linux-kernel - Re: [PATCH v3 5/8] tracing/fprobe: Enable fprobe events with CONFIG_DYNAMIC_FTRACE_WITH

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Date:   Fri, 18 Aug 2023 20:11:15 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Florent Revest <revest@...omium.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 5/8] tracing/fprobe: Enable fprobe events with
 CONFIG_DYNAMIC_FTRACE_WITH_ARGS

On Thu, 17 Aug 2023 10:57:50 +0200
Florent Revest <revest@...omium.org> wrote:

> On Sat, Aug 12, 2023 at 7:37 AM Masami Hiramatsu (Google)
> <mhiramat@...nel.org> wrote:
> >
> > diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> > index d56304276318..6fb4ecf8767d 100644
> > --- a/kernel/trace/Kconfig
> > +++ b/kernel/trace/Kconfig
> > @@ -679,7 +679,6 @@ config FPROBE_EVENTS
> >         select TRACING
> >         select PROBE_EVENTS
> >         select DYNAMIC_EVENTS
> > -       depends on DYNAMIC_FTRACE_WITH_REGS
> 
> I believe that, in practice, fprobe events still rely on WITH_REGS:
> 
> > diff --git a/kernel/trace/trace_fprobe.c b/kernel/trace/trace_fprobe.c
> > index f440c97e050f..94c01dc061ec 100644
> > --- a/kernel/trace/trace_fprobe.c
> > +++ b/kernel/trace/trace_fprobe.c
> > @@ -327,14 +328,15 @@ static int fentry_dispatcher(struct fprobe *fp, unsigned long entry_ip,
> >         struct pt_regs *regs = ftrace_get_regs(fregs);
> 
> Because here you require the entry handler needs ftrace_regs that are
> full pt_regs.

Ah, that is for perf events. Yes, that is the problematic point.
Since perf's interfaces are depending on the pt_regs (especially stacktrace)
I can not remove this part. This is the next issue to be solved.
Maybe we can use partial pt_regs for stack tracing, so we can swap the order
of the patches to introduce ftrace_partial_regs() before this and use it for
perf event.

> 
> >         int ret = 0;
> >
> > +       if (trace_probe_test_flag(&tf->tp, TP_FLAG_TRACE))
> > +               fentry_trace_func(tf, entry_ip, fregs);
> > +
> > +#ifdef CONFIG_PERF_EVENTS
> >         if (!regs)
> >                 return 0;
> >
> > -       if (trace_probe_test_flag(&tf->tp, TP_FLAG_TRACE))
> > -               fentry_trace_func(tf, entry_ip, regs);
> > -#ifdef CONFIG_PERF_EVENTS
> >         if (trace_probe_test_flag(&tf->tp, TP_FLAG_PROFILE))
> > -               ret = fentry_perf_func(tf, entry_ip, regs);
> > +               ret = fentry_perf_func(tf, entry_ip, fregs, regs);
> >  #endif
> >         return ret;
> >  }
> > @@ -347,14 +349,15 @@ static void fexit_dispatcher(struct fprobe *fp, unsigned long entry_ip,
> >         struct trace_fprobe *tf = container_of(fp, struct trace_fprobe, fp);
> >         struct pt_regs *regs = ftrace_get_regs(fregs);
> 
> And same here with the return handler
> 
> I think fprobe events would need the same sort of refactoring as
> kprobe_multi bpf: using ftrace_partial_regs so they work on build
> !WITH_REGS.

Actually, kprobe_multi is using fprobe directly, so this is not related
to bpf part.

Thank you,


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