On 12/02/2011 11:23 PM, OGAWA Hirofumi wrote:

OGAWA Hirofumi[off-list ref]  writes:

quoted

Kai Meyer[off-list ref]  writes:

quoted

Thanks for the helpful response. I'm not entirely sure I understand the
next part though. I hacked a dirty entry dumper tool:

#include<stdio.h>
#include<linux/msdos_fs.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<unistd.h>
#include<string.h>

int main(int argc, char** argv)
{
          off_t pos = atoi(argv[2]);
          unsigned long block;
          off_t sector;
          unsigned int offset;
          int fd = open(argv[1], O_RDONLY);
          char buf[512];
          struct msdos_dir_entry dirent;
          block = pos / (4096 / 32);
          sector = block * 8;
          offset = pos % (4096 / 32);
          printf("block %lu, sector %lu, offset %u\n", block, sector,
offset);
          lseek(fd, sector * 512, SEEK_SET);
          if (read(fd, buf, 512)<  0) {
                  fprintf(stderr, "Unable to read from device %s\n",
argv[1]);
                  return -1;
          }
          memcpy(&dirent, buf + offset, sizeof(dirent));
          printf("name      %s\n", dirent.name);
          printf("attr      %u\n", dirent.attr);
          printf("lcase     %u\n", dirent.lcase);
          printf("ctime_cs  %u\n", dirent.ctime_cs);
          printf("ctime     %u\n", dirent.ctime);
          printf("cdate     %u\n", dirent.cdate);
          printf("adate     %u\n", dirent.adate);
          printf("starthi   %u\n", dirent.starthi);
          printf("time      %u\n", dirent.time);
          printf("date      %u\n", dirent.date);
          printf("start     %u\n", dirent.start);
          printf("size      %u\n", dirent.size);
}

Here's what it outputs:

./vfat_entry /dev/sblsnap0 523793
block 4092, sector 32736, offset 17
name
attr      255
lcase     255
ctime_cs  255
ctime     12799
cdate     12670
adate     8224
starthi   8224
time      23072
date      21061
start     32
size      2171155456

So, I take starthi, and shift 16 bits left, then and in the start value.
That should give me the byte address of the first cluster of the file,
correct?

Then I need to follow the cluster chain until I get a bad value.

It looks like wrong as dirent. Did you use 523793 really? If so, I think
523791 is correct value. :)

And I didn't mention about offset correctly. offset means number of
entries, not bytes offset. So, bytes offset is "buf + offset * 32".
(32 == sizeof(struct msdos_dir_entry))

Thanks.

Ok, I fixed the buf + offset * 32. I have a new volume, so the error is now:
fat_get_cluster: invalid cluster chain (i_pos 523781)

I added a few lines at the end to print the start value:
         pos = dirent.starthi << 16;
         pos |= dirent.start;
         printf("next pos: %u\n", sector);

[root at dev1 sblsnap]# ./vfat_entry /dev/sblsnap0 523781
block 4092, sector 32736, offset 5
name      3~1     ZER
attr      32
lcase     0
ctime_cs  100
ctime     29092
cdate     16264
adate     16264
starthi   4
time      29092
date      16264
start     7427
size      37748736
next pos: 32736
[root@dev1 sblsnap]# ./vfat_entry /dev/sblsnap0 32736
block 255, sector 2040, offset 96
name
attr      0
lcase     0
ctime_cs  0
ctime     0
cdate     0
adate     0
starthi   0
time      0
date      0
start     0
size      0
next pos: 2040

Does that look like what would be causing my error? meaning, sector 2040 
has bad data?