VFAT i_pos value

[OGAWA Hirofumi]

OGAWA Hirofumi <hirofumi at mail.parknet.co.jp> writes:

> Kai Meyer <kai at gnukai.com> writes:
>
>> 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.
-- 
OGAWA Hirofumi <hirofumi at mail.parknet.co.jp>