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

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Date: Mon, 29 Apr 2024 23:57:14 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
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 Thu, 25 Apr 2024 13:09:32 -0700
Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:

> 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?

Good catch! Yes, we can 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)?

Indeed.

Thank you!

> 
> 
> >         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;
> >
> >


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