linux-kernel - Re: Have RESOLVE_* flags superseded AT

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Message-ID: <20200302151021.x5mm54jtoukg4tdk@yavin>
Date:   Tue, 3 Mar 2020 02:10:21 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     David Howells <dhowells@...hat.com>
Cc:     Christian Brauner <christian.brauner@...ntu.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Subject: Re: Have RESOLVE_* flags superseded AT_* flags for new syscalls?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, David Howells <dhowells@...hat.com> 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()

Yes, though this only happens if you're opening "/home/dhowells/foobar".
If you are doing "./foobar" from within "/home/dhowells" it will work
(or if you open a dirfd to "/home/dhowells") -- because no symlink
resolution is done as part of that openat2() call.

> So RESOLVE_NO_SYMLINKS is not a substitute for AT_SYMLINK_NOFOLLOW
> (O_NOFOLLOW would not come into it).

This is what I was saying up-thread -- the semantics are not the same
*on purpose*. If you want "don't follow symlinks *only for the final
component*" then you need to have an AT_SYMLINK_NOFOLLOW equivalent.

My counter-argument is that most people actually want
RESOLVE_NO_SYMLINKS (as evidenced by the countless symlink-related
security bugs -- many of which used O_NOFOLLOW incorrectly), it just
wasn't available before Linux 5.6.

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

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