bochs: Unify header structs and make them QEMU_PACKED

This is an on-disk structure, so offsets must be accurate.

Before this patch, sizeof(bochs) != sizeof(header_v1), which makes the
memcpy() between both invalid. We're lucky enough that the destination
buffer happened to be the larger one, and the memcpy size to be taken
from the smaller one, so we didn't get a buffer overflow in practice.

This patch unifies the both structures, eliminating the need to do a
memcpy in the first place. The common fields are extracted to the top
level of the struct and the actually differing part gets a union of the
two versions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/bochs.c

@@ -38,46 +38,31 @@
 
 // not allocated: 0xffffffff
 
-// always little-endian
-struct bochs_header_v1 {
-    char magic[32]; // "Bochs Virtual HD Image"
-    char type[16]; // "Redolog"
-    char subtype[16]; // "Undoable" / "Volatile" / "Growing"
-    uint32_t version;
-    uint32_t header; // size of header
-
-    union {
-	struct {
-	    uint32_t catalog; // num of entries
-	    uint32_t bitmap; // bitmap size
-	    uint32_t extent; // extent size
-	    uint64_t disk; // disk size
-	    char padding[HEADER_SIZE - 64 - 8 - 20];
-	} redolog;
-	char padding[HEADER_SIZE - 64 - 8];
-    } extra;
-};
-
 // always little-endian
 struct bochs_header {
-    char magic[32]; // "Bochs Virtual HD Image"
-    char type[16]; // "Redolog"
-    char subtype[16]; // "Undoable" / "Volatile" / "Growing"
+    char magic[32];     /* "Bochs Virtual HD Image" */
+    char type[16];      /* "Redolog" */
+    char subtype[16];   /* "Undoable" / "Volatile" / "Growing" */
     uint32_t version;
-    uint32_t header; // size of header
+    uint32_t header;    /* size of header */
+
+    uint32_t catalog;   /* num of entries */
+    uint32_t bitmap;    /* bitmap size */
+    uint32_t extent;    /* extent size */
 
     union {
-	struct {
-	    uint32_t catalog; // num of entries
-	    uint32_t bitmap; // bitmap size
-	    uint32_t extent; // extent size
-	    uint32_t reserved; // for ???
-	    uint64_t disk; // disk size
-	    char padding[HEADER_SIZE - 64 - 8 - 24];
-	} redolog;
-	char padding[HEADER_SIZE - 64 - 8];
+        struct {
+            uint32_t reserved;  /* for ??? */
+            uint64_t disk;      /* disk size */
+            char padding[HEADER_SIZE - 64 - 20 - 12];
+        } QEMU_PACKED redolog;
+        struct {
+            uint64_t disk;      /* disk size */
+            char padding[HEADER_SIZE - 64 - 20 - 8];
+        } QEMU_PACKED redolog_v1;
+        char padding[HEADER_SIZE - 64 - 20];
     } extra;
-};
+} QEMU_PACKED;
 
 typedef struct BDRVBochsState {
     CoMutex lock;
@@ -114,7 +99,6 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
     BDRVBochsState *s = bs->opaque;
     int i;
     struct bochs_header bochs;
-    struct bochs_header_v1 header_v1;
     int ret;
 
     bs->read_only = 1; // no write support yet
@@ -134,13 +118,12 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
     }
 
     if (le32_to_cpu(bochs.version) == HEADER_V1) {
-      memcpy(&header_v1, &bochs, sizeof(bochs));
-      bs->total_sectors = le64_to_cpu(header_v1.extra.redolog.disk) / 512;
+        bs->total_sectors = le64_to_cpu(bochs.extra.redolog_v1.disk) / 512;
     } else {
-      bs->total_sectors = le64_to_cpu(bochs.extra.redolog.disk) / 512;
+        bs->total_sectors = le64_to_cpu(bochs.extra.redolog.disk) / 512;
     }
 
-    s->catalog_size = le32_to_cpu(bochs.extra.redolog.catalog);
+    s->catalog_size = le32_to_cpu(bochs.catalog);
     s->catalog_bitmap = g_malloc(s->catalog_size * 4);
 
     ret = bdrv_pread(bs->file, le32_to_cpu(bochs.header), s->catalog_bitmap,
@@ -154,10 +137,10 @@ static int bochs_open(BlockDriverState *bs, QDict *options, int flags,
 
     s->data_offset = le32_to_cpu(bochs.header) + (s->catalog_size * 4);
 
-    s->bitmap_blocks = 1 + (le32_to_cpu(bochs.extra.redolog.bitmap) - 1) / 512;
-    s->extent_blocks = 1 + (le32_to_cpu(bochs.extra.redolog.extent) - 1) / 512;
+    s->bitmap_blocks = 1 + (le32_to_cpu(bochs.bitmap) - 1) / 512;
+    s->extent_blocks = 1 + (le32_to_cpu(bochs.extent) - 1) / 512;
 
-    s->extent_size = le32_to_cpu(bochs.extra.redolog.extent);
+    s->extent_size = le32_to_cpu(bochs.extent);
 
     qemu_co_mutex_init(&s->lock);
     return 0;