On Fri, Jul 06, 2018 at 02:41:44PM +0100, Mark Rutland wrote: > When idr_alloc() is called for the first time on an IDR (which has no > nodes in its radix tree), we end up with calculate_count() calling > get_slot_offset() with a NULL node, leading to a NULL pointer > dereference caught by UBSAN:
Thanks! > The result of the load is passed into node_tag_get(), which ignores the > value when node is NULL. Typically, the compiler inlines both > get_slot_offset() and node_tag_get() into calculate_count(), optimizing > away the NULL-pointer dereference, and hence this doesn't typically > result in a boot failure. > > We can't rely on the compiler always doing this, and must avoid > dereferencing fields from node when it is potentially NULL. > > To do so, this patch folds the generation of offset into tag_get(), such > that this only happens when node is not NULL. Callers are updated to > pass the relevant slot, rather than the offset derived from it. I did think about this ... honest! My reasoning clearly glitched at some point. I think this is the right way to fix the problem. Acked-by: Matthew Wilcox <wi...@infradead.org>