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

----- On Apr 13, 2016, at 8:57 AM, Richard W.M. Jones rjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org wrote:

v1 -> v2:

- Use current_umask() instead of current->fs->umask.

- Retested it.

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

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 trivial system call "getumask" which returns
the umask of the current process.

In addition to this system call, we could extend a variation of my
thread_local_abi system call (https://lkml.org/lkml/2016/4/4/455)
(could be without features flags, or an entirely new system call
specifically for a umask cache) to register a "current umask" cache
located in a TLS area.

Basically, reading the current umask value would be a simple load from
a TLS variable. This could also allow quickly blocking and unblocking
signal delivery from user-space by storing a mask to this TLS area.

The kernel could then look into the signal mask in this TLS area whenever
it needs to deliver a signal (assuming this code path can take
user-space faults), in addition to the mask kept within the
task struct.

This "tls cache" idea could also apply to setting a CPU affinity to the
currently running CPU for short user-space critical sections.

The benefit here is to get _very_ fast operations on the thread umask
and cpu affinity.

Are those ideas too far-fetched ?

Thanks,

Mathieu

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.

Typical test script:

#include <stdio.h>
#include <stdlib.h>
#include <linux/unistd.h>
#include <sys/syscall.h>

int main(int argc, char *argv[])
{
 int r = syscall(329);
 if (r == -1) {
   perror("getumask");
   exit(1);
 }
 printf("umask = %o\n", r);
 exit(0);
}

$ ./getumask
umask = 22

Rich.

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com