Re: [LTP-FAIL][02/21] fs: refactor ksys_umount

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

On Thu, Aug 06, 2020 at 04:17:32PM +0200, Christoph Hellwig wrote:
> Fix for umount03 below.  The other one works fine here, but from
> your logs this might be a follow on if you run it after umount without
> the fix.

Ugh...

How about 
static int may_umount(const struct path *path, int flags)
{
        struct mount *mnt = real_mount(path->mnt);

        if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
                return -EINVAL;
        if (!may_mount())
                return -EPERM;
        if (path->dentry != path->mnt->mnt_root)
                return -EINVAL;
        if (!check_mnt(mnt))
                return -EINVAL;
        if (mnt->mnt.mnt_flags & MNT_LOCKED) /* Check optimistically */
                return -EINVAL;
        if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
                return -EPERM;
	return 0;
}

int path_umount(const struct path *path, int flags)
{
        struct mount *mnt = real_mount(path->mnt);
        int err;

	err = may_umount(path, flags);
	if (!err)
		err = do_umount(mnt, flags);

        /* we mustn't call path_put() as that would clear mnt_expiry_mark */
        dput(path->dentry);
        mntput_no_expire(mnt);
        return err;
}

instead?