netdev - Re: [PATCH v2 bpf-next 3/5] ftrace: introduce FTRACE_OPS_FL_SHARE

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Date:   Fri, 15 Jul 2022 17:29:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Song Liu <songliubraving@...com>
Cc:     Song Liu <song@...nel.org>, Networking <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, lkml <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "mhiramat@...nel.org" <mhiramat@...nel.org>
Subject: Re: [PATCH v2 bpf-next 3/5] ftrace: introduce
 FTRACE_OPS_FL_SHARE_IPMODIFY

On Fri, 15 Jul 2022 20:21:49 +0000
Song Liu <songliubraving@...com> wrote:

> >>> Wouldn't this need to be done anyway if BPF was first and live kernel
> >>> patching needed the update? An -EAGAIN would not suffice.    
> >> 
> >> prepare_direct_functions_for_ipmodify handles BPF-first-livepatch-later
> >> case. The benefit of prepare_direct_functions_for_ipmodify() is that it 
> >> holds direct_mutex before ftrace_lock, and keeps holding it if necessary. 
> >> This is enough to make sure we don't need the wash-rinse-repeat. 
> >> 
> >> OTOH, if we wait until __ftrace_hash_update_ipmodify(), we already hold
> >> ftrace_lock, but not direct_mutex. To make changes to bpf trampoline, we
> >> have to unlock ftrace_lock and lock direct_mutex to avoid deadlock. 
> >> However, this means we will need the wash-rinse-repeat.   
> 
> What do you think about the prepare_direct_functions_for_ipmodify() 
> approach? If this is not ideal, maybe we can simplify it so that it only
> holds direct_mutex (when necessary). The benefit is that we are sure
> direct_mutex is already held in __ftrace_hash_update_ipmodify(). However, 
> I think it is not safe to unlock ftrace_lock in __ftrace_hash_update_ipmodify(). 
> We can get parallel do_for_each_ftrace_rec(), which is dangerous, no? 

I'm fine with it. But one nit on the logic:

>  int register_ftrace_function(struct ftrace_ops *ops)
> +	__releases(&direct_mutex)
>  {
> +	bool direct_mutex_locked;
>  	int ret;
>  
>  	ftrace_ops_init(ops);
>  
> +	ret = prepare_direct_functions_for_ipmodify(ops);
> +	if (ret < 0)
> +		return ret;
> +
> +	direct_mutex_locked = ret == 1;
> +

Please make the above:

	if (ret < 0)
		return ret;
	else if (ret == 1)
		direct_mutex_locked = true;

It's much easier to read that way.

-- Steve

>  	mutex_lock(&ftrace_lock);
>  
>  	ret = ftrace_startup(ops, 0);
>  
>  	mutex_unlock(&ftrace_lock);
>  
> +	if (direct_mutex_locked)
> +		mutex_unlock(&direct_mutex);
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(register_ftrace_function);
> --