btrfs-progs: Fix bus error on sparc

Currently, as of 8cae1840afb3ea44dcc298f32983e577480dfee4 when running
btrfs-convert I get a bus error.

The problem is that struct btrfs_key has __attribute__ ((__packed__))
so it is not aligned. Then, a pointer to it's objectid field is taken,
cast to a  void*, then eventually cast back to a u64* and
dereferenced. The problem is that the dereferenced u64* is not
necessarily aligned (ie, not necessarily a valid u64*), resulting in
undefined behavior.

This patch adds a local u64 variable which would of course be properly
aligned and then uses a pointer to that.

I did not modify the call from btrfs_fs_roots_compare_roots as that
uses struct btrfs_root which is a regular struct and would thus have
it's members correctly aligned to begin with.

After patching this I realized Liu Bo had already written a similar
patch, but I think mine is cleaner, so I'm sending it anyway.

Signed-off-by: Ivan Jager <aij+@mrph.org>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/disk-io.c b/disk-io.c
index e4550bd4..e840177f 100644
--- a/disk-io.c
+++ b/disk-io.c
@@ -683,6 +683,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 	struct btrfs_root *root;
 	struct rb_node *node;
 	int ret;
+	u64 objectid = location->objectid;
 
 	if (location->objectid == BTRFS_ROOT_TREE_OBJECTID)
 		return fs_info->tree_root;
@@ -698,7 +699,7 @@ struct btrfs_root *btrfs_read_fs_root(struct btrfs_fs_info *fs_info,
 	BUG_ON(location->objectid == BTRFS_TREE_RELOC_OBJECTID ||
 	       location->offset != (u64)-1);
 
-	node = rb_search(&fs_info->fs_root_tree, (void *)&location->objectid,
+	node = rb_search(&fs_info->fs_root_tree, (void *)&objectid,
 			 btrfs_fs_roots_compare_objectids, NULL);
 	if (node)
 		return container_of(node, struct btrfs_root, rb_node);