Commit f2ea3940 authored by Paolo 'Blaisorblade' Giarrusso's avatar Paolo 'Blaisorblade' Giarrusso Committed by Linus Torvalds
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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: default avatarPaolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent cda402b2

arch/um/drivers/cow_user.c

+76 −15
Original line number Diff line number Diff line
@@ -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, 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;

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