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SubjectRe: [PATCH v5 3.1.0-rc4-tip 3/26] Uprobes: register/unregister probes.
* Peter Zijlstra <peterz@infradead.org> [2011-09-26 15:15:00]:

> On Tue, 2011-09-20 at 17:30 +0530, Srikar Dronamraju wrote:
>
> > +static struct vma_info *__find_next_vma_info(struct list_head *head,
> > + loff_t offset, struct address_space *mapping,
> > + struct vma_info *vi)
> > +{
> > + struct prio_tree_iter iter;
> > + struct vm_area_struct *vma;
> > + struct vma_info *tmpvi;
> > + loff_t vaddr;
> > + unsigned long pgoff = offset >> PAGE_SHIFT;
> > + int existing_vma;
> > +
> > + vma_prio_tree_foreach(vma, &iter, &mapping->i_mmap, pgoff, pgoff) {
> > + if (!vma || !valid_vma(vma))
> > + return NULL;
> > +
> > + existing_vma = 0;
> > + vaddr = vma->vm_start + offset;
> > + vaddr -= vma->vm_pgoff << PAGE_SHIFT;
> > + list_for_each_entry(tmpvi, head, probe_list) {
> > + if (tmpvi->mm == vma->vm_mm && tmpvi->vaddr == vaddr) {
> > + existing_vma = 1;
> > + break;
> > + }
> > + }
> > + if (!existing_vma &&
> > + atomic_inc_not_zero(&vma->vm_mm->mm_users)) {
> > + vi->mm = vma->vm_mm;
> > + vi->vaddr = vaddr;
> > + list_add(&vi->probe_list, head);
> > + return vi;
>
> The the sole purpose of actually having that list is the above linear
> was to test if we've already had this one?
>
> Does that really matter? After all, if the probe is already installed
> installing it again will return with -EEXIST, which should be easy
> enough to deal with.
>

No, There is a possibility of going in a forever loop.
Since the the priotree can change when we drop the mapping->mutex, we
dont pass the hint to vma_prio_tree_foreach.
So we might keep getting the same vma again and again.

> > + }
> > + }
> > + return NULL;
> > +}
> > +
> > +/*
> > + * Iterate in the rmap prio tree and find a vma where a probe has not
> > + * yet been inserted.
> > + */
> > +static struct vma_info *find_next_vma_info(struct list_head *head,
> > + loff_t offset, struct address_space *mapping)
> > +{
> > + struct vma_info *vi, *retvi;
> > + vi = kzalloc(sizeof(struct vma_info), GFP_KERNEL);
> > + if (!vi)
> > + return ERR_PTR(-ENOMEM);
> > +
> > + INIT_LIST_HEAD(&vi->probe_list);
>
> weird place for the INIT_LIST_HEAD, I would have expected it near where
> the rest of vi is initialized, although it looks to be superfluous
> anyway, since list_add() can handle an uninitialized entry.
>
>
> > + mutex_lock(&mapping->i_mmap_mutex);
> > + retvi = __find_next_vma_info(head, offset, mapping, vi);
> > + mutex_unlock(&mapping->i_mmap_mutex);
> > +
> > + if (!retvi)
> > + kfree(vi);
> > + return retvi;
> > +}
> > +
> > +static int __register_uprobe(struct inode *inode, loff_t offset,
> > + struct uprobe *uprobe)
> > +{
> > + struct list_head try_list;
> > + struct vm_area_struct *vma;
> > + struct address_space *mapping;
> > + struct vma_info *vi, *tmpvi;
> > + struct mm_struct *mm;
> > + int ret = 0;
> > +
> > + mapping = inode->i_mapping;
> > + INIT_LIST_HEAD(&try_list);
> > + while ((vi = find_next_vma_info(&try_list, offset,
> > + mapping)) != NULL) {
> > + if (IS_ERR(vi)) {
> > + ret = -ENOMEM;
> > + break;
> > + }
>
> Here we hold neither i_mmap_mutex nor mmap_sem, so everything can change
> under our feet. See below..
>
> > + mm = vi->mm;
> > + down_read(&mm->mmap_sem);
> > + vma = find_vma(mm, (unsigned long) vi->vaddr);
> > + if (!vma || !valid_vma(vma)) {
>
> No validation if its indeed the same vma you found earlier? At the very
> least we should validate the vma returned from find_vma() is indeed a
> mapping of the inode we're after and that the offset is still to be
> found at vaddr.
>

Yes, this can be done.

> > + list_del(&vi->probe_list);
> > + kfree(vi);
> > + up_read(&mm->mmap_sem);
> > + mmput(mm);
> > + continue;
> > + }
> > + ret = install_breakpoint(mm);
> > + if (ret && (ret != -ESRCH || ret != -EEXIST)) {
> > + up_read(&mm->mmap_sem);
> > + mmput(mm);
> > + break;
> > + }
>
> Right, so you already deal with -EEXIST, so why do we need that list at
> all then?
>
> Aah, its to make fwd progress, without it we would keep retrying the
> same vma over and over,.. hmm?
>

Yes.

> > + ret = 0;
> > + up_read(&mm->mmap_sem);
> > + mmput(mm);
> > + }
> > + list_for_each_entry_safe(vi, tmpvi, &try_list, probe_list) {
> > + list_del(&vi->probe_list);
> > + kfree(vi);
> > + }
> > + return ret;
> > +}
>

--
Thanks and Regards
Srikar


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