From: Paolo 'Blaisorblade' Giarrusso <[email protected]>
*) Correct the layout of all header versions - make all them well-specified for
any external event. As we don't have 1-byte or 2-byte wide fields, the 32-bit
layout (historical one) has no extra padding, so we can safely add
__attribute__((packed)).
*) Add detection and reading of the broken 64-bit COW format which has been
around for a while - to allow safe migration to the correct 32-bit format. Safe
detection is possible, thanks to some luck with the existing format, and it
works in practice.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <[email protected]>
---
arch/um/drivers/cow_user.c | 91 +++++++++++++++++++++++++++++++++++++-------
1 files changed, 76 insertions(+), 15 deletions(-)
diff --git a/arch/um/drivers/cow_user.c b/arch/um/drivers/cow_user.c
index afdf1ea..a9afccf 100644
--- a/arch/um/drivers/cow_user.c
+++ b/arch/um/drivers/cow_user.c
@@ -17,30 +17,34 @@
#define PATH_LEN_V1 256
+typedef __u32 time32_t;
+
struct cow_header_v1 {
- int magic;
- int version;
+ __s32 magic;
+ __s32 version;
char backing_file[PATH_LEN_V1];
- time_t mtime;
+ time32_t mtime;
__u64 size;
- int sectorsize;
-};
+ __s32 sectorsize;
+} __attribute__((packed));
-#define PATH_LEN_V2 MAXPATHLEN
+/* Define PATH_LEN_V3 as the usual value of MAXPATHLEN, just hard-code it in
+ * case other systems have different values for MAXPATHLEN.
+ *
+ * The same must hold for V2 - we want file format compatibility, not anything
+ * else.
+ */
+#define PATH_LEN_V3 4096
+#define PATH_LEN_V2 PATH_LEN_V3
struct cow_header_v2 {
__u32 magic;
__u32 version;
char backing_file[PATH_LEN_V2];
- time_t mtime;
+ time32_t mtime;
__u64 size;
- int sectorsize;
-};
-
-/* Define PATH_LEN_V3 as the usual value of MAXPATHLEN, just hard-code it in
- * case other systems have different values for MAXPATHLEN
- */
-#define PATH_LEN_V3 4096
+ __s32 sectorsize;
+} __attribute__((packed));
/* Changes from V2 -
* PATH_LEN_V3 as described above
@@ -66,6 +70,15 @@ struct cow_header_v2 {
* Fixed (finally!) the rounding bug
*/
+/* Until Dec2005, __attribute__((packed)) was left out from the below
+ * definition, leading on 64-bit systems to 4 bytes of padding after mtime, to
+ * align size to 8-byte alignment. This shifted all fields above (no padding
+ * was present on 32-bit, no other padding was added).
+ *
+ * However, this _can be detected_: it means that cow_format (always 0 until
+ * now) is shifted onto the first 4 bytes of backing_file, where it is otherwise
+ * impossible to find 4 zeros. -bb */
+
struct cow_header_v3 {
__u32 magic;
__u32 version;
@@ -77,6 +90,18 @@ struct cow_header_v3 {
char backing_file[PATH_LEN_V3];
} __attribute__((packed));
+/* This is the broken layout used by some 64-bit binaries. */
+struct cow_header_v3_broken {
+ __u32 magic;
+ __u32 version;
+ __s64 mtime;
+ __u64 size;
+ __u32 sectorsize;
+ __u32 alignment;
+ __u32 cow_format;
+ char backing_file[PATH_LEN_V3];
+};
+
/* COW format definitions - for now, we have only the usual COW bitmap */
#define COW_BITMAP 0
@@ -84,6 +109,7 @@ union cow_header {
struct cow_header_v1 v1;
struct cow_header_v2 v2;
struct cow_header_v3 v3;
+ struct cow_header_v3_broken v3_b;
};
#define COW_MAGIC 0x4f4f4f4d /* MOOO */
@@ -300,7 +326,8 @@ int read_cow_header(int (*reader)(__u64,
*align_out = *sectorsize_out;
file = header->v2.backing_file;
}
- else if(version == 3){
+ /* This is very subtle - see above at union cow_header definition */
+ else if(version == 3 && (*((int*)header->v3.backing_file) != 0)){
if(n < sizeof(header->v3)){
cow_printf("read_cow_header - failed to read V3 "
"header\n");
@@ -310,9 +337,43 @@ int read_cow_header(int (*reader)(__u64,
*size_out = ntohll(header->v3.size);
*sectorsize_out = ntohl(header->v3.sectorsize);
*align_out = ntohl(header->v3.alignment);
+ if (*align_out == 0) {
+ cow_printf("read_cow_header - invalid COW header, "
+ "align == 0\n");
+ }
*bitmap_offset_out = ROUND_UP(sizeof(header->v3), *align_out);
file = header->v3.backing_file;
}
+ else if(version == 3){
+ cow_printf("read_cow_header - broken V3 file with"
+ " 64-bit layout - recovering content.\n");
+
+ if(n < sizeof(header->v3_b)){
+ cow_printf("read_cow_header - failed to read V3 "
+ "header\n");
+ goto out;
+ }
+
+ /* this was used until Dec2005 - 64bits are needed to represent
+ * 2038+. I.e. we can safely do this truncating cast.
+ *
+ * Additionally, we must use ntohl() instead of ntohll(), since
+ * the program used to use the former (tested - I got mtime
+ * mismatch "0 vs whatever").
+ *
+ * Ever heard about bug-to-bug-compatibility ? ;-) */
+ *mtime_out = (time32_t) ntohl(header->v3_b.mtime);
+
+ *size_out = ntohll(header->v3_b.size);
+ *sectorsize_out = ntohl(header->v3_b.sectorsize);
+ *align_out = ntohl(header->v3_b.alignment);
+ if (*align_out == 0) {
+ cow_printf("read_cow_header - invalid COW header, "
+ "align == 0\n");
+ }
+ *bitmap_offset_out = ROUND_UP(sizeof(header->v3_b), *align_out);
+ file = header->v3_b.backing_file;
+ }
else {
cow_printf("read_cow_header - invalid COW version\n");
goto out;
-
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