Re: [PATCH 03/19] fs: release anon dev_t in deactivate_locked_super

[Al Viro]

On Fri, Sep 15, 2023 at 04:12:07PM +0200, Christian Brauner wrote:
> +	static void some_fs_kill_sb(struct super_block *sb)
> +	{
> +		struct some_fs_info *info = sb->s_fs_info;
> +
> +		kill_*_super(sb);
> +		kfree(info);
> +	}
> +
> +It's best practice to never deviate from this pattern.

The last part is flat-out incorrect.  If e.g. fatfs or cifs ever switches
to that pattern, you'll get UAF - they need freeing of ->s_fs_info
of anything that ever had been mounted done with RCU delay; moreover,
unload_nls() in fatfs needs to be behind the same.

Lifetime rules for fs-private parts of superblock are really private to
filesystem; their use by sget/sget_fc callbacks might impose restrictions
on those, but that again is none of the VFS business.