linux-kernel - Re: [PATCH v9 29/36] bpf: Enable kprobe_multi feature if CONFIG

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Date: Thu, 25 Apr 2024 13:09:32 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Steven Rostedt <rostedt@...dmis.org>, 
	Florent Revest <revest@...omium.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>, Guo Ren <guoren@...nel.org>
Subject: Re: [PATCH v9 29/36] bpf: Enable kprobe_multi feature if
 CONFIG_FPROBE is enabled

On Mon, Apr 15, 2024 at 6:22 AM Masami Hiramatsu (Google)
<mhiramat@...nel.org> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
>
> Enable kprobe_multi feature if CONFIG_FPROBE is enabled. The pt_regs is
> converted from ftrace_regs by ftrace_partial_regs(), thus some registers
> may always returns 0. But it should be enough for function entry (access
> arguments) and exit (access return value).
>
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> Acked-by: Florent Revest <revest@...omium.org>
> ---
>  Changes from previous series: NOTHING, Update against the new series.
> ---
>  kernel/trace/bpf_trace.c |   22 +++++++++-------------
>  1 file changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index e51a6ef87167..57b1174030c9 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2577,7 +2577,7 @@ static int __init bpf_event_init(void)
>  fs_initcall(bpf_event_init);
>  #endif /* CONFIG_MODULES */
>
> -#if defined(CONFIG_FPROBE) && defined(CONFIG_DYNAMIC_FTRACE_WITH_REGS)
> +#ifdef CONFIG_FPROBE
>  struct bpf_kprobe_multi_link {
>         struct bpf_link link;
>         struct fprobe fp;
> @@ -2600,6 +2600,8 @@ struct user_syms {
>         char *buf;
>  };
>
> +static DEFINE_PER_CPU(struct pt_regs, bpf_kprobe_multi_pt_regs);

this is a waste if CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST=y, right?
Can we guard it?


> +
>  static int copy_user_syms(struct user_syms *us, unsigned long __user *usyms, u32 cnt)
>  {
>         unsigned long __user usymbol;
> @@ -2792,13 +2794,14 @@ static u64 bpf_kprobe_multi_entry_ip(struct bpf_run_ctx *ctx)
>
>  static int
>  kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
> -                          unsigned long entry_ip, struct pt_regs *regs)
> +                          unsigned long entry_ip, struct ftrace_regs *fregs)
>  {
>         struct bpf_kprobe_multi_run_ctx run_ctx = {
>                 .link = link,
>                 .entry_ip = entry_ip,
>         };
>         struct bpf_run_ctx *old_run_ctx;
> +       struct pt_regs *regs;
>         int err;
>
>         if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> @@ -2809,6 +2812,7 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link,
>
>         migrate_disable();
>         rcu_read_lock();
> +       regs = ftrace_partial_regs(fregs, this_cpu_ptr(&bpf_kprobe_multi_pt_regs));

and then pass NULL if defined(CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST)?


>         old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
>         err = bpf_prog_run(link->link.prog, regs);
>         bpf_reset_run_ctx(old_run_ctx);
> @@ -2826,13 +2830,9 @@ kprobe_multi_link_handler(struct fprobe *fp, unsigned long fentry_ip,
>                           void *data)
>  {
>         struct bpf_kprobe_multi_link *link;
> -       struct pt_regs *regs = ftrace_get_regs(fregs);
> -
> -       if (!regs)
> -               return 0;
>
>         link = container_of(fp, struct bpf_kprobe_multi_link, fp);
> -       kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), regs);
> +       kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), fregs);
>         return 0;
>  }
>
> @@ -2842,13 +2842,9 @@ kprobe_multi_link_exit_handler(struct fprobe *fp, unsigned long fentry_ip,
>                                void *data)
>  {
>         struct bpf_kprobe_multi_link *link;
> -       struct pt_regs *regs = ftrace_get_regs(fregs);
> -
> -       if (!regs)
> -               return;
>
>         link = container_of(fp, struct bpf_kprobe_multi_link, fp);
> -       kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), regs);
> +       kprobe_multi_link_prog_run(link, get_entry_ip(fentry_ip), fregs);
>  }
>
>  static int symbols_cmp_r(const void *a, const void *b, const void *priv)
> @@ -3107,7 +3103,7 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
>         kvfree(cookies);
>         return err;
>  }
> -#else /* !CONFIG_FPROBE || !CONFIG_DYNAMIC_FTRACE_WITH_REGS */
> +#else /* !CONFIG_FPROBE */
>  int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
>  {
>         return -EOPNOTSUPP;
>
>