linux-kernel - Re: [RFC PATCH 2/6] uretprobes: trampoline implementation

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Date:	Sat, 22 Dec 2012 17:02:28 +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>,
	Anithra P Janakiraman <anithra@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 2/6] uretprobes: trampoline implementation

On 12/21, Anton Arapov wrote:
>
> +static unsigned long xol_get_trampoline_slot(void)
> +{
> +	struct xol_area *area;
> +	unsigned long offset;
> +	void *vaddr;
> +	uprobe_opcode_t bp_insn = UPROBE_SWBP_INSN;
> +
> +	area = get_xol_area(current->mm);
> +	if (!area) {
> +		area = xol_alloc_area();
> +		if (!area)
> +			return 0;
> +	}
> +
> +	if (!area->rp_trampoline_vaddr)
> +		area->rp_trampoline_vaddr = xol_take_insn_slot(area);
> +
> +	/*
> +	 * Initialize the slot if rp_trampoline_vaddr points to valid
> +	 * instruction slot.
> +	 */
> +	if (unlikely(!area->rp_trampoline_vaddr))
> +		return 0;
> +
> +	offset = area->rp_trampoline_vaddr & ~PAGE_MASK;
> +	vaddr = kmap_atomic(area->page);
> +	memcpy(vaddr + offset, &bp_insn, UPROBE_SWBP_INSN_SIZE);
> +	kunmap_atomic(vaddr);
> +
> +	return area->rp_trampoline_vaddr;
> +}

Oh, this should be unified with xol_get_insn_slot(), we certainly do not
want to copy-and-paste it.

Or. Perhaps even better, do not add this helper at all. xol_alloc_area()
could reserve the first slot/bit for trampoline. And note that in this
case we do not need xol_area->rp_trampoline_vaddr, it is always equal
to xol_area->vaddr.

Oleg.

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