[PATCH v2] procfs: expose umask in /proc/<PID>/status

[Richard W.M. Jones]

v1 -> v2:

 - Change printf format to %#04o.

 - Retest and update examples accordingly.

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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.

Add a new status line ("Umask") in /proc/<PID>/status.  It contains
the file mode creation mask (umask) in octal.  It is only shown for
tasks which have task->fs.

For the library this allows me to read the umask from
/proc/self/status.

This patch is adapted from one originally written by Pierre Carrier:
https://lkml.org/lkml/2012/5/4/451

Example usage:

$ grep Umask /proc/1/status 
Umask:	0022
$ grep Umask /proc/2/status 
Umask:	0022
$ grep Umask /proc/self/status 
Umask:	0022
$ umask 002
$ grep Umask /proc/self/status 
Umask:	0002

Rich.