linux-kernel - Re: [PATCHv3 1/7] uprobe: Add support for session consumer

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Message-ID: <20240912162028.GD27648@redhat.com>
Date: Thu, 12 Sep 2024 18:20:29 +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@...gle.com>, 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: [PATCHv3 1/7] uprobe: Add support for session consumer

On 09/09, Jiri Olsa wrote:
>
>  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  {
>  	struct uprobe_consumer *uc;
>  	int remove = UPROBE_HANDLER_REMOVE;
> -	bool need_prep = false; /* prepare return uprobe, when needed */
> +	struct return_consumer *ric = NULL;
> +	struct return_instance *ri = NULL;
>  	bool has_consumers = false;
>
>  	current->utask->auprobe = &uprobe->arch;
>
>  	list_for_each_entry_srcu(uc, &uprobe->consumers, cons_node,
>  				 srcu_read_lock_held(&uprobes_srcu)) {
> +		__u64 cookie = 0;
>  		int rc = 0;
>
>  		if (uc->handler) {
> -			rc = uc->handler(uc, regs);
> -			WARN(rc & ~UPROBE_HANDLER_MASK,
> +			rc = uc->handler(uc, regs, &cookie);
> +			WARN(rc < 0 || rc > 2,
>  				"bad rc=0x%x from %ps()\n", rc, uc->handler);
>  		}
>
> -		if (uc->ret_handler)
> -			need_prep = true;
> -
> +		/*
> +		 * The handler can return following values:
> +		 * 0 - execute ret_handler (if it's defined)
> +		 * 1 - remove uprobe
> +		 * 2 - do nothing (ignore ret_handler)
> +		 */
>  		remove &= rc;
>  		has_consumers = true;
> +
> +		if (rc == 0 && uc->ret_handler) {

should we enter this block if uc->handler == NULL?

> +			/*
> +			 * Preallocate return_instance object optimistically with
> +			 * all possible consumers, so we allocate just once.
> +			 */
> +			if (!ri) {
> +				ri = alloc_return_instance(uprobe->consumers_cnt);

This doesn't look right...

Suppose we have a single consumer C1, so uprobe->consumers_cnt == 1 and
alloc_return_instance() allocates return_instance with for a single consumer,
so that only ri->consumers[0] is valid.

Right after that uprobe_register()->consumer_add() adds another consumer
C2 with ->ret_handler != NULL.

On the next iteration return_consumer_next() will return the invalid addr
== &ri->consumers[1].

perhaps this needs krealloc() ?

> +				if (!ri)
> +					return;

Not sure we should simply return if kzalloc fails... at least it would be better
to clear current->utask->auprobe.

> +	if (ri && !remove)
> +		prepare_uretprobe(uprobe, regs, ri); /* put bp at return */
> +	else
> +		kfree(ri);

Well, if ri != NULL then remove is not possible, afaics... ri != NULL means
that at least one ->handler() returned rc = 0, thus "remove" must be zero.

So it seems you can just do

	if (ri)
		prepare_uretprobe(...);


Didn't read other parts of your patch yet ;)

Oleg.