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

[Srikar Dronamraju]

* Oleg Nesterov <oleg@xxxxxxxxxx> [2011-10-07 20:37:40]:

> 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?
> 

Yes, xol_vma->vm_file is always NULL.
previously we used shmem_file_setup before we map the XOL area.
However we now use init_creds instead, so this should also change
accordingly. Will correct this.

> > +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().

Agree, 
I think its better to use cmpxchg instead of xchg(). Otherwise,
(using xchg), I would set area to new value, but the old area might be in
use already. So I cant unmap the old area.

If I use cmpxchg, I can free up the new area if previous area is non
NULL.

However setting uprobes_xol_area in xol_add_vma() where we already take
mmap_sem for write while maping the xol_area is the best option.

> 
> >  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.
> 

Agree.

-- 
Thanks and Regards
Srikar
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