linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Message-ID: <20200302121959.it3iophjavbhtoyp@wittgenstein>
Date:   Mon, 2 Mar 2020 13:19:59 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     David Howells <dhowells@...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Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?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 01:09:06PM +0100, Florian Weimer wrote:
> * Christian Brauner:
> 
> >> But that's inconsistent with the rest of the system.  And for example,
> >> if you make /etc/resolv.conf a symbolic link, a program which uses a new
> >> I/O library (with the new interfaces) will not be able to read it.
> >
> > Fair, but I expect that e.g. a C library would simply implement openat()
> > on top of openat2() if the latter is available and thus could simply
> > pass RESOLVE_SYMLINKS so any new I/O library not making use of the
> > syscall directly would simply get the old behavior. For anyone using the
> > syscall directly they need to know about its exact semantics anyway. But
> > again, maybe just having it opt-in is fine.
> 
> I'm more worried about fancy new libraries which go directly to the new
> system calls, but set the wrong defaults for a general-purpose open
> operation.
> 
> Can we pass RESOLVE_SYMLINKS with O_NOFLLOW, so that we can easily
> implement open/openat for architectures that provide only the openat2
> system call?

You can currently do RESOLVE_NO_SYMLINKS | O_NOFOLLOW. So I'd expect
RESOLVE_SYMLINKS | O_NOFOLLOW would work as well. But from what it looks
like having no symlink resolution be opt-in seems more likely.

> 
> >> AT_SYMLINK_NOFOLLOW only applies to the last pathname component anyway,
> >> so it's relatively little protection.
> >
> > So this is partially why I think it's at least worth considerings: the
> > new RESOLVE_NO_SYMLINKS flag does block all symlink resolution, not just
> > for the last component in contrast to AT_SYMLINK_NOFOLLOW. This is
> > 278121417a72d87fb29dd8c48801f80821e8f75a
> 
> RESOLVE_NO_SYMLINKS shouldn't be the default, though (whoever is
> responsible for applying that default).  Otherwise system administrators
> can no longer move around data between different file systems and set
> symbolic links accordingly.

Ok, maybe then we'll just leave RESOLVE_NO_SYMLINKS as opt-in.