btree: fix tree corruption in btree_get_prev()

The memory the parameter __key points to is used as an iterator in
btree_get_prev(), so if we save off a bkey() pointer in retry_key and
then assign that to __key, we'll end up corrupting the btree internals
when we do eg

	longcpy(__key, bkey(geo, node, i), geo->keylen);

to return the key value.  What we should do instead is use longcpy() to
copy the key value that retry_key points to __key.

This can cause a btree to get corrupted by seemingly read-only
operations such as btree_for_each_safe.

[akpm@linux-foundation.org: avoid the double longcpy()]
Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: Joern Engel <joern@logfs.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/btree.c b/lib/btree.c
index e5ec1e9c1aa..5cf9e74ec3f 100644
--- a/lib/btree.c
+++ b/lib/btree.c
@@ -319,8 +319,8 @@ void *btree_get_prev(struct btree_head *head, struct btree_geo *geo,
 
         if (head->height == 0)
                 return NULL;
-retry:
         longcpy(key, __key, geo->keylen);
+retry:
         dec_key(geo, key);
 
         node = head->node;
@@ -351,7 +351,7 @@ retry:
         }
 miss:
         if (retry_key) {
-                __key = retry_key;
+                longcpy(key, retry_key, geo->keylen);
                 retry_key = NULL;
                 goto retry;
         }