Re: problem with exfat on 4k logical sector devices

[Namjae Jeon <linkinjeon@kernel.org>]

> Hi Namjae -
Hi Eric,
>
> It seems that exfat is unhappy on 4k logical sector size devices:
Thanks for your report!
We have got same report from Christophe Vu-Brugier. And he sent us
the patch(https://github.com/exfatprogs/exfatprogs/pull/164) to fix it
yesterday.(Thanks Christophe!), I will check it today.
>
> [root@big18 exfatprogs]# modprobe scsi_debug sector_size=4096
> dev_size_mb=256
> [root@big18 exfatprogs]# dmesg | tail -n 1
> [933449.931608] sd 16:0:0:0: [sdh] Attached SCSI disk
> [root@big18 exfatprogs]# mkfs.exfat /dev/sdh
> exfatprogs version : 1.0.4
> Creating exFAT filesystem(/dev/sdh, cluster size=4096)
>
> Writing volume boot record: done
> Writing backup volume boot record: done
> Fat table creation: done
> Allocation bitmap creation: done
> Upcase table creation: done
> Writing root directory entry: done
> Synchronizing...
>
> exFAT format complete!
>
> [root@big18 exfatprogs]# fsck.exfat /dev/sdh
> exfatprogs version : 1.0.4
> checksums of boot sector are not correct. 0x44e6c5, but expected 0xda55694.
> Fix (y/N)? n
>
> [root@big18 exfatprogs]# mount /dev/sdh /mnt/test
> mount: /mnt/test: wrong fs type, bad option, bad superblock on /dev/sdh,
> missing codepage or helper program, or other error.
> [root@big18 exfatprogs]# dmesg
> <...>
> [933485.685102] exFAT-fs (sdh): Invalid exboot-signature(sector = 1):
> 0x00000000
> [933485.686134] exFAT-fs (sdh): Invalid exboot-signature(sector = 2):
> 0x00000000
> [933485.687173] exFAT-fs (sdh): Invalid exboot-signature(sector = 3):
> 0x00000000
> [933485.688209] exFAT-fs (sdh): Invalid exboot-signature(sector = 4):
> 0x00000000
> [933485.689247] exFAT-fs (sdh): Invalid exboot-signature(sector = 5):
> 0x00000000
> [933485.690283] exFAT-fs (sdh): Invalid exboot-signature(sector = 6):
> 0x00000000
> [933485.691318] exFAT-fs (sdh): Invalid exboot-signature(sector = 7):
> 0x00000000
> [933485.692352] exFAT-fs (sdh): Invalid exboot-signature(sector = 8):
> 0x00000000
> [933485.695450] exFAT-fs (sdh): Invalid boot checksum (boot checksum :
> 0x0044e6c5, checksum : 0x04653cbf)
> [933485.695452] exFAT-fs (sdh): invalid boot region
> [933485.695453] exFAT-fs (sdh): failed to recognize exfat type
>
> I think the primary problem here is that the boot sector disk structures are
> always 512 bytes, even if the underlying disk sectors are 4k. There is a
> mismatch between mkfs, fsck, and the kernel in this respect. mkfs calculates
> checksums using 512 bytes for all but the OEM and reserved sectors, where it
> uses sector_size instead (which may be 4k)
Right.
>
> 3 mkfs/mkfs.c    exfat_write_boot_sector            129
> boot_calc_checksum((unsigned char *)ppbr, sizeof(struct pbr),
> 4 mkfs/mkfs.c    exfat_write_extended_boot_sectors  155
> boot_calc_checksum((unsigned char *) &eb, sizeof(struct exbs),
> 5 mkfs/mkfs.c    exfat_write_oem_sector             184
> boot_calc_checksum((unsigned char *)oem, bd->sector_size, false,
> 6 mkfs/mkfs.c    exfat_write_oem_sector             196
> boot_calc_checksum((unsigned char *)oem, bd->sector_size, false,
>
> but fsck uses the disk sector size (4k) for everything, so there is a
> checksum mismatch and failure.
>
> exfat_update_boot_checksum()
> ...
>         int sector_size = bd->sector_size;
> ...
>                 boot_calc_checksum(buf, sector_size, is_boot_sec,
>                         &checksum);
>
> The kernel has similar problems at mount time.
>
> (the kernel also has an issue where exfat_verify_boot_region is looking at 4
> bytes from the end of the sector for EXBOOT_SIGNATURE, rather than 4 bytes
> from the end of the boot_sector.  Also, s_blocksize_bits never gets set...)
>
> Anyway, this can all be fixed, but first there is a question about what is
> proper:
Okay.
>
> For these 11 regions (main boot sector, main extended boot sectors, OEM, and
> reserved, I think?) should the checksums be calculated on the full sector
> size (possibly 4k) or 512, or is it calculated on 512 (structure size) for
> all but the OEM sector as mkfs does? It's not clear to me from a quick read
> of the spec.
>
> (But from the example at
> https://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification#figure-1-boot-checksum-computation
> it almost looks like the checksum calculation should cover the entire 4k for
> all these regions, even if the disk structure itself is smaller?)
In this case,  We can check a dump of boot sectors formatted from Windows.
Let me check it more.

Thanks Eric!
>
> Thanks,
> -Eric
>