linux-kernel - Re: [PATCHv4 02/14] uprobe: Add support for session consumer

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Message-ID: <20240917120250.GA7752@redhat.com>
Date: Tue, 17 Sep 2024 14:03:17 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <jolsa@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
	Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>,
	Yonghong Song <yhs@...com>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...omium.org>,
	Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCHv4 02/14] uprobe: Add support for session consumer

I don't see anything wrong after a quick glance, but I don't
really understand the UPROBE_HANDLER_IGNORE logic, see below.

On 09/17, Jiri Olsa wrote:
>
> + * UPROBE_HANDLER_IWANTMYCOOKIE
> + * - Store cookie and pass it to ret_handler (if defined).

Cough ;) yes it was me who used this name in the previous discussion, but maybe

	UPROBE_HANDLER_COOKIE

will look a bit better? Feel free to ignore.

>  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
...
> +		if (!uc->ret_handler || rc == UPROBE_HANDLER_REMOVE)
> +			continue;
> +
> +		/*
> +		 * If alloc_return_instance and push_consumer fail, the return probe
> +		 * won't be prepared, but we'll finish to execute all entry handlers.
> +		 *
> +		 * We need to store handler's return value in case the return uprobe
> +		 * gets installed and contains consumers that need to be ignored.
> +		 */
> +		if (!ri)
> +			ri = alloc_return_instance();
> +
> +		if (rc == UPROBE_HANDLER_IWANTMYCOOKIE || rc == UPROBE_HANDLER_IGNORE)
> +			ri = push_consumer(ri, push_idx++, uc->id, cookie, rc);

So this code allocates ri (which implies prepare_uretprobe!) and calls push_consumer()
even if rc == UPROBE_HANDLER_IGNORE.

Why? The comment in uprobes.h says:

	UPROBE_HANDLER_IGNORE
	- Ignore ret_handler callback for this consumer

but the ret_handler callback won't be ignored?

To me this code should do:

		if (!uc->ret_handler || UPROBE_HANDLER_REMOVE || UPROBE_HANDLER_IGNORE)
			continue;

		if (!ri)
			ri = alloc_return_instance();

		if (rc == UPROBE_HANDLER_IWANTMYCOOKIE)
			ri = push_consumer(...);

And,

>  handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
...
>  	list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
>  				 srcu_read_lock_held(&uprobes_srcu)) {
> +		ric = return_consumer_find(ri, &ric_idx, uc->id);
> +		if (ric && ric->rc == UPROBE_HANDLER_IGNORE)
> +			continue;
>  		if (uc->ret_handler)
> -			uc->ret_handler(uc, ri->func, regs);
> +			uc->ret_handler(uc, ri->func, regs, ric ? &ric->cookie : NULL);
>  	}

the UPROBE_HANDLER_IGNORE check above and the new ric->rc member should die,

		if (!uc->ret_handler)
			continue;

		ric = return_consumer_find(...);
		uc->ret_handler(..., ric ? &ric->cookie : NULL);

as we have already discussed, the session ret_handler(data) can simply do

		// my ->handler() wasn't called or it didn't return
		// UPROBE_HANDLER_IWANTMYCOOKIE
		if (!data)
			return;

at the start.

Could you explain why this can't work?

Oleg.