Re: [PATCH -next] fs: sysv: check sbi->s_firstdatazone in complete_read_super

[Liu Shixin <liushixin2@huawei.com>]

The type I used is sysv v7.

I discovered this problem through a syzkaller use case. The use case first create a file
via memfd and bind it to loop0. Then mount it to a folder with type v7. No error is
reported for preceding operations. Finally when create a file, the warning in inc_nlink
is triggered. I want to look the filesystem information, so I execute 'df' command and
find this problem.

Btw, I think the warning may be a follow-up problem caused by the filesystem being
created or mounted incorrectly.

Thanks,

On 2022/3/30 23:42, Christoph Hellwig wrote:
> On Wed, Mar 30, 2022 at 06:42:15PM +0800, Liu Shixin wrote:
>> sbi->s_firstinodezone is initialized to 2 and sbi->s_firstdatazone is read
>> from sbd. There's no guarantee that sbi->s_firstdatazone must bigger than
>> sbi->s_firstinodezone. If sbi->s_firstdatazone less than 2, the filesystem
>> can still be mounted unexpetly. At this point, sbi->s_ninodes flip to very
>> large value and this filesystem is broken. We can observe this by
>> executing 'df' command. When we execute, we will get an error message:
>> 	"sysv_count_free_inodes: unable to read inode table"
> This looks fine.  Just curious: which variant of the sysv fs do you
> use and what is the use case?
> .
>