Re: [PATCH v5 3.1.0-rc4-tip 3/26] Uprobes: register/unregister probes.

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* 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