linux-kernel - Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

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Message-ID: <CAHk-=wiF6en6WD7JdYAUxHnzeTgs3P08ysDQPv4504hQ2qUcmA@mail.gmail.com>
Date:   Thu, 9 Jan 2020 21:03:00 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        stable <stable@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ian Kent <raven@...maw.net>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

On Thu, Jan 9, 2020 at 8:15 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > Hmm. If that's the case, maybe they should be marked implicitly as
> > O_PATH when opened?
>
> I thought you wanted O_PATH as starting point to have mounts traversed?
> Confused...

No, I'm confused.  I meant "non-O_PATH", just got the rules reversed in my mind.

So cwd/root would always act as it non-O_PATH, and only using an
actual fd would look at the O_PATH flag, and if it was set would walk
the mountpoints.

> <grabs Bach> Right, he simply transcribes v7 iget().
>
> So I suspect that you are right - your variant of iget was pretty much
> one-to-one implementation of Bach's description of v7 iget.

Ok, that makes sense. My copy of Bach literally had the system call
list "marked off" when I implemented them back when.

I may still have that paperbook copy somewhere. I don't _think_ I'd
have thrown it out, it has sentimental value.

> > I think that in a perfect world, the O_PATH'ness of '42' would be the
> > deciding factor. Wouldn't those be the best and most consistent
> > semantics?
> >
> > And then 'cwd'/'root' always have the O_PATH behavior.
>
> See above - unless I'm misparsing you, you wanted mount traversals in the
> starting point if it's ...at() with O_PATH fd.

.. and see above, it was just my confusion about the sense of O_PATH.

> For cwd and root the situation is opposite - we do NOT traverse mounts
> for those.  And that's really too late to change.

Oh, absolutely.

[ snip some more about your automount digging. Looks about right, but
I'm not going to make a peep after getting O_PATH reversed ;) ]

            Linus