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

From: Richard W.M. Jones
Date: Wed Apr 13 2016 - 08:58:10 EST


v1 -> v2:

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

- Retested it.

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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.

This patch series adds a trivial system call "getumask" which returns
the umask of the current process.

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.