linux-kernel - Re: [RFC PATCH v2 4/6] tracing/fprobe: Enable fprobe events with CONFIG_DYNAMIC_FTRACE_WITH

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Message-ID: <CABRcYm+8-zYRGjKSPtWQ8_Vq2649=vi71fGvFx2aWM1tnOMYQQ@mail.gmail.com>
Date:   Wed, 9 Aug 2023 12:31:00 +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: [RFC PATCH v2 4/6] tracing/fprobe: Enable fprobe events with CONFIG_DYNAMIC_FTRACE_WITH_ARGS

On Mon, Aug 7, 2023 at 8:49 AM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Allow fprobe events to be enabled with CONFIG_DYNAMIC_FTRACE_WITH_ARGS.
> With this change, fprobe events mostly use ftrace_regs instead of pt_regs.
> Note that if the arch doesn't enable HAVE_PT_REGS_COMPAT_FTRACE_REGS,
> fprobe events will not be able to use from perf.

nit: "to be used from perf" ?

> --- a/kernel/trace/trace_fprobe.c
> +++ b/kernel/trace/trace_fprobe.c
> @@ -132,25 +132,30 @@ static int
>  process_fetch_insn(struct fetch_insn *code, void *rec, void *dest,
>                    void *base)
>  {
> -       struct pt_regs *regs = rec;
> -       unsigned long val;
> +       struct ftrace_regs *fregs = rec;
> +       unsigned long val, *stackp;
>         int ret;
>
>  retry:
>         /* 1st stage: get value from context */
>         switch (code->op) {
>         case FETCH_OP_STACK:
> -               val = regs_get_kernel_stack_nth(regs, code->param);
> +               stackp = (unsigned long *)ftrace_regs_get_stack_pointer(fregs);
> +               if (((unsigned long)(stackp + code->param) & ~(THREAD_SIZE - 1)) ==
> +                   ((unsigned long)stackp & ~(THREAD_SIZE - 1)))

Maybe it'd be worth extracting a local
"ftrace_regs_get_kernel_stack_nth_addr" helper function and/or
"ftrace_regs_within_kernel_stack" ?