From: Paolo 'Blaisorblade' Giarrusso Date: Tue, 11 Apr 2006 05:53:30 +0000 (-0700) Subject: [PATCH] uml: safe migration path to the correct V3 COW format X-Git-Tag: v2.6.17-rc2~240 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=f2ea394082c5d1682e6a131c5981085b8752c6e9;p=linux-2.6 [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 Cc: Jeff Dike Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/arch/um/drivers/cow_user.c b/arch/um/drivers/cow_user.c index afdf1ea0f5..a9afccf68e 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, 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;