There are four cases I can see where we could end up with a NULL 'slot' in
radix_tree_next_slot(). This unit test exercises all four of them, making
sure that if in the future we have an unsafe path through
radix_tree_next_slot(), we'll catch it.
Here are details on the four cases:
1) radix_tree_iter_retry() via a non-tagged iteration like
radix_tree_for_each_slot(). In this case we currently aren't seeing a bug
because radix_tree_iter_retry() sets
iter->next_index = iter->index;
which means that in in the else case in radix_tree_next_slot(), 'count' is
zero, so we skip over the while() loop and effectively just return NULL
without ever dereferencing 'slot'.
2) radix_tree_iter_retry() via tagged iteration like
radix_tree_for_each_tagged(). This case was giving us NULL pointer
dereferences in testing, and was fixed with this commit:
commit 3cb9185c67 ("radix-tree: fix radix_tree_iter_retry() for tagged
iterators.")
This fix doesn't explicitly check for 'slot' being NULL, though, it works
around the NULL pointer dereference by instead zeroing iter->tags in
radix_tree_iter_retry(), which makes us bail out of the if() case in
radix_tree_next_slot() before we dereference 'slot'.
3) radix_tree_iter_next() via via a non-tagged iteration like
radix_tree_for_each_slot(). This currently happens in shmem_tag_pins()
and shmem_partial_swap_usage().
As with non-tagged iteration, 'count' in the else case of
radix_tree_next_slot() is zero, so we skip over the while() loop and
effectively just return NULL without ever dereferencing 'slot'.
4) radix_tree_iter_next() via tagged iteration like
radix_tree_for_each_tagged(). This happens in shmem_wait_for_pins().
radix_tree_iter_next() zeros out iter->tags, so we end up exiting
radix_tree_next_slot() here:
if (flags & RADIX_TREE_ITER_TAGGED) {
void *canon = slot;
iter->tags >>= 1;
if (unlikely(!iter->tags))
return NULL;
Link: http://lkml.kernel.org/r/20160815194237.25967-3-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Ross Zwisler