linux-kernel - Re: [PATCH bpf-next 2/3] uprobes: prepare uprobe args buffer lazily

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Date: Wed, 13 Mar 2024 16:47:16 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, rostedt@...dmis.org,
	mhiramat@...nel.org, bpf@...r.kernel.org,
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 2/3] uprobes: prepare uprobe args buffer lazily

Again, looks good to me, but I have a minor nit. Feel free to ignore.

On 03/12, Andrii Nakryiko wrote:
>
>  static void __uprobe_trace_func(struct trace_uprobe *tu,
>  				unsigned long func, struct pt_regs *regs,
> -				struct uprobe_cpu_buffer *ucb,
> +				struct uprobe_cpu_buffer **ucbp,
>  				struct trace_event_file *trace_file)
>  {
>  	struct uprobe_trace_entry_head *entry;
>  	struct trace_event_buffer fbuffer;
> +	struct uprobe_cpu_buffer *ucb;
>  	void *data;
>  	int size, esize;
>  	struct trace_event_call *call = trace_probe_event_call(&tu->tp);
>  
> +	ucb = *ucbp;
> +	if (!ucb) {
> +		ucb = prepare_uprobe_buffer(tu, regs);
> +		*ucbp = ucb;
> +	}

perhaps it would be more clean to pass ucbp to prepare_uprobe_buffer()
and change it to do

	if (*ucbp)
		return *ucbp;

at the start. Then __uprobe_trace_func() and __uprobe_perf_func() can
simply do

	ucb = prepare_uprobe_buffer(tu, regs, ucbp);

> -	uprobe_buffer_put(ucb);
> +	if (ucb)
> +		uprobe_buffer_put(ucb);

Similarly, I think the "ucb != NULL" check should be shifted into
uprobe_buffer_put().

Oleg.