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

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Date:	Wed, 13 Apr 2016 06:59:50 -0700
From:	Greg KH <greg@...ah.com>
To:	"Richard W.M. Jones" <rjones@...hat.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, akpm@...ux-foundation.org, luto@...nel.org,
	viro@...iv.linux.org.uk, mathieu.desnoyers@...icios.com,
	zab@...hat.com, emunson@...mai.com, paulmck@...ux.vnet.ibm.com,
	aarcange@...hat.com, josh@...htriplett.org, xemul@...allels.com,
	sfr@...b.auug.org.au, milosz@...in.com, rostedt@...dmis.org,
	arnd@...db.de, ebiederm@...ssion.com, gorcunov@...nvz.org,
	iulia.manda21@...il.com, dave.hansen@...ux.intel.com,
	mguzik@...hat.com, adobriyan@...il.com, dave@...olabs.net,
	linux-api@...r.kernel.org, gorcunov@...il.com, fw@...eb.enyo.de
Subject: Re: [PATCH v2 0/2] vfs: Define new syscall getumask.

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