ipc: add shmdt syscall wrapper

Provide ksys_shmdt() 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_shmdt().

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

@@ -1481,7 +1481,7 @@ COMPAT_SYSCALL_DEFINE3(shmat, int, shmid, compat_uptr_t, shmaddr, int, shmflg)
  * detach and kill segment if marked destroyed.
  * The work is done in shm_close.
  */
-SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
+long ksys_shmdt(char __user *shmaddr)
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma;
@@ -1588,6 +1588,11 @@ SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
 	return retval;
 }
 
+SYSCALL_DEFINE1(shmdt, char __user *, shmaddr)
+{
+	return ksys_shmdt(shmaddr);
+}
+
 #ifdef CONFIG_PROC_FS
 static int sysvipc_shm_proc_show(struct seq_file *s, void *it)
 {

ipc/syscall.c

@@ -90,7 +90,7 @@ SYSCALL_DEFINE6(ipc, unsigned int, call, int, first, unsigned long, second,
 			return -EINVAL;
 		}
 	case SHMDT:
-		return sys_shmdt((char __user *)ptr);
+		return ksys_shmdt((char __user *)ptr);
 	case SHMGET:
 		return ksys_shmget(first, second, third);
 	case SHMCTL:
@@ -178,7 +178,7 @@ COMPAT_SYSCALL_DEFINE6(ipc, u32, call, int, first, int, second,
 		return put_user(raddr, (compat_ulong_t __user *)compat_ptr(third));
 	}
 	case SHMDT:
-		return sys_shmdt(compat_ptr(ptr));
+		return ksys_shmdt(compat_ptr(ptr));
 	case SHMGET:
 		return ksys_shmget(first, (unsigned int)second, third);
 	case SHMCTL:

ipc/util.h

@@ -244,6 +244,7 @@ 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);
+long ksys_shmdt(char __user *shmaddr);
 
 /* for CONFIG_ARCH_WANT_OLD_COMPAT_IPC */
 #ifdef CONFIG_COMPAT