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cow_user.c

/*
 * Copyright (C) 2007 Jeff Dike (jdike@{linux.intel,addtoit}.com)
 * Licensed under the GPL
 */

/*
 * _XOPEN_SOURCE is needed for pread, but we define _GNU_SOURCE, which defines
 * that.
 */
#include <unistd.h>
#include <byteswap.h>
#include <errno.h>
#include <string.h>
#include <arpa/inet.h>
#include <asm/types.h>
#include "cow.h"
#include "cow_sys.h"

#define PATH_LEN_V1 256

typedef __u32 time32_t;

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];
      time32_t mtime;
      __u64 size;
      __s32 sectorsize;
} __attribute__((packed));

/*
 * Changes from V2 -
 *    PATH_LEN_V3 as described above
 *    Explicitly specify field bit lengths for systems with different
 *          lengths for the usual C types.  Not sure whether char or
 *          time_t should be changed, this can be changed later without
 *          breaking compatibility
 *    Add alignment field so that different alignments can be used for the
 *          bitmap and data
 *    Add cow_format field to allow for the possibility of different ways
 *          of specifying the COW blocks.  For now, the only value is 0,
 *          for the traditional COW bitmap.
 *    Move the backing_file field to the end of the header.  This allows
 *          for the possibility of expanding it into the padding required
 *          by the bitmap alignment.
 *    The bitmap and data portions of the file will be aligned as specified
 *          by the alignment field.  This is to allow COW files to be
 *          put on devices with restrictions on access alignments, such as
 *          /dev/raw, with a 512 byte alignment restriction.  This also
 *          allows the data to be more aligned more strictly than on
 *          sector boundaries.  This is needed for ubd-mmap, which needs
 *          the data to be page aligned.
 *    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;
      __u32 mtime;
      __u64 size;
      __u32 sectorsize;
      __u32 alignment;
      __u32 cow_format;
      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

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 */
#define COW_VERSION 3

#define DIV_ROUND(x, len) (((x) + (len) - 1) / (len))
#define ROUND_UP(x, align) DIV_ROUND(x, align) * (align)

void cow_sizes(int version, __u64 size, int sectorsize, int align,
             int bitmap_offset, unsigned long *bitmap_len_out,
             int *data_offset_out)
{
      if (version < 3) {
            *bitmap_len_out = (size + sectorsize - 1) / (8 * sectorsize);

            *data_offset_out = bitmap_offset + *bitmap_len_out;
            *data_offset_out = (*data_offset_out + sectorsize - 1) /
                  sectorsize;
            *data_offset_out *= sectorsize;
      }
      else {
            *bitmap_len_out = DIV_ROUND(size, sectorsize);
            *bitmap_len_out = DIV_ROUND(*bitmap_len_out, 8);

            *data_offset_out = bitmap_offset + *bitmap_len_out;
            *data_offset_out = ROUND_UP(*data_offset_out, align);
      }
}

static int absolutize(char *to, int size, char *from)
{
      char save_cwd[256], *slash;
      int remaining;

      if (getcwd(save_cwd, sizeof(save_cwd)) == NULL) {
            cow_printf("absolutize : unable to get cwd - errno = %d\n",
                     errno);
            return -1;
      }
      slash = strrchr(from, '/');
      if (slash != NULL) {
            *slash = '\0';
            if (chdir(from)) {
                  *slash = '/';
                  cow_printf("absolutize : Can't cd to '%s' - "
                           "errno = %d\n", from, errno);
                  return -1;
            }
            *slash = '/';
            if (getcwd(to, size) == NULL) {
                  cow_printf("absolutize : unable to get cwd of '%s' - "
                         "errno = %d\n", from, errno);
                  return -1;
            }
            remaining = size - strlen(to);
            if (strlen(slash) + 1 > remaining) {
                  cow_printf("absolutize : unable to fit '%s' into %d "
                         "chars\n", from, size);
                  return -1;
            }
            strcat(to, slash);
      }
      else {
            if (strlen(save_cwd) + 1 + strlen(from) + 1 > size) {
                  cow_printf("absolutize : unable to fit '%s' into %d "
                         "chars\n", from, size);
                  return -1;
            }
            strcpy(to, save_cwd);
            strcat(to, "/");
            strcat(to, from);
      }
      chdir(save_cwd);
      return 0;
}

int write_cow_header(char *cow_file, int fd, char *backing_file,
                 int sectorsize, int alignment, unsigned long long *size)
{
      struct cow_header_v3 *header;
      unsigned long modtime;
      int err;

      err = cow_seek_file(fd, 0);
      if (err < 0) {
            cow_printf("write_cow_header - lseek failed, err = %d\n", -err);
            goto out;
      }

      err = -ENOMEM;
      header = cow_malloc(sizeof(*header));
      if (header == NULL) {
            cow_printf("write_cow_header - failed to allocate COW V3 "
                     "header\n");
            goto out;
      }
      header->magic = htonl(COW_MAGIC);
      header->version = htonl(COW_VERSION);

      err = -EINVAL;
      if (strlen(backing_file) > sizeof(header->backing_file) - 1) {
            /* Below, %zd is for a size_t value */
            cow_printf("Backing file name \"%s\" is too long - names are "
                     "limited to %zd characters\n", backing_file,
                     sizeof(header->backing_file) - 1);
            goto out_free;
      }

      if (absolutize(header->backing_file, sizeof(header->backing_file),
                  backing_file))
            goto out_free;

      err = os_file_modtime(header->backing_file, &modtime);
      if (err < 0) {
            cow_printf("write_cow_header - backing file '%s' mtime "
                     "request failed, err = %d\n", header->backing_file,
                     -err);
            goto out_free;
      }

      err = cow_file_size(header->backing_file, size);
      if (err < 0) {
            cow_printf("write_cow_header - couldn't get size of "
                     "backing file '%s', err = %d\n",
                     header->backing_file, -err);
            goto out_free;
      }

      header->mtime = htonl(modtime);
      header->size = htonll(*size);
      header->sectorsize = htonl(sectorsize);
      header->alignment = htonl(alignment);
      header->cow_format = COW_BITMAP;

      err = cow_write_file(fd, header, sizeof(*header));
      if (err != sizeof(*header)) {
            cow_printf("write_cow_header - write of header to "
                     "new COW file '%s' failed, err = %d\n", cow_file,
                     -err);
            goto out_free;
      }
      err = 0;
 out_free:
      cow_free(header);
 out:
      return err;
}

int file_reader(__u64 offset, char *buf, int len, void *arg)
{
      int fd = *((int *) arg);

      return pread(fd, buf, len, offset);
}

/* XXX Need to sanity-check the values read from the header */

int read_cow_header(int (*reader)(__u64, char *, int, void *), void *arg,
                __u32 *version_out, char **backing_file_out,
                time_t *mtime_out, unsigned long long *size_out,
                int *sectorsize_out, __u32 *align_out,
                int *bitmap_offset_out)
{
      union cow_header *header;
      char *file;
      int err, n;
      unsigned long version, magic;

      header = cow_malloc(sizeof(*header));
      if (header == NULL) {
              cow_printf("read_cow_header - Failed to allocate header\n");
            return -ENOMEM;
      }
      err = -EINVAL;
      n = (*reader)(0, (char *) header, sizeof(*header), arg);
      if (n < offsetof(typeof(header->v1), backing_file)) {
            cow_printf("read_cow_header - short header\n");
            goto out;
      }

      magic = header->v1.magic;
      if (magic == COW_MAGIC)
            version = header->v1.version;
      else if (magic == ntohl(COW_MAGIC))
            version = ntohl(header->v1.version);
      /* No error printed because the non-COW case comes through here */
      else goto out;

      *version_out = version;

      if (version == 1) {
            if (n < sizeof(header->v1)) {
                  cow_printf("read_cow_header - failed to read V1 "
                           "header\n");
                  goto out;
            }
            *mtime_out = header->v1.mtime;
            *size_out = header->v1.size;
            *sectorsize_out = header->v1.sectorsize;
            *bitmap_offset_out = sizeof(header->v1);
            *align_out = *sectorsize_out;
            file = header->v1.backing_file;
      }
      else if (version == 2) {
            if (n < sizeof(header->v2)) {
                  cow_printf("read_cow_header - failed to read V2 "
                           "header\n");
                  goto out;
            }
            *mtime_out = ntohl(header->v2.mtime);
            *size_out = ntohll(header->v2.size);
            *sectorsize_out = ntohl(header->v2.sectorsize);
            *bitmap_offset_out = sizeof(header->v2);
            *align_out = *sectorsize_out;
            file = header->v2.backing_file;
      }
      /* 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");
                  goto out;
            }
            *mtime_out = ntohl(header->v3.mtime);
            *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;
      }
      err = -ENOMEM;
      *backing_file_out = cow_strdup(file);
      if (*backing_file_out == NULL) {
            cow_printf("read_cow_header - failed to allocate backing "
                     "file\n");
            goto out;
      }
      err = 0;
 out:
      cow_free(header);
      return err;
}

int init_cow_file(int fd, char *cow_file, char *backing_file, int sectorsize,
              int alignment, int *bitmap_offset_out,
              unsigned long *bitmap_len_out, int *data_offset_out)
{
      unsigned long long size, offset;
      char zero = 0;
      int err;

      err = write_cow_header(cow_file, fd, backing_file, sectorsize,
                         alignment, &size);
      if (err)
            goto out;

      *bitmap_offset_out = ROUND_UP(sizeof(struct cow_header_v3), alignment);
      cow_sizes(COW_VERSION, size, sectorsize, alignment, *bitmap_offset_out,
              bitmap_len_out, data_offset_out);

      offset = *data_offset_out + size - sizeof(zero);
      err = cow_seek_file(fd, offset);
      if (err < 0) {
            cow_printf("cow bitmap lseek failed : err = %d\n", -err);
            goto out;
      }

      /*
       * does not really matter how much we write it is just to set EOF
       * this also sets the entire COW bitmap
       * to zero without having to allocate it
       */
      err = cow_write_file(fd, &zero, sizeof(zero));
      if (err != sizeof(zero)) {
            cow_printf("Write of bitmap to new COW file '%s' failed, "
                     "err = %d\n", cow_file, -err);
            if (err >= 0)
                  err = -EINVAL;
            goto out;
      }

      return 0;
 out:
      return err;
}

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