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

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

[Richard W.M. Jones]

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 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 +
 kernel/sys.c             | 5 +++++
 2 files changed, 6 insertions(+)

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/kernel/sys.c b/kernel/sys.c
index cf8ba54..026c146 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->fs->umask;
+}
+
 static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 {
 	struct fd exe;
-- 
2.7.4

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

[Florian Weimer]

* Richard W. M. Jones:

> +SYSCALL_DEFINE0(getumask)
> +{
> +	return current->fs->umask;
> +}

The convention seems to be to call current_umask(), instead of
inlining its contents.

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

[Cyrill Gorcunov]

On Wed, Apr 13, 2016 at 02:21:35PM +0200, Florian Weimer wrote:
> * Richard W. M. Jones:
> 
> > +SYSCALL_DEFINE0(getumask)
> > +{
> > +	return current->fs->umask;
> > +}
> 
> The convention seems to be to call current_umask(), instead of
> inlining its contents.

Yes, please use existing.

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

[kbuild test robot]

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Hi Richard,

[auto build test WARNING on v4.6-rc3]
[also build test WARNING on next-20160413]
[cannot apply to tip/x86/core tip/auto-latest]
[if your patch is applied to the wrong git tree, please drop us a note to help improving the system]

url:    https://github.com/0day-ci/linux/commits/Richard-W-M-Jones/vfs-Define-new-syscall-getumask/20160413-194722
config: xtensa-allmodconfig (attached as .config)
reproduce:
        wget https://git.kernel.org/cgit/linux/kernel/git/wfg/lkp-tests.git/plain/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # save the attached .config to linux build tree
        make.cross ARCH=xtensa 

All warnings (new ones prefixed by >>):

   <stdin>:1298:2: warning: #warning syscall userfaultfd not implemented [-Wcpp]
   <stdin>:1301:2: warning: #warning syscall membarrier not implemented [-Wcpp]
   <stdin>:1304:2: warning: #warning syscall mlock2 not implemented [-Wcpp]
   <stdin>:1307:2: warning: #warning syscall copy_file_range not implemented [-Wcpp]
   <stdin>:1310:2: warning: #warning syscall preadv2 not implemented [-Wcpp]
   <stdin>:1313:2: warning: #warning syscall pwritev2 not implemented [-Wcpp]
>> <stdin>:1316:2: warning: #warning syscall getumask not implemented [-Wcpp]

---
0-DAY kernel test infrastructure                Open Source Technology Center
https://lists.01.org/pipermail/kbuild-all                   Intel Corporation

[-- Attachment #2: .config.gz --]
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Re: [PATCH 0/2] vfs: Define new syscall getumask.

[Pavel Emelyanov]

On 04/13/2016 02:43 PM, Richard W.M. Jones wrote:
> 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.

Ah! Thanks for this :)

Acked-by: Pavel Emelyanov <xemul@virtuozzo.com>