linux-kernel - Re: [PATCH v2 0/2] vfs: Define new syscall getumask.

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Date:	Mon, 18 Apr 2016 10:14:12 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Colin Walters <walters@...bum.org>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	akpm@...ux-foundation.org, luto@...nel.org,
	viro@...iv.linux.org.uk, mathieu.desnoyers@...icios.com,
	zab@...hat.com, emunson@...mai.com, paulmck@...ux.vnet.ibm.com,
	aarcange@...hat.com, xemul@...allels.com, sfr@...b.auug.org.au,
	milosz@...in.com, rostedt@...dmis.org, arnd@...db.de,
	ebiederm@...ssion.com, gorcunov@...nvz.org,
	iulia.manda21@...il.com, dave.hansen@...ux.intel.com,
	mguzik@...hat.com, adobriyan@...il.com, dave@...olabs.net,
	linux-api@...r.kernel.org, gorcunov@...il.com, fw@...eb.enyo.de
Subject: Re: [PATCH v2 0/2] vfs: Define new syscall getumask.

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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