linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Date:   Mon, 2 Mar 2020 16:05:28 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Florian Weimer <fweimer@...hat.com>, linux-api@...r.kernel.org,
        viro@...iv.linux.org.uk, metze@...ba.org,
        torvalds@...ux-foundation.org, cyphar@...har.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?

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...

Christian