Date: Mon, 18 Apr 2016 10:14:12 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Colin Walters <walters@verbum.org>,
        linux-kernel@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com,
        akpm@linux-foundation.org, luto@kernel.org, viro@zeniv.linux.org.uk,
        mathieu.desnoyers@efficios.com, zab@redhat.com, emunson@akamai.com,
        paulmck@linux.vnet.ibm.com, aarcange@redhat.com, xemul@parallels.com,
        sfr@canb.auug.org.au, milosz@adfin.com, rostedt@goodmis.org,
        arnd@arndb.de, ebiederm@xmission.com, gorcunov@openvz.org,
        iulia.manda21@gmail.com, dave.hansen@linux.intel.com,
        mguzik@redhat.com, adobriyan@gmail.com, dave@stgolabs.net,
        linux-api@vger.kernel.org, gorcunov@gmail.com, fw@deneb.enyo.de
Subject: Re: [PATCH v2 0/2] vfs: Define new syscall getumask.
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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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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