On 12/01/2011 07:38 AM, OGAWA Hirofumi wrote:

Kai Meyer<kai@gnukai.com> writes:

...

I'm getting this error: FAT: Filesystem error (dev sblsnap0) fat_get_cluster: invalid cluster chain (i_pos 523791)

I'm wondering if there was a way to figure out what sector is causing the error? I would like to try and track down what is changing that sector and fix the problem. Is there a straight forward way to convert i_pos to a sector value? I've been staring at the fat.c and fat.h code all morning, and I'm having trouble grok'ing the flow. 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. So if the block_number is 4092, I would multiply that by 8 (sectors per logical block) to get the sector number: 32736 Does the error indicate that sector contains the corrupted data? Or is it the sector that contains the information that points to the corrupted data? Or is it something entirely different? -Kai Meyer

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

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

Kai Meyer <kai@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@mail.parknet.co.jp>

Kai Meyer <kai@gnukai.com> writes:

[root@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

Looks like sane entry.

[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?

The next pos means the position of data cluster (i.e that entry is using the cluster of this position to store data). You have to walk the cluster position chain on FAT table, not directory entry. -- OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>