btrfs-progs: find-root: fix a bug that will cause wrong result

[BUG]
btrfs-find-root may not output desire result, as due to
search_extent_cache() may return a result that doesn't cover the desired
range, generation cache can be screwed up if higher generation tree root
is found before lower generation tree root.

For example:
=======
./btrfs-find-root  /dev/sda6  -a
Superblock thinks the generation is 8
Superblock thinks the level is 0
adding bytenr: 4194304, gen: 8	<<< Debug output
adding bytenr: 24715264, gen: 7 <<< gen is 7 at read_tree_block time
Well block 4194304(gen: 8 level: 0) seems good, and it matches
superblock
Well block 24715264(gen: 8 level: 0) seems good, and it matches
superblock  <<< But its gen is wrong at result output time
=======

[Fix]
Add a new check to make sure the search_extent_cache() is returning the
desired result.

Reported-by: Marc Merlin <marc@merlins.org>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/find-root.c b/find-root.c
index 1af37b5..55e7942 100644 (file)
--- a/find-root.c
+++ b/find-root.c
@@ -46,9 +46,14 @@ static int add_eb_to_result(struct extent_buffer *eb,
            generation < filter->generation)
                return ret;
 
-       /* Get the generation cache or create one */
+       /*
+        * Get the generation cache or create one
+        *
+        * NOTE: search_cache_extent() may return cache that doesn't cover
+        * the range. So we need an extra check to make sure it's the right one.
+        */
        cache = search_cache_extent(result, generation);
-       if (!cache) {
+       if (!cache || cache->start != generation) {
                gen_cache = malloc(sizeof(*gen_cache));
                BUG_ON(!gen_cache);
                cache = &gen_cache->cache;