Date: Sun, 17 Apr 2016 18:57:36 -0700
From: Josh Triplett <josh@joshtriplett.org>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Colin Walters <walters@verbum.org>,
        "Richard W.M. Jones" <rjones@redhat.com>, 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 06:42:12PM -0700, H. Peter Anvin wrote:
> On 04/13/16 08:41, Colin Walters wrote:
> > On Wed, Apr 13, 2016, at 08:57 AM, Richard W.M. Jones wrote:
> > 
> >> It's not possible to read the process umask without also modifying it,
> >> which is what umask(2) does.  A library cannot read umask safely,
> >> especially if the main program might be multithreaded.
> > 
> > I assume you just want to do this from a shared library so you can
> > determine whether or not you need to call fchown() after making files
> > and the like?  If that's the case it'd be good to note it in the commit
> > message.
> > 
> > BTW...it might be a good idea to add a flags argument:
> > https://lwn.net/Articles/585415/
> > 
> > Did you consider calling this `umask2`, having the initial version only support
> > retrieving it via a UMASK_GET flag, and lay the groundwork to support
> > setting a threadsafe umask with a UMASK_SET_THREAD flag?
> > 
> 
> The comments on that article also list a number of problems with this
> approach, related to how undefined flags are handled.
> 
> In fact, if it wasn't for this exact problem then umask(-1) would have
> been the logical way to deal with this, but because umask(2) is defined
> to have an internal & 07777 it becomes infeasible at least in theory.
> In practice it might work...
> 
> However, see previous discussions about making this available in /proc.
>  Also, I really think there is something to be said for a O_NOUMASK
> option...

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.

- Josh Triplett