linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Date:   Tue, 3 Mar 2020 02:24:58 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     David Howells <dhowells@...hat.com>,
        Florian Weimer <fweimer@...hat.com>, linux-api@...r.kernel.org,
        viro@...iv.linux.org.uk, metze@...ba.org,
        torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?

On 2020-03-02, Christian Brauner <christian.brauner@...ntu.com> wrote:
> On Mon, Mar 02, 2020 at 02:50:03PM +0000, David Howells wrote:
> > Christian Brauner <christian.brauner@...ntu.com> wrote:
> > 
> > > I think we settled this and can agree on RESOLVE_NO_SYMLINKS being the
> > > right thing to do, i.e. not resolving symlinks will stay opt-in.
> > > Or is your worry even with the current semantics of openat2()? I don't
> > > see the issue since O_NOFOLLOW still works with openat2().
> > 
> > Say, for example, my home dir is on a network volume somewhere and /home has a
> > symlink pointing to it.  RESOLVE_NO_SYMLINKS cannot be used to access a file
> > inside my homedir if the pathwalk would go through /home/dhowells - this would
> > affect fsinfo() - so RESOLVE_NO_SYMLINKS is not a substitute for
> > AT_SYMLINK_NOFOLLOW (O_NOFOLLOW would not come into it).
> 
> I think we didn't really have this issue/face that question because
> openat() never supported AT_SYMLINK_{NO}FOLLOW. Whereas e.g. fsinfo()
> does. So in such cases we are back to: either allow both AT_* and
> RESOLVE_* flags (imho not the best option) or add (a) new RESOLVE_*
> variant(s). It seems we leaned toward the latter so far...

So, RESOLVE_NO_TRAILING_SYMLINKS?

... *sigh*. Yeah, okay I'm fine (though not super happy) with that. We'd
also presumably need RESOLVE_NO_TRAILING_AUTOMOUNTS for David's
AT_NO_AUTOMOUNT usecases -- as well as RESOLVE_NO_AUTOMOUNTS eventually.

Now let's just hope no new syscalls need both AT_RECURSIVE and
RESOLVE_NO_SYMLINKS -- that will put us in a very interesting situation
where you have two ways of specifying "don't follow trailing
symlinks"...

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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