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

["Richard W.M. Jones" <rjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

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.

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