ipc: add shmget syscall wrapper

Provide ksys_shmget() 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_shmget().

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: Dominik Brodowski <linux@dominikbrodowski.net>

ipc/shm.c

@@ -656,7 +656,7 @@ static inline int shm_more_checks(struct kern_ipc_perm *ipcp,
 	return 0;
 }
 
-SYSCALL_DEFINE3(shmget, key_t, key, size_t, size, int, shmflg)
+long ksys_shmget(key_t key, size_t size, int shmflg)
 {
 	struct ipc_namespace *ns;
 	static const struct ipc_ops shm_ops = {
@@ -675,6 +675,11 @@ SYSCALL_DEFINE3(shmget, key_t, key, size_t, size, int, shmflg)
 	return ipcget(ns, &shm_ids(ns), &shm_ops, &shm_params);
 }
 
+SYSCALL_DEFINE3(shmget, key_t, key, size_t, size, int, shmflg)
+{
+	return ksys_shmget(key, size, shmflg);
+}
+
 static inline unsigned long copy_shmid_to_user(void __user *buf, struct shmid64_ds *in, int version)
 {
 	switch (version) {

ipc/syscall.c

@@ -92,7 +92,7 @@ SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
 	case SHMDT:
 		return sys_shmdt((char __user *)ptr);
 	case SHMGET:
-		return sys_shmget(first, second, third);
+		return ksys_shmget(first, second, third);
 	case SHMCTL:
 		return sys_shmctl(first, second,
 				   (struct shmid_ds __user *) ptr);
@@ -180,7 +180,7 @@ COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
 	case SHMDT:
 		return sys_shmdt(compat_ptr(ptr));
 	case SHMGET:
-		return sys_shmget(first, (unsigned)second, third);
+		return ksys_shmget(first, (unsigned int)second, third);
 	case SHMCTL:
 		return compat_sys_shmctl(first, second, compat_ptr(ptr));
 	}

ipc/util.h

@@ -243,6 +243,7 @@ long ksys_semtimedop(int semid, struct sembuf __user *tsops,
 long ksys_semget(key_t key, int nsems, int semflg);
 long ksys_semctl(int semid, int semnum, int cmd, unsigned long arg);
 long ksys_msgget(key_t key, int msgflg);
+long ksys_shmget(key_t key, size_t size, int shmflg);
 
 /* for CONFIG_ARCH_WANT_OLD_COMPAT_IPC */
 #ifdef CONFIG_COMPAT