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

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Message-ID: <CABRcYm+ayJwS+YMaKBF9pdnHYcJvioOoOrXHWOeRAg1hPacYiA@mail.gmail.com>
Date:   Thu, 17 Aug 2023 10:57:50 +0200
From:   Florent Revest <revest@...omium.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.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 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.

>         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.