linux-kernel - Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism

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Date:   Tue, 18 Apr 2023 09:49:14 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, rostedt@...dmis.org,
        mhiramat@...nel.org, bpf@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism

On Tue, Apr 18, 2023 at 4:15 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Mon, Apr 17, 2023 at 03:47:36PM +0000, Yafang Shao wrote:
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index f61d513..3df39a5 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -842,15 +842,21 @@ static __always_inline u64 notrace bpf_prog_start_time(void)
> >  static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tramp_run_ctx *run_ctx)
> >       __acquires(RCU)
> >  {
> > -     rcu_read_lock();
> > -     migrate_disable();
> > -
> > -     run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx);
> > +     int bit;
> >
> > -     if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) {
> > +     rcu_read_lock();
> > +     bit = test_recursion_try_acquire(_THIS_IP_, _RET_IP_);
>
> and bpf will prevent ftrace to run and vice versa.
> Not a good idea.
>
> One bpf prog will prevent different bpf prog to run since they share current task.
> Not a good idea either.

That shouldn't happen. test_recursion_try_acquire() uses a
per-task_struct value. One single task_struct can't run in parallel,
right?
Note that the bpf program running in softirq or irq context won't be
prevented by it.
IIUC, the bpf program should run in serial in one single task, right?
That said, one bpf program can only run after another bpf program
finished in the same task?

-- 
Regards
Yafang