ipc: add semget syscall wrapper

Provide ksys_semget() wrapper to avoid in-kernel calls to this syscall.
The ksys_ prefix denotes that this function is meant as a drop-in
replacement for the syscall. In particular, it uses the same calling
convention as sys_semget().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: NDominik Brodowski <linux@dominikbrodowski.net>

ipc/sem.c

@@ -556,7 +556,7 @@ static inline int sem_more_checks(struct kern_ipc_perm *ipcp,
 	return 0;
 }
 
-SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
+long ksys_semget(key_t key, int nsems, int semflg)
 {
 	struct ipc_namespace *ns;
 	static const struct ipc_ops sem_ops = {
@@ -578,6 +578,11 @@ SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
 	return ipcget(ns, &sem_ids(ns), &sem_ops, &sem_params);
 }
 
+SYSCALL_DEFINE3(semget, key_t, key, int, nsems, int, semflg)
+{
+	return ksys_semget(key, nsems, semflg);
+}
+
 /**
  * perform_atomic_semop[_slow] - Attempt to perform semaphore
  *                               operations on a given array.

ipc/syscall.c

@@ -35,7 +35,7 @@ SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
 				       (const struct timespec __user *)fifth);
 
 	case SEMGET:
-		return sys_semget(first, second, third);
+		return ksys_semget(first, second, third);
 	case SEMCTL: {
 		unsigned long arg;
 		if (!ptr)
@@ -132,7 +132,7 @@ COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
 		return compat_ksys_semtimedop(first, compat_ptr(ptr), second,
 						compat_ptr(fifth));
 	case SEMGET:
-		return sys_semget(first, second, third);
+		return ksys_semget(first, second, third);
 	case SEMCTL:
 		if (!ptr)
 			return -EINVAL;

ipc/util.h

@@ -240,6 +240,7 @@ static inline int compat_ipc_parse_version(int *cmd)
 long ksys_semtimedop(int semid, struct sembuf __user *tsops,
 		     unsigned int nsops,
 		     const struct timespec __user *timeout);
+long ksys_semget(key_t key, int nsems, int semflg);
 
 /* for CONFIG_ARCH_WANT_OLD_COMPAT_IPC */
 #ifdef CONFIG_COMPAT