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

Jonathan Corbet
2011-03-15T19:10:24Z

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:

+/* 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.

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

jon