Re: [PATCH v2 2.6.38-rc8-tip 11/20] 11: uprobes: slot allocation for uprobes

Srikar Dronamraju
2011-03-16T05:04:36Z

* Jonathan Corbet <corbet@lwn.net> [2011-03-15 13:10:20]:

> Just a couple of minor notes while I was looking at this code...
>
> > +static struct uprobes_xol_area *xol_alloc_area(void)
> > +{
> > + struct uprobes_xol_area *area = NULL;
> > +
> > + area = kzalloc(sizeof(*area), GFP_USER);
> > + if (unlikely(!area))
> > + return NULL;
> > +
> > + area->bitmap = kzalloc(BITS_TO_LONGS(UINSNS_PER_PAGE) * sizeof(long),
> > + GFP_USER);
>
> Why GFP_USER? That causes extra allocation limits to be enforced. Given
> that in part 14 you have:

Okay, Will use GFP_KERNEL.
We used GFP_USER because we thought its going to represent part of
process address space;

>
> +/* Prepare to single-step probed instruction out of line. */
> +static int pre_ssout(struct uprobe *uprobe, struct pt_regs *regs,
> + unsigned long vaddr)
> +{
> + xol_get_insn_slot(uprobe, vaddr);
> + BUG_ON(!current->utask->xol_vaddr);
>
> It seems to me that you really don't want those allocations to fail.
>
> back to xol_alloc_area():
>
> > + if (!area->bitmap)
> > + goto fail;
> > +
> > + 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);
> > + }
> > +
> > +fail:
> > + if (area) {
> > + if (area->bitmap)
> > + kfree(area->bitmap);
> > + kfree(area);
> > + }
>
> You've already checked area against NULL, and kfree() can handle null
> pointers, so both of those tests are unneeded.

Okay,

>
> > + return current->mm->uprobes_xol_area;
> > +}
>
> jon

--
Thanks and Regards
Srikar