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.
Define a system call for reading the current umask value.
Signed-off-by: Richard W.M. Jones <[email protected]>
---
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
Signed-off-by: Richard W.M. Jones <[email protected]>
---
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
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 <[email protected]>
Thank you!
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,
bindings from many languages. http://libguestfs.org
----- On Apr 13, 2016, at 3:06 PM, Richard W.M. Jones [email protected] 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