block/cow.c

   1 /*
   2  * Block driver for the COW format
   3  *
   4  * Copyright (c) 2004 Fabrice Bellard
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24 #include "qemu-common.h"
  25 #include "block_int.h"
  26 #include "module.h"
  27
  28 /**************************************************************/
  29 /* COW block driver using file system holes */
  30
  31 /* user mode linux compatible COW file */
  32 #define COW_MAGIC 0x4f4f4f4d  /* MOOO */
  33 #define COW_VERSION 2
  34
  35 struct cow_header_v2 {
  36     uint32_t magic;
  37     uint32_t version;
  38     char backing_file[1024];
  39     int32_t mtime;
  40     uint64_t size;
  41     uint32_t sectorsize;
  42 };
  43
  44 typedef struct BDRVCowState {
  45     int64_t cow_sectors_offset;
  46 } BDRVCowState;
  47
  48 static int cow_probe(const uint8_t *buf, int buf_size, const char *filename)
  49 {
  50     const struct cow_header_v2 *cow_header = (const void *)buf;
  51
  52     if (buf_size >= sizeof(struct cow_header_v2) &&
  53         be32_to_cpu(cow_header->magic) == COW_MAGIC &&
  54         be32_to_cpu(cow_header->version) == COW_VERSION)
  55         return 100;
  56     else
  57         return 0;
  58 }
  59
  60 static int cow_open(BlockDriverState *bs, int flags)
  61 {
  62     BDRVCowState *s = bs->opaque;
  63     struct cow_header_v2 cow_header;
  64     int bitmap_size;
  65     int64_t size;
  66
  67     /* see if it is a cow image */
  68     if (bdrv_pread(bs->file, 0, &cow_header, sizeof(cow_header)) !=
  69             sizeof(cow_header)) {
  70         goto fail;
  71     }
  72
  73     if (be32_to_cpu(cow_header.magic) != COW_MAGIC ||
  74         be32_to_cpu(cow_header.version) != COW_VERSION) {
  75         goto fail;
  76     }
  77
  78     /* cow image found */
  79     size = be64_to_cpu(cow_header.size);
  80     bs->total_sectors = size / 512;
  81
  82     pstrcpy(bs->backing_file, sizeof(bs->backing_file),
  83             cow_header.backing_file);
  84
  85     bitmap_size = ((bs->total_sectors + 7) >> 3) + sizeof(cow_header);
  86     s->cow_sectors_offset = (bitmap_size + 511) & ~511;
  87     return 0;
  88  fail:
  89     return -1;
  90 }
  91
  92 /*
  93  * XXX(hch): right now these functions are extremly ineffcient.
  94  * We should just read the whole bitmap we'll need in one go instead.
  95  */
  96 static inline int cow_set_bit(BlockDriverState *bs, int64_t bitnum)
  97 {
  98     uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
  99     uint8_t bitmap;
 100
 101     if (bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap)) !=
 102             sizeof(bitmap)) {
 103        return -errno;
 104     }
 105
 106     bitmap |= (1 << (bitnum % 8));
 107
 108     if (bdrv_pwrite(bs->file, offset, &bitmap, sizeof(bitmap)) !=
 109             sizeof(bitmap)) {
 110        return -errno;
 111     }
 112     return 0;
 113 }
 114
 115 static inline int is_bit_set(BlockDriverState *bs, int64_t bitnum)
 116 {
 117     uint64_t offset = sizeof(struct cow_header_v2) + bitnum / 8;
 118     uint8_t bitmap;
 119
 120     if (bdrv_pread(bs->file, offset, &bitmap, sizeof(bitmap)) !=
 121             sizeof(bitmap)) {
 122        return -errno;
 123     }
 124
 125     return !!(bitmap & (1 << (bitnum % 8)));
 126 }
 127
 128 /* Return true if first block has been changed (ie. current version is
 129  * in COW file).  Set the number of continuous blocks for which that
 130  * is true. */
 131 static int cow_is_allocated(BlockDriverState *bs, int64_t sector_num,
 132         int nb_sectors, int *num_same)
 133 {
 134     int changed;
 135
 136     if (nb_sectors == 0) {
 137         *num_same = nb_sectors;
 138         return 0;
 139     }
 140
 141     changed = is_bit_set(bs, sector_num);
 142     if (changed < 0) {
 143         return 0; /* XXX: how to return I/O errors? */
 144     }
 145
 146     for (*num_same = 1; *num_same < nb_sectors; (*num_same)++) {
 147         if (is_bit_set(bs, sector_num + *num_same) != changed)
 148             break;
 149     }
 150
 151     return changed;
 152 }
 153
 154 static int cow_update_bitmap(BlockDriverState *bs, int64_t sector_num,
 155         int nb_sectors)
 156 {
 157     int error = 0;
 158     int i;
 159
 160     for (i = 0; i < nb_sectors; i++) {
 161         error = cow_set_bit(bs, sector_num + i);
 162         if (error) {
 163             break;
 164         }
 165     }
 166
 167     return error;
 168 }
 169
 170 static int cow_read(BlockDriverState *bs, int64_t sector_num,
 171                     uint8_t *buf, int nb_sectors)
 172 {
 173     BDRVCowState *s = bs->opaque;
 174     int ret, n;
 175
 176     while (nb_sectors > 0) {
 177         if (cow_is_allocated(bs, sector_num, nb_sectors, &n)) {
 178             ret = bdrv_pread(bs->file,
 179                         s->cow_sectors_offset + sector_num * 512,
 180                         buf, n * 512);
 181             if (ret != n * 512)
 182                 return -1;
 183         } else {
 184             if (bs->backing_hd) {
 185                 /* read from the base image */
 186                 ret = bdrv_read(bs->backing_hd, sector_num, buf, n);
 187                 if (ret < 0)
 188                     return -1;
 189             } else {
 190             memset(buf, 0, n * 512);
 191         }
 192         }
 193         nb_sectors -= n;
 194         sector_num += n;
 195         buf += n * 512;
 196     }
 197     return 0;
 198 }
 199
 200 static int cow_write(BlockDriverState *bs, int64_t sector_num,
 201                      const uint8_t *buf, int nb_sectors)
 202 {
 203     BDRVCowState *s = bs->opaque;
 204     int ret;
 205
 206     ret = bdrv_pwrite(bs->file, s->cow_sectors_offset + sector_num * 512,
 207                       buf, nb_sectors * 512);
 208     if (ret != nb_sectors * 512)
 209         return -1;
 210
 211     return cow_update_bitmap(bs, sector_num, nb_sectors);
 212 }
 213
 214 static void cow_close(BlockDriverState *bs)
 215 {
 216 }
 217
 218 static int cow_create(const char *filename, QEMUOptionParameter *options)
 219 {
 220     int fd, cow_fd;
 221     struct cow_header_v2 cow_header;
 222     struct stat st;
 223     int64_t image_sectors = 0;
 224     const char *image_filename = NULL;
 225     int ret;
 226
 227     /* Read out options */
 228     while (options && options->name) {
 229         if (!strcmp(options->name, BLOCK_OPT_SIZE)) {
 230             image_sectors = options->value.n / 512;
 231         } else if (!strcmp(options->name, BLOCK_OPT_BACKING_FILE)) {
 232             image_filename = options->value.s;
 233         }
 234         options++;
 235     }
 236
 237     cow_fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY,
 238               0644);
 239     if (cow_fd < 0)
 240         return -errno;
 241     memset(&cow_header, 0, sizeof(cow_header));
 242     cow_header.magic = cpu_to_be32(COW_MAGIC);
 243     cow_header.version = cpu_to_be32(COW_VERSION);
 244     if (image_filename) {
 245         /* Note: if no file, we put a dummy mtime */
 246         cow_header.mtime = cpu_to_be32(0);
 247
 248         fd = open(image_filename, O_RDONLY | O_BINARY);
 249         if (fd < 0) {
 250             close(cow_fd);
 251             goto mtime_fail;
 252         }
 253         if (fstat(fd, &st) != 0) {
 254             close(fd);
 255             goto mtime_fail;
 256         }
 257         close(fd);
 258         cow_header.mtime = cpu_to_be32(st.st_mtime);
 259     mtime_fail:
 260         pstrcpy(cow_header.backing_file, sizeof(cow_header.backing_file),
 261                 image_filename);
 262     }
 263     cow_header.sectorsize = cpu_to_be32(512);
 264     cow_header.size = cpu_to_be64(image_sectors * 512);
 265     ret = qemu_write_full(cow_fd, &cow_header, sizeof(cow_header));
 266     if (ret != sizeof(cow_header)) {
 267         ret = -errno;
 268         goto exit;
 269     }
 270
 271     /* resize to include at least all the bitmap */
 272     ret = ftruncate(cow_fd, sizeof(cow_header) + ((image_sectors + 7) >> 3));
 273     if (ret) {
 274         ret = -errno;
 275         goto exit;
 276     }
 277
 278 exit:
 279     close(cow_fd);
 280     return ret;
 281 }
 282
 283 static void cow_flush(BlockDriverState *bs)
 284 {
 285     bdrv_flush(bs->file);
 286 }
 287
 288 static QEMUOptionParameter cow_create_options[] = {
 289     {
 290         .name = BLOCK_OPT_SIZE,
 291         .type = OPT_SIZE,
 292         .help = "Virtual disk size"
 293     },
 294     {
 295         .name = BLOCK_OPT_BACKING_FILE,
 296         .type = OPT_STRING,
 297         .help = "File name of a base image"
 298     },
 299     { NULL }
 300 };
 301
 302 static BlockDriver bdrv_cow = {
 303     .format_name        = "cow",
 304     .instance_size      = sizeof(BDRVCowState),
 305     .bdrv_probe         = cow_probe,
 306     .bdrv_open          = cow_open,
 307     .bdrv_read          = cow_read,
 308     .bdrv_write         = cow_write,
 309     .bdrv_close         = cow_close,
 310     .bdrv_create        = cow_create,
 311     .bdrv_flush         = cow_flush,
 312     .bdrv_is_allocated  = cow_is_allocated,
 313
 314     .create_options = cow_create_options,
 315 };
 316
 317 static void bdrv_cow_init(void)
 318 {
 319     bdrv_register(&bdrv_cow);
 320 }
 321
 322 block_init(bdrv_cow_init);