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Date:   Thu, 6 Aug 2020 15:32:21 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Christoph Hellwig <hch@lst.de>
Cc:     Vikas Kumar <vikas.kumar2@arm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-api@vger.kernel.org, rafael@kernel.org
Subject: Re: [LTP-FAIL][02/21] fs: refactor ksys_umount
Message-ID: <20200806143221.GQ1236603@ZenIV.linux.org.uk>
References: <d28d2235-9b1c-0403-59ca-e57ac5d0460e@arm.com>
 <20200806141732.GA5902@lst.de>
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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?