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[PATCH] uml: safe migration path to the correct V3 COW format 

- 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 <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Paolo 'Blaisorblade' Giarrusso 2006-04-10 22:53:30 -07:00 committed by Linus Torvalds
parent cda402b283
commit f2ea394082
1 changed files with 79 additions and 18 deletions
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97
arch/um/drivers/cow_user.c
View File

@ -17,30 +17,34 @@
#define PATH_LEN_V1 256
struct cow_header_v1 {
int magic;
int version;
char backing_file[PATH_LEN_V1];
time_t mtime;
__u64 size;
int sectorsize;
};
typedef __u32 time32_t;
#define PATH_LEN_V2 MAXPATHLEN
struct cow_header_v1 {
__s32 magic;
__s32 version;
char backing_file[PATH_LEN_V1];
time32_t mtime;
__u64 size;
__s32 sectorsize;
} __attribute__((packed));
/* 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, char *, int, void *), void *arg,
*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, char *, int, void *), void *arg,
*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;