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

[Greg KH <greg-U8xfFu+wG4EAvxtiuMwx3w@public.gmane.org>]

On Wed, Apr 13, 2016 at 01:57:50PM +0100, Richard W.M. Jones 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.
> 
> 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);
> }

Why not add this to the ktest infrastructure, we strongly encourage that
for new syscalls, along with a man page patch.

thanks,

greg k-h