s390/entry: use assignment to read intcode / asm to copy gprs

arch/s390/kernel/syscall.c: In function __do_syscall:
arch/s390/kernel/syscall.c:147:9: warning: memcpy reading 64 bytes from a region of size 0 [-Wstringop-overread]
  147 |         memcpy(&regs->gprs[8], S390_lowcore.save_area_sync, 8 * sizeof(unsigned long));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/s390/kernel/syscall.c:148:9: warning: memcpy reading 4 bytes from a region of size 0 [-Wstringop-overread]
  148 |         memcpy(&regs->int_code, &S390_lowcore.svc_ilc, sizeof(regs->int_code));
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by moving the gprs restore from C to assembly, and use a assignment
for int_code instead of memcpy.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: svens-s390

arch/s390/include/asm/lowcore.h

@@ -24,8 +24,7 @@ struct lowcore {
 	__u32	ext_params;			/* 0x0080 */
 	__u16	ext_cpu_addr;			/* 0x0084 */
 	__u16	ext_int_code;			/* 0x0086 */
-	__u16	svc_ilc;			/* 0x0088 */
-	__u16	svc_code;			/* 0x008a */
+	__u32	svc_int_code;			/* 0x0088 */
 	__u16	pgm_ilc;			/* 0x008c */
 	__u16	pgm_code;			/* 0x008e */
 	__u32	data_exc_code;			/* 0x0090 */

arch/s390/kernel/asm-offsets.c

@@ -58,8 +58,6 @@ int main(void)
 	OFFSET(__LC_EXT_PARAMS, lowcore, ext_params);
 	OFFSET(__LC_EXT_CPU_ADDR, lowcore, ext_cpu_addr);
 	OFFSET(__LC_EXT_INT_CODE, lowcore, ext_int_code);
-	OFFSET(__LC_SVC_ILC, lowcore, svc_ilc);
-	OFFSET(__LC_SVC_INT_CODE, lowcore, svc_code);
 	OFFSET(__LC_PGM_ILC, lowcore, pgm_ilc);
 	OFFSET(__LC_PGM_INT_CODE, lowcore, pgm_code);
 	OFFSET(__LC_DATA_EXC_CODE, lowcore, data_exc_code);

arch/s390/kernel/entry.S

@@ -276,6 +276,7 @@ ENTRY(system_call)
 	xgr	%r10,%r10
 	xgr	%r11,%r11
 	la	%r2,STACK_FRAME_OVERHEAD(%r15)	# pointer to pt_regs
+	mvc	__PT_R8(64,%r2),__LC_SAVE_AREA_SYNC
 	lgr	%r3,%r14
 	brasl	%r14,__do_syscall
 	lctlg	%c1,%c1,__LC_USER_ASCE

arch/s390/kernel/syscall.c

@@ -144,11 +144,8 @@ void noinstr __do_syscall(struct pt_regs *regs, int per_trap)
 {
 	add_random_kstack_offset();
 	enter_from_user_mode(regs);
-
-	memcpy(&regs->gprs[8], S390_lowcore.save_area_sync, 8 * sizeof(unsigned long));
-	memcpy(&regs->int_code, &S390_lowcore.svc_ilc, sizeof(regs->int_code));
 	regs->psw = S390_lowcore.svc_old_psw;
-
+	regs->int_code = S390_lowcore.svc_int_code;
 	update_timer_sys();
 
 	local_irq_enable();