Re: [PATCH v3] fs: don't scan the inode cache before SB_BORN is set

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, May 10, 2018 at 02:21:33PM +1000, Dave Chinner wrote:

> Setting sb->s_fs_info to NULL on xfs_mount setup failure only solves
> the use-after-free part of the problem - it doesn't solve the
> use-before-initialisation part. To solve that we need to check the
> SB_BORN flag in super_cache_count().
> 
> The SB_BORN flag is not set until ->fs_mount() completes
> successfully and trylock_super() won't succeed until it is set.
> Hence super_cache_scan() will not run until SB_BORN is set, so it
> makes sense to not allow super_cache_scan to run and enter the
> filesystem until it is set, too. This prevents the superblock
> shrinker from entering the filesystem while it is being set up and
> so avoids the use-before-initialisation issue.

I'm fine with the first part of that (fs/super.c, that is), but
I don't understand why do you need the xfs side of the patch with
that.  Confused...