linux-kernel - Re: [PATCH v5 3.1.0-rc4-tip 18/26] uprobes: slot allocation.

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Date:	Fri, 7 Oct 2011 20:37:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux-mm <linux-mm@...ck.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonathan Corbet <corbet@....net>,
	Hugh Dickins <hughd@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 3.1.0-rc4-tip 18/26]   uprobes: slot allocation.

On 09/20, Srikar Dronamraju wrote:
>
> - * valid_vma: Verify if the specified vma is an executable vma
> + * valid_vma: Verify if the specified vma is an executable vma,
> + * but not an XOL vma.
>   *	- Return 1 if the specified virtual address is in an
> - *	  executable vma.
> + *	  executable vma, but not in an XOL vma.
>   */
>  static bool valid_vma(struct vm_area_struct *vma)
>  {
> +	struct uprobes_xol_area *area = vma->vm_mm->uprobes_xol_area;
> +
>  	if (!vma->vm_file)
>  		return false;
>
> +	if (area && (area->vaddr == vma->vm_start))
> +			return false;

Could you explain why do we need this "but not an XOL vma" check?
xol_vma->vm_file is always NULL, no?

> +static struct uprobes_xol_area *xol_alloc_area(void)
> +{
> +	struct uprobes_xol_area *area = NULL;
> +
> +	area = kzalloc(sizeof(*area), GFP_KERNEL);
> +	if (unlikely(!area))
> +		return NULL;
> +
> +	area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
> +								GFP_KERNEL);
> +
> +	if (!area->bitmap)
> +		goto fail;
> +
> +	init_waitqueue_head(&area->wq);
> +	spin_lock_init(&area->slot_lock);
> +	if (!xol_add_vma(area) && !current->mm->uprobes_xol_area) {
> +		task_lock(current);
> +		if (!current->mm->uprobes_xol_area) {
> +			current->mm->uprobes_xol_area = area;
> +			task_unlock(current);
> +			return area;
> +		}
> +		task_unlock(current);

But you can't rely on task_lock(), you can race with another thread
with the same ->mm. I guess you need mmap_sem or xchg().

>  static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
>  				unsigned long vaddr)
>  {
> -	/* TODO: Yet to be implemented */
> +	if (xol_get_insn_slot(uprobe, vaddr) && !pre_xol(uprobe, regs)) {
> +		set_instruction_pointer(regs, current->utask->xol_vaddr);

set_instruction_pointer() looks unneded, pre_xol() has already changed
regs->ip.

Oleg.

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