On Wed, Sep 18, 2019 at 04:50:58AM -0700, Matthew Wilcox wrote:
> On Tue, Sep 17, 2019 at 03:48:42PM -0600, Jordan Crouse wrote:
> > It is possible for unaware callers of several idr functions to accidentally
> > underflow the index by specifying a id that is less than the idr base.
>
> Hi Jordan. Thanks for the patch, but this seems like a distinction
> without a difference.
>
> > void *idr_remove(struct idr *idr, unsigned long id)
> > {
> > + if (id < idr->idr_base)
> > + return NULL;
> > +
> > return radix_tree_delete_item(&idr->idr_rt, id - idr->idr_base, NULL);
>
> If this underflows, we'll try to delete an index which doesn't exist,
> which will return NULL.
>
> > void *idr_find(const struct idr *idr, unsigned long id)
> > {
> > + if (id < idr->idr_base)
> > + return NULL;
> > +
> > return radix_tree_lookup(&idr->idr_rt, id - idr->idr_base);
>
> If this underflows, we'll look up an entry which doesn't exist, which
> will return NULL.
>
> > @@ -302,6 +308,9 @@ void *idr_replace(struct idr *idr, void *ptr, unsigned long id)
> > void __rcu **slot = NULL;
> > void *entry;
> >
> > + if (id < idr->idr_base)
> > + return ERR_PTR(-ENOENT);
> > +
> > id -= idr->idr_base;
> >
> > entry = __radix_tree_lookup(&idr->idr_rt, id, &node, &slot);
>
> ... just outside the context is this line:
> if (!slot || radix_tree_tag_get(&idr->idr_rt, id, IDR_FREE))
> return ERR_PTR(-ENOENT);
>
> Looking up an index which doesn't exist gets you a NULL slot, so you get
> -ENOENT anyway.
>
> I did think about these possibilities when I was writing the code and
> convinced myself I didn't need them. If you have an example of a case
> where I got thast wrong, I'd love to see it.
>
> More generally, the IDR is deprecated; I'm trying to convert users to
> the XArray. If you're adding a new user, can you use the XArray API
> instead?
Thanks for the explanation. I happened to walk by while code inspecting an
existing out-of-tree user and thought there might be a small hole to fill
but I agree it is unlikely that the underflow is likely to be a valid id.
Jordan
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