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Subject: Re: [PATCH v2 0/2] vfs: Define new syscall getumask.
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 18 Apr 2016 03:04:02 -0700
To: "Richard W.M. Jones" <rjones@redhat.com>,
        Josh Triplett <josh@joshtriplett.org>
CC: Colin Walters <walters@verbum.org>, linux-kernel@vger.kernel.org,
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On April 18, 2016 2:14:12 AM PDT, "Richard W.M. Jones" <rjones@redhat.com> wrote:
>On Sun, Apr 17, 2016 at 06:57:36PM -0700, Josh Triplett wrote:
>> O_NOUMASK seems potentially useful to support implementation of umask
>> entirely in userspace, which also addresses thread-safety.  A program
>> could read its process umask out at startup, handle umask entirely in
>> userspace (including for threads), and only interact with the system
>> umask after fork and before exec.
>
>I had a look at O_NOUMASK and there are a few problems:
>
>It's relatively easy to implement for open(2).  A few filesystems
>implement their own open so I had to go into those filesystems and
>modify how they handle current_umask too.  And FUSE support is tricky
>so I passed on that.
>
>The real problem is that mkdir/mkdirat/mknod/mknodat are affected by
>umask, but there is no convenient flags parameter to pass the
>O_NOUMASK flag.  So I think the patch only half-solves the problem.
>
>I have a patch which needs a bit more testing, which I can post if you
>think that's helpful, but I don't think it would be acceptable in its
>current state.
>
>Rich.

Ironically this illustrates one of the limitations with flags arguments: this really belongs in the S_-flags, but we can't assume userspace is clean there... anymore than we can repurpose umask(-1).
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