linux-kernel - [PATCH v4 0/3] vfs: Define new syscall umask2 [formerly getumask]

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Date:	Wed, 13 Apr 2016 20:05:33 +0100
From:	"Richard W.M. Jones" <rjones@...hat.com>
To:	linux-kernel@...r.kernel.org
Cc:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	akpm@...ux-foundation.org, luto@...nel.org,
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Subject: [PATCH v4 0/3] vfs: Define new syscall umask2 [formerly getumask]

v3 -> v4:

 - Rename the syscall: getumask becomes umask2.

 - Add flags parameter, with one flag (UMASK_GET_MASK).

 - Expand the rationale for this change in the first commit message.

 - Add a selftest.

 - Retest everything.

--------------------

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.

This patch series adds a new system call "umask2".  This adds a flags
parameter.  Specifying flags=UMASK_GET_MASK allows the umask of the
current process to be read without modifying it.

This leaves open the possibility in future of adding a per-thread
umask, set or read with other flags.  This is not implemented.

Another approach to this has been attempted before, adding something
to /proc, although it didn't go anywhere.  See:

  http://comments.gmane.org/gmane.linux.kernel/1292109

Another way to solve this would be to add a thread-safe getumask to
glibc.  Since glibc could own the mutex, this would permit libraries
linked to this glibc to read umask safely.  I should also note that
man-pages documents getumask(3), but no version of glibc has ever
implemented it.

Rich.