>
Date:   Wed, 9 Aug 2023 18:17:47 +0200
Message-ID: <CABRcYmLHfQsjwf7dk+A0Q96iANhj60M0g_xRjVyMUY9LJPybNg@mail.gmail.com>
Subject: Re: [RFC PATCH v2 1/6] fprobe: Use fprobe_regs in fprobe entry handler
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-trace-kernel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        bpf <bpf@vger.kernel.org>, Sven Schnelle <svens@linux.ibm.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Alan Maguire <alan.maguire@oracle.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>
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On Wed, Aug 9, 2023 at 6:09=E2=80=AFPM Florent Revest <revest@chromium.org>=
 wrote:
>
> On Wed, Aug 9, 2023 at 4:10=E2=80=AFPM 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=E2=80=AFAM 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 fpro=
be
> > > > 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 register=
s
> > > 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_r=
egs
> > 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/k=
ernel/trace/fprobe.c?h=3Dtopic/fprobe-ftrace-regs&id=3D2ca022b2753ae0d2a251=
3c95f7ed5b5b727fb2c4#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/k=
ernel/trace/ftrace.c?h=3Dtopic/fprobe-ftrace-regs&id=3D2ca022b2753ae0d2a251=
3c95f7ed5b5b727fb2c4#n338
>
> As documented here:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git/tree/i=
nclude/linux/ftrace.h?h=3Dtopic/fprobe-ftrace-regs&id=3D2ca022b2753ae0d2a25=
13c95f7ed5b5b727fb2c4#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