On 12/01/2011 12:20 PM, OGAWA Hirofumi wrote:
Kai Meyer<kai@gnukai.com> writes:
The i_pos means directory entry (contains inode information in unix-fs) position,
block number == i_pos / (logical-blocksize / 32) offset == i_pos& (logical-blocksize / 32)
the above position's directory entry contains information for problematic file. This is how to use i_pos information.
FWIW, in this error case, the cluster chain in FAT table which is pointed by that entry, it has invalid cluster value.
Thanks. If you would verify my math for me, I would appreciate it.
In this case, my logical block size is 4096, because byte 13 of the 8Gb file system is 8, and I take that to be 8 * 512, which is 4096. So:
block_number = 523791 / (4096 / 32) = 4092 offset = 523791 % (4096 / 32) = 15 // I assume you meant modulo in your original post, and not binary AND. Whoops, you are right. (I forgot "-1")
So if the block_number is 4092, I would multiply that by 8 (sectors per logical block) to get the sector number: 32736 Right.
Does the error indicate that sector contains the corrupted data? No.
Or is it the sector that contains the information that points to the corrupted data? Right.
The i_pos is pointing a directory entry (include/linux/msdos_fs.h: struct msdos_dir_entry).
And starthi (if FAT32) and start contain the pointer to next cluster number. That message was outputted when walking in cluster chain.
If you want to see actual corrupted data, you can check the cluster chain by pointing from that directory entry.
Thanks.
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. Thanks