btrfs-progs: check: do not dereference tree_refs as data_refs

In a filesystem corrupted by a faulty memory module, btrfsck would get
very confused attempting to access backrefs that weren't data backrefs
as if they were.  Besides invoking undefined behavior for accessing
potentially-uninitialized data past the end of objects, or with
dynamic types unrelated with the static types held in the
corresponding memory, it used offsets and lengths from such fields
that did not correspond to anything in the filesystem proper.

Moving the test for full backrefs and checking that they're data
backrefs earlier avoided the crash I was running into, but that was
not enough to make the filesystem complete a successful repair.

Signed-off-by: Alexandre Oliva <oliva@gnu.org>
Signed-off-by: David Sterba <dsterba@suse.cz>

diff --git a/cmds-check.c b/cmds-check.c
index 7a86077..f6fde23 100644 (file)
--- a/cmds-check.c
+++ b/cmds-check.c
@@ -4911,15 +4911,17 @@ static int verify_backrefs(struct btrfs_trans_handle *trans,
                return 0;
 
        list_for_each_entry(back, &rec->backrefs, list) {
+               if (back->full_backref || !back->is_data)
+                       continue;
+
                dback = (struct data_backref *)back;
+
                /*
                 * We only pay attention to backrefs that we found a real
                 * backref for.
                 */
                if (dback->found_ref == 0)
                        continue;
-               if (back->full_backref)
-                       continue;
 
                /*
                 * For now we only catch when the bytes don't match, not the
@@ -5035,6 +5037,9 @@ static int verify_backrefs(struct btrfs_trans_handle *trans,
         * references and fix up the ones that don't match.
         */
        list_for_each_entry(back, &rec->backrefs, list) {
+               if (back->full_backref || !back->is_data)
+                       continue;
+
                dback = (struct data_backref *)back;
 
                /*
@@ -5043,8 +5048,6 @@ static int verify_backrefs(struct btrfs_trans_handle *trans,
                 */
                if (dback->found_ref == 0)
                        continue;
-               if (back->full_backref)
-                       continue;
 
                if (dback->bytes == best->bytes &&
                    dback->disk_bytenr == best->bytenr)
@@ -5264,14 +5267,16 @@ static int find_possible_backrefs(struct btrfs_trans_handle *trans,
        int ret;
 
        list_for_each_entry(back, &rec->backrefs, list) {
+               /* Don't care about full backrefs (poor unloved backrefs) */
+               if (back->full_backref || !back->is_data)
+                       continue;
+
                dback = (struct data_backref *)back;
 
                /* We found this one, we don't need to do a lookup */
                if (dback->found_ref)
                        continue;
-               /* Don't care about full backrefs (poor unloved backrefs) */
-               if (back->full_backref)
-                       continue;
+
                key.objectid = dback->root;
                key.type = BTRFS_ROOT_ITEM_KEY;
                key.offset = (u64)-1;