On-disk structures should be marked packed so the compiler does not
insert padding for field alignment. Padding should be explicit so
on-disk layout is obvious and we don't rely on the architecture-specific
ABI for alignment rules.
The pahole(1) diff shows that the padding is now explicit and offsets
are unchanged:
char backing_file[1024]; /* 8 1024 */
/* --- cacheline 16 boundary (1024 bytes) was 8 bytes ago --- */
int32_t mtime; /* 1032 4 */
-
- /* XXX 4 bytes hole, try to pack */
-
+ uint32_t padding; /* 1036 4 */
uint64_t size; /* 1040 8 */
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
block/cow.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/block/cow.c b/block/cow.c
index 6ee4833..5ee9363 100644
--- a/block/cow.c
+++ b/block/cow.c
@@ -37,9 +37,10 @@ struct cow_header_v2 {
uint32_t version;
char backing_file[1024];
int32_t mtime;
+ uint32_t padding;
uint64_t size;
uint32_t sectorsize;
-};
+} QEMU_PACKED;
typedef struct BDRVCowState {
CoMutex lock;
--
1.9.3
