[PATCH v3 0/2] vfs: Define new syscall getumask.

[PATCH v3 0/2] vfs: Define new syscall getumask.

[Richard W.M. Jones]

v2 -> v3:

 - Add the syscall to uapi/asm-generic/unistd.h.

 - Retest.

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

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.

[PATCH v3 1/2] vfs: Define new syscall getumask.

[Richard W.M. Jones]

Define a system call for reading the current umask value.

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
 include/linux/syscalls.h          | 1 +
 include/uapi/asm-generic/unistd.h | 4 +++-
 kernel/sys.c                      | 5 +++++
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index d795472..e96e88f 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -659,6 +659,7 @@ asmlinkage long sys_prlimit64(pid_t pid, unsigned int resource,
 				struct rlimit64 __user *old_rlim);
 asmlinkage long sys_getrusage(int who, struct rusage __user *ru);
 asmlinkage long sys_umask(int mask);
+asmlinkage long sys_getumask(void);
 
 asmlinkage long sys_msgget(key_t key, int msgflg);
 asmlinkage long sys_msgsnd(int msqid, struct msgbuf __user *msgp,
diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
index 2622b33..e59e880 100644
--- a/include/uapi/asm-generic/unistd.h
+++ b/include/uapi/asm-generic/unistd.h
@@ -717,9 +717,11 @@ __SYSCALL(__NR_membarrier, sys_membarrier)
 __SYSCALL(__NR_mlock2, sys_mlock2)
 #define __NR_copy_file_range 285
 __SYSCALL(__NR_copy_file_range, sys_copy_file_range)
+#define __NR_getumask 286
+__SYSCALL(__NR_getumask, sys_getumask)
 
 #undef __NR_syscalls
-#define __NR_syscalls 286
+#define __NR_syscalls 287
 
 /*
  * All syscalls below here should go away really,
diff --git a/kernel/sys.c b/kernel/sys.c
index cf8ba54..9db526c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1649,6 +1649,11 @@ SYSCALL_DEFINE1(umask, int, mask)
 	return mask;
 }
 
+SYSCALL_DEFINE0(getumask)
+{
+	return current_umask();
+}
+
 static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 {
 	struct fd exe;
-- 
2.7.4

[PATCH v3 2/2] x86: Wire up new getumask system call on x86.

[Richard W.M. Jones]

Signed-off-by: Richard W.M. Jones <rjones@redhat.com>
---
 arch/x86/entry/syscalls/syscall_32.tbl | 1 +
 arch/x86/entry/syscalls/syscall_64.tbl | 1 +
 2 files changed, 2 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
index b30dd81..af0a032 100644
--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl
@@ -386,3 +386,4 @@
 377	i386	copy_file_range		sys_copy_file_range
 378	i386	preadv2			sys_preadv2
 379	i386	pwritev2		sys_pwritev2
+380	i386	getumask		sys_getumask
diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index cac6d17..47c1579 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -335,6 +335,7 @@
 326	common	copy_file_range		sys_copy_file_range
 327	64	preadv2			sys_preadv2
 328	64	pwritev2		sys_pwritev2
+329	common	getumask		sys_getumask
 
 #
 # x32-specific system call numbers start at 512 to avoid cache impact
-- 
2.7.4

Re: [PATCH v3 0/2] vfs: Define new syscall getumask.

[Cyrill Gorcunov]

On Wed, Apr 13, 2016 at 02:57:55PM +0100, Richard W.M. Jones wrote:
> v2 -> v3:
> 
>  - Add the syscall to uapi/asm-generic/unistd.h.
> 
>  - Retest.

Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>

Thank you!

Re: [PATCH v3 0/2] vfs: Define new syscall getumask.

[Davidlohr Bueso]

ENOMANPAGE

Re: [PATCH v3 0/2] vfs: Define new syscall getumask.

[Richard W.M. Jones]

On Wed, Apr 13, 2016 at 11:52:53AM -0700, Davidlohr Bueso wrote:
> ENOMANPAGE

Where do man pages go?  In the man-pages project?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
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Re: [PATCH v3 0/2] vfs: Define new syscall getumask.

[Mathieu Desnoyers]

----- On Apr 13, 2016, at 3:06 PM, Richard W.M. Jones rjones@redhat.com wrote:

> On Wed, Apr 13, 2016 at 11:52:53AM -0700, Davidlohr Bueso wrote:
>> ENOMANPAGE
> 
> Where do man pages go?  In the man-pages project?

Yes. I usually also put a rendered version of the man page after the
changelog of the patch that implements the new system call. This facilitates
the discussion.

Thanks,

Mathieu


> 
> Rich.
> 
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> libguestfs lets you edit virtual machines.  Supports shell scripting,
> bindings from many languages.  http://libguestfs.org

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