linux-kernel - Re: [RFC PATCH v3 4/6] uretprobes: return probe entry, prepare uretprobe

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Date:	Thu, 28 Feb 2013 21:10:24 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Anton Arapov <anton@...hat.com>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Josh Stone <jistone@...hat.com>,
	Frank Eigler <fche@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [RFC PATCH v3 4/6] uretprobes: return probe entry, prepare
	uretprobe

On 02/28, Anton Arapov wrote:
>
> +static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
> +{
> +	struct return_uprobe_i *ri;
> +	struct uprobe_task *utask;
> +	struct xol_area *area;
> +	unsigned long rp_trampoline_vaddr = 0;
> +	uprobe_opcode_t insn = UPROBE_SWBP_INSN;
> +
> +	area = get_xol_area();
> +	if (area)
> +		rp_trampoline_vaddr = area->rp_trampoline_vaddr;
> +	if (!rp_trampoline_vaddr) {
> +		rp_trampoline_vaddr = xol_get_insn_slot(&insn);
> +		if (!rp_trampoline_vaddr)
> +			return;
> +	}
> +	area->rp_trampoline_vaddr = rp_trampoline_vaddr;

This is called under down_read(), so 2 threads can race with each other
and use the different rp_trampoline_vaddr's if ->rp_trampoline_vaddr was
NULL.

And again, I think ->rp_trampoline_vaddr is simply unneeded, see my
reply to 3/6.

>  static void handler_chain(struct uprobe *uprobe, struct pt_regs *regs)
>  {
> +	int rc = 0;
>  	struct uprobe_consumer *uc;
>  	int remove = UPROBE_HANDLER_REMOVE;
>  
>  	down_read(&uprobe->register_rwsem);
>  	for (uc = uprobe->consumers; uc; uc = uc->next) {
> -		int rc = uc->handler(uc, regs);
> +		if (uc->handler)
> +			rc = uc->handler(uc, regs);
> +
> +		if (uc->rp_handler)
> +			prepare_uretprobe(uprobe, regs); /* put bp at return */

Hmm. I didn't read this series yet. But at first glance I am not
sure prepare_uretprobe() should be called every time we see
->rp_handler != NULL, there could be multiple consumers...

Oleg.

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