powerpc: Wire up clone3 syscall

Wire up the new clone3 syscall added in commit 7f192e3c ("fork:
add clone3").

This requires a ppc_clone3 wrapper, in order to save the non-volatile
GPRs before calling into the generic syscall code. Otherwise we hit
the BUG_ON in CHECK_FULL_REGS in copy_thread().

Lightly tested using Christian's test code on a Power8 LE VM.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Christian Brauner <christian@brauner.io>
Link: https://lore.kernel.org/r/20190724140259.23554-1-mpe@ellerman.id.au

arch/powerpc/include/asm/unistd.h

@@ -49,6 +49,7 @@
 #define __ARCH_WANT_SYS_FORK
 #define __ARCH_WANT_SYS_VFORK
 #define __ARCH_WANT_SYS_CLONE
+#define __ARCH_WANT_SYS_CLONE3
 
 #endif		/* __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_UNISTD_H_ */

arch/powerpc/kernel/entry_32.S

@@ -597,6 +597,14 @@ ppc_clone:
 	stw	r0,_TRAP(r1)		/* register set saved */
 	b	sys_clone
 
+	.globl	ppc_clone3
+ppc_clone3:
+	SAVE_NVGPRS(r1)
+	lwz	r0,_TRAP(r1)
+	rlwinm	r0,r0,0,0,30		/* clear LSB to indicate full */
+	stw	r0,_TRAP(r1)		/* register set saved */
+	b	sys_clone3
+
 	.globl	ppc_swapcontext
 ppc_swapcontext:
 	SAVE_NVGPRS(r1)

arch/powerpc/kernel/entry_64.S

@@ -487,6 +487,11 @@ _GLOBAL(ppc_clone)
 	bl	sys_clone
 	b	.Lsyscall_exit
 
+_GLOBAL(ppc_clone3)
+       bl      save_nvgprs
+       bl      sys_clone3
+       b       .Lsyscall_exit
+
 _GLOBAL(ppc32_swapcontext)
 	bl	save_nvgprs
 	bl	compat_sys_swapcontext

arch/powerpc/kernel/syscalls/syscall.tbl

@@ -516,4 +516,4 @@
 432	common	fsmount				sys_fsmount
 433	common	fspick				sys_fspick
 434	common	pidfd_open			sys_pidfd_open
-# 435 reserved for clone3
+435	nospu	clone3				ppc_clone3