linux-kernel - Re: [PATCH 01/14] VFS: Add additional RESOLVE

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Date:   Wed, 11 Mar 2020 10:59:51 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Stefan Metzmacher <metze@...ba.org>,
        Ian Kent <raven@...maw.net>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <christian@...uner.io>,
        Jann Horn <jannh@...gle.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Karel Zak <kzak@...hat.com>, jlayton@...hat.com,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 01/14] VFS: Add additional RESOLVE_* flags [ver #18]

On Tue, Mar 10, 2020 at 12:25 AM David Howells <dhowells@...hat.com> wrote:
?
> Okay.  So what's the equivalent of AT_SYMLINK_NOFOLLOW in RESOLVE_* flag
> terms?

Nothing.

openat2() takes two sets of flags. We'll never get rid of
AT_SYMLINK_NOFOLLOW / O_NOFOLLOW, and we've added RESOLVE_NO_SYMLINKS
to the new set of flags. It's just a separate namespace.

We will _not_ be adding a RESOLVE_XYZ flag for O_NOFOLLOW or
AT_SYMLINK_NOFOLLOW. At least not visible to user space - because as
people already figured out, that just causes problems with consistency
issues.

And yes, the fact that we then have three different user-visible
namespaces (O_xyz flags for open(), AT_xyz flags for linkat(), and now
RESOLVE_xyz flags for openat2()) is sad and messy. But it's an
inherent messiness from just how the world works. We can't get rid of
it.

If we need linkat2() and friends, so be it. Do we?

Could we have a _fourth_ set of flags that are simply for internal use
that is a superset of them all? Sure. But no, it's almost certainly
not worth it. Four is not better than three.

Now, some type-safety in the kernel to make sure that we can't mix
AT_xyz with O_xyz or RESOLVE_xyz - that might be worth it. Although
judging by past experience, not enough people run sparse for it to
really be worth it.

               Linus

PS. Yeah, we also have that LOOKUP_xyz namespace, and the access mode
namespace, so we already have those internal format versions too.