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

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Message-ID: <20230417201457.c43xfcukjzm4u6vx@dhcp-172-26-102-232.dhcp.thefacebook.com>
Date:   Mon, 17 Apr 2023 13:14:57 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yafang Shao <laoar.shao@...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 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.