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Date:   Sat, 27 Jul 2019 03:28:26 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Christian Brauner <christian@brauner.io>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
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        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
Subject: Re: Regression in 5.3 for some FS_USERNS_MOUNT (aka
 user-namespace-mountable) filesystems
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On Fri, Jul 26, 2019 at 07:46:18PM -0500, Eric W. Biederman wrote:

> If someone had bothered to actually look at how I was proposing to clean
> things up before the new mount api we would already have that.  Sigh.
> 
> You should be able to get away with something like this which moves the
> checks earlier and makes things clearer.  My old patch against the pre
> new mount api code.

Check your instances of ->permission(); AFAICS in all cases it's (in
current terms)
	return ns_capable(fc->user_ns, CAP_SYS_ADMIN) ? 0 : -EPERM;

In principle I like killing FS_USERNS_MOUNT flag, but when a method
is always either NULL or exact same function...