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

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Message-ID: <20240913105750.GC19305@redhat.com>
Date: Fri, 13 Sep 2024 12:57:51 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Jiri Olsa <olsajiri@...il.com>
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 09/13, Jiri Olsa wrote:
>
> I'm not sure the realloc will help, I feel like we need to allocate return
> consumer for each called handler separately to be safe

How about something like the (pseudo) code below? Note that this way
we do not need uprobe->consumers_cnt. Note also that krealloc() should
be unlikely and it checks ksize() before it does another allocation.

Oleg.

static size_t ri_size(int consumers_cnt)
{
	return sizeof(struct return_instance) +
		      sizeof(struct return_consumer) * consumers_cnt;
}

#define DEF_CNT	4	// arbitrary value

static struct return_instance *alloc_return_instance(void)
{
	struct return_instance *ri;

	ri = kzalloc(ri_size(DEF_CNT), GFP_KERNEL);
	if (!ri)
		return ZERO_SIZE_PTR;

	ri->consumers_cnt = DEF_CNT;
	return ri;
}

static struct return_instance *push_id_cookie(struct return_instance *ri, int idx,
						__u64 id, __u64 cookie)
{
	if (unlikely(ri == ZERO_SIZE_PTR))
		return ri;

	if (unlikely(idx >= ri->consumers_cnt)) {
		ri->consumers_cnt += DEF_CNT;
		ri = krealloc(ri, ri_size(ri->consumers_cnt), GFP_KERNEL);
		if (!ri) {
			kfree(ri);
			return ZERO_SIZE_PTR;
		}
	}

	ri->consumers[idx].id = id;
	ri->consumers[idx].cookie = cookie;
	return ri;
}

static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
{
	...
	struct return_instance *ri = NULL;
	int push_idx = 0;

	list_for_each_entry_rcu(uc, &uprobe->consumers, cons_node, rcu_read_lock_trace_held()) {
		__u64 cookie = 0;
		int rc = 0;

		if (uc->handler)
			rc = uc->handler(uc, regs, &cookie);

		remove &= rc;
		has_consumers = true;

		if (!uc->ret_handler || rc == UPROBE_HANDLER_REMOVE || rc == 2)
			continue;

		if (!ri)
			ri = alloc_return_instance();

		// or, better if (rc = UPROBE_HANDLER_I_WANT_MY_COOKIE)
		if (uc->handler))
			ri = push_id_cookie(ri, push_idx++, uc->id, cookie);
	}

	if (!ZERO_OR_NULL_PTR(ri)) {
		ri->consumers_cnt = push_idx;
		prepare_uretprobe(uprobe, regs, ri);
	}

	...
}