m68k: Wire up clone3() syscall

Wire up the clone3() syscall for m68k. The special entry point is done in
assembler as was done for clone() as well. This is needed because all
registers need to be saved. The C wrapper then calls the generic
sys_clone3() with the correct arguments.

Tested on A1200 using the simple test program from:

  https://lore.kernel.org/lkml/20190716130631.tohj4ub54md25dys@brauner.io/Signed-off-by: NKars de Jong <jongk@linux-m68k.org>
Link: https://lore.kernel.org/r/20191124195225.31230-1-jongk@linux-m68k.orgSigned-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/unistd.h

@@ -30,5 +30,6 @@
 #define __ARCH_WANT_SYS_SIGPROCMASK
 #define __ARCH_WANT_SYS_FORK
 #define __ARCH_WANT_SYS_VFORK
+#define __ARCH_WANT_SYS_CLONE3
 
 #endif /* _ASM_M68K_UNISTD_H_ */

arch/m68k/kernel/entry.S

@@ -69,6 +69,13 @@ ENTRY(__sys_vfork)
 	lea     %sp@(24),%sp
 	rts
 
+ENTRY(__sys_clone3)
+	SAVE_SWITCH_STACK
+	pea	%sp@(SWITCH_STACK_SIZE)
+	jbsr	m68k_clone3
+	lea	%sp@(28),%sp
+	rts
+
 ENTRY(sys_sigreturn)
 	SAVE_SWITCH_STACK
 	movel	%sp,%sp@-		  | switch_stack pointer

arch/m68k/kernel/process.c

@@ -30,8 +30,9 @@
 #include <linux/init_task.h>
 #include <linux/mqueue.h>
 #include <linux/rcupdate.h>
-
+#include <linux/syscalls.h>
 #include <linux/uaccess.h>
+
 #include <asm/traps.h>
 #include <asm/machdep.h>
 #include <asm/setup.h>
@@ -119,6 +120,16 @@ asmlinkage int m68k_clone(struct pt_regs *regs)
 		       (int __user *)regs->d3, (int __user *)regs->d4);
 }
 
+/*
+ * Because extra registers are saved on the stack after the sys_clone3()
+ * arguments, this C wrapper extracts them from pt_regs * and then calls the
+ * generic sys_clone3() implementation.
+ */
+asmlinkage int m68k_clone3(struct pt_regs *regs)
+{
+	return sys_clone3((struct clone_args __user *)regs->d1, regs->d2);
+}
+
 int copy_thread(unsigned long clone_flags, unsigned long usp,
 		 unsigned long arg, struct task_struct *p)
 {

arch/m68k/kernel/syscalls/syscall.tbl

@@ -434,4 +434,4 @@
 432	common	fsmount				sys_fsmount
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
-# 435 reserved for clone3
+435	common	clone3				__sys_clone3