Btrfs: teach backref walking about backrefs with underflowed offset values

[cascardo/linux.git] / fs / btrfs / backref.c

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index 802fabb..a0ca575 100644 (file)
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -206,10 +206,33 @@ static int __add_prelim_ref(struct list_head *head, u64 root_id,
                return -ENOMEM;
 
        ref->root_id = root_id;
-       if (key)
+       if (key) {
                ref->key_for_search = *key;
-       else
+               /*
+                * We can often find data backrefs with an offset that is too
+                * large (>= LLONG_MAX, maximum allowed file offset) due to
+                * underflows when subtracting a file's offset with the data
+                * offset of its corresponding extent data item. This can
+                * happen for example in the clone ioctl.
+                * So if we detect such case we set the search key's offset to
+                * zero to make sure we will find the matching file extent item
+                * at add_all_parents(), otherwise we will miss it because the
+                * offset taken form the backref is much larger then the offset
+                * of the file extent item. This can make us scan a very large
+                * number of file extent items, but at least it will not make
+                * us miss any.
+                * This is an ugly workaround for a behaviour that should have
+                * never existed, but it does and a fix for the clone ioctl
+                * would touch a lot of places, cause backwards incompatibility
+                * and would not fix the problem for extents cloned with older
+                * kernels.
+                */
+               if (ref->key_for_search.type == BTRFS_EXTENT_DATA_KEY &&
+                   ref->key_for_search.offset >= LLONG_MAX)
+                       ref->key_for_search.offset = 0;
+       } else {
                memset(&ref->key_for_search, 0, sizeof(ref->key_for_search));
+       }
 
        ref->inode_list = NULL;
        ref->level = level;