[PATCH v2] Btrfs: qgroup: add BUILD_BUG to report pointer cast breakage

[Daniel Dressler <danieru.dressler@gmail.com>]

Our ulist data structure stores at max 64bit
values. qgroup has used this structure to store
pointers. In the future when we upgrade to 128bit
this casting of pointers to uint64_t will break.

This patch adds a BUILD_BUG ensuring that this
code will not be left untouched in the upgrade.

It also marks this issue on the TODO list so it
may be addressed before such an upgrade.

Signed-off-by: Daniel Dressler <danieru.dressler@gmail.com>
---
 fs/btrfs/qgroup.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 48b60db..a9a4cab 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -44,6 +44,7 @@
  *  - caches fuer ulists
  *  - performance benchmarks
  *  - check all ioctl parameters
+ *  - do not cast uintptr_t to uint64_t in ulist usage
  */
 
 /*
@@ -99,8 +100,12 @@ struct btrfs_qgroup_list {
 	struct btrfs_qgroup *member;
 };
 
-#define ptr_to_u64(x) ((u64)(uintptr_t)x)
-#define u64_to_ptr(x) ((struct btrfs_qgroup *)(uintptr_t)x)
+#define ptr_to_u64(x) \
+	(BUILD_BUG_ON_ZERO(sizeof(uintptr_t) > sizeof(u64)) + \
+	((u64)(uintptr_t)x))
+#define u64_to_ptr(x) \
+	(BUILD_BUG_ON_ZERO(sizeof(uintptr_t) > sizeof(u64)) + \
+	((struct btrfs_qgroup *)(uintptr_t)x))
 
 static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
-- 
2.1.0