Mount empty UBI volume attached via fastmap may not work

[Richard Weinberger]

Jimmy,

Am 26.04.2017 um 09:54 schrieb Richard Weinberger:
> Jimmy,
> 
> Am 26.04.2017 um 09:46 schrieb Jimmy Perchet:
>> Hello,
>>
>> I ran into a problem I'd like to share. If Fastmap is used and if an
>> unclean shutdown happens after a volume has been truncated, the
>> subsequent mounts will fail, even though the status is 'OK'.
>> In this case, UBIFS does not seem to see the volume as empty
>> and fails with the following log :
>>
>> UBIFS error (ubi0:0 pid 9258): ubifs_read_node [ubifs]: bad node type (255 but expected 6)
>> UBIFS error (ubi0:0 pid 9258): ubifs_read_node [ubifs]: bad node at LEB 0:0, LEB mapping status 1
>> Not a node, first 24 bytes:
>> 00000000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ........................
>> CPU: 0 PID: 9258 Comm: mount Tainted: G           OE 4.9.0-2-amd64 #1 Debian 4.9.13-1
>> Hardware name: Hewlett-Packard HP Compaq 8200 Elite SFF PC/1495, BIOS J01 v02.06 06/09/2011
>>  0000000000000000 ffffffff9d928594 0000000000000000 ffff8abc0e07c000
>>  ffffffffc0d18686 ffff8abc85800300 ffffffff9d7dd937 ffff8abbba252000
>>  ffff8abc0e07c000 ffff8abb53b5c4c0 ffff8abc0e07c008 0000000000000000
>> Call Trace:
>>  [<ffffffff9d928594>] ? dump_stack+0x5c/0x78
>>  [<ffffffffc0d18686>] ? ubifs_read_node+0x256/0x2e0 [ubifs]
>>  [<ffffffff9d7dd937>] ? cache_grow_end+0xa7/0xc0
>>  [<ffffffffc0d15492>] ? ubifs_read_sb_node+0x52/0x70 [ubifs]
>>  [<ffffffffc0d15b5e>] ? ubifs_read_superblock+0x66e/0xe50 [ubifs]
>>  [<ffffffffc0d13ffb>] ? ubifs_mount+0xb2b/0x1d10 [ubifs]
>>  [<ffffffff9d7a32ee>] ? pcpu_alloc+0x38e/0x660
>>  [<ffffffff9d805fd6>] ? mount_fs+0x36/0x150
>>  [<ffffffff9d822f62>] ? vfs_kern_mount+0x62/0x100
>>  [<ffffffff9d82564f>] ? do_mount+0x1cf/0xc80
>>  [<ffffffff9d804001>] ? __fput+0x131/0x1e0
>>  [<ffffffff9d79dcea>] ? memdup_user+0x4a/0x70
>>  [<ffffffff9d82642e>] ? SyS_mount+0x7e/0xd0
>>  [<ffffffff9d603b1c>] ? do_syscall_64+0x7c/0xf0
>>  [<ffffffff9dbfaf6f>] ? entry_SYSCALL64_slow_path+0x25/0x25

I'm slow today. I think now I got your problem scenario.
You erase all blocks of a volume, face a power-cut before all blocks
got erased and then UBIFS fails upon mount since it thinks everything
is fine.

Thanks,
//richard