s390: fix __EMIT_BUG() macro

Setting a kprobe on getname_flags() failed:

$ echo 'p:tmr1 getname_flags +0(%r2):ustring' > kprobe_events
-bash: echo: write error: Invalid argument

Debugging the kprobes code showed that the address of
getname_flags() is contained in the __bug_table. Kprobes
doesn't allow to set probes at BUG() locations.

$ objdump -j  __bug_table -x build/fs/namei.o
[..]
0000000000000108 R_390_PC32        .text+0x00000000000075a8
000000000000010c R_390_PC32        .L223+0x0000000000000004

I was expecting getname_flags() to start with a BUG(), but:

7598:       e3 20 10 00 00 04       lg      %r2,0(%r1)
759e:       c0 f4 00 00 00 00       jg      759e <putname+0x7e>
75a0: R_390_PLT32DBL    kmem_cache_free+0x2
75a4:       a7 f4 00 01             j       75a6 <putname+0x86>

00000000000075a8 <getname_flags>:
75a8:       c0 04 00 00 00 00       brcl    0,75a8 <getname_flags>
75ae:       eb 6f f0 48 00 24       stmg    %r6,%r15,72(%r15)
75b4:       b9 04 00 ef             lgr     %r14,%r15
75b8:       e3 f0 ff a8 ff 71       lay     %r15,-88(%r15)

So the BUG() is actually the last opcode of the previous function.
Fix this by switching to using the MONITOR CALL (MC) instruction,
and set the entry in __bug_table to the beginning of that MC.

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

arch/s390/boot/head.S

@@ -329,7 +329,7 @@ ENTRY(startup_kdump)
 	.quad	.Lduct			# cr5: primary-aste origin
 	.quad	0			# cr6:	I/O interrupts
 	.quad	0			# cr7:	secondary space segment table
-	.quad	0			# cr8:	access registers translation
+	.quad	0x0000000000008000	# cr8:	access registers translation
 	.quad	0			# cr9:	tracing off
 	.quad	0			# cr10: tracing off
 	.quad	0			# cr11: tracing off

arch/s390/include/asm/bug.h

@@ -10,15 +10,14 @@
 
 #define __EMIT_BUG(x) do {					\
 	asm_inline volatile(					\
-		"0:	j	0b+2\n"				\
-		"1:\n"						\
+		"0:	mc	0,0\n"				\
 		".section .rodata.str,\"aMS\",@progbits,1\n"	\
-		"2:	.asciz	\""__FILE__"\"\n"		\
+		"1:	.asciz	\""__FILE__"\"\n"		\
 		".previous\n"					\
 		".section __bug_table,\"awM\",@progbits,%2\n"	\
-		"3:	.long	1b-3b,2b-3b\n"			\
+		"2:	.long	0b-2b,1b-2b\n"			\
 		"	.short	%0,%1\n"			\
-		"	.org	3b+%2\n"			\
+		"	.org	2b+%2\n"			\
 		".previous\n"					\
 		: : "i" (__LINE__),				\
 		    "i" (x),					\
@@ -29,12 +28,11 @@
 
 #define __EMIT_BUG(x) do {					\
 	asm_inline volatile(					\
-		"0:	j	0b+2\n"				\
-		"1:\n"						\
+		"0:	mc	0,0\n"				\
 		".section __bug_table,\"awM\",@progbits,%1\n"	\
-		"2:	.long	1b-2b\n"			\
+		"1:	.long	0b-1b\n"			\
 		"	.short	%0\n"				\
-		"	.org	2b+%1\n"			\
+		"	.org	1b+%1\n"			\
 		".previous\n"					\
 		: : "i" (x),					\
 		    "i" (sizeof(struct bug_entry)));		\

arch/s390/kernel/entry.h

@@ -45,6 +45,7 @@ void specification_exception(struct pt_regs *regs);
 void transaction_exception(struct pt_regs *regs);
 void translation_exception(struct pt_regs *regs);
 void vector_exception(struct pt_regs *regs);
+void monitor_event_exception(struct pt_regs *regs);
 
 void do_per_trap(struct pt_regs *regs);
 void do_report_trap(struct pt_regs *regs, int si_signo, int si_code, char *str);

arch/s390/kernel/pgm_check.S

@@ -81,7 +81,7 @@ PGM_CHECK_DEFAULT			/* 3c */
 PGM_CHECK_DEFAULT			/* 3d */
 PGM_CHECK_DEFAULT			/* 3e */
 PGM_CHECK_DEFAULT			/* 3f */
-PGM_CHECK_DEFAULT			/* 40 */
+PGM_CHECK(monitor_event_exception)	/* 40 */
 PGM_CHECK_DEFAULT			/* 41 */
 PGM_CHECK_DEFAULT			/* 42 */
 PGM_CHECK_DEFAULT			/* 43 */

arch/s390/kernel/traps.c

@@ -53,11 +53,6 @@ void do_report_trap(struct pt_regs *regs, int si_signo, int si_code, char *str)
                 if (fixup)
 			regs->psw.addr = extable_fixup(fixup);
 		else {
-			enum bug_trap_type btt;
-
-			btt = report_bug(regs->psw.addr, regs);
-			if (btt == BUG_TRAP_TYPE_WARN)
-				return;
 			die(regs, str);
 		}
         }
@@ -245,6 +240,27 @@ void space_switch_exception(struct pt_regs *regs)
 	do_trap(regs, SIGILL, ILL_PRVOPC, "space switch event");
 }
 
+void monitor_event_exception(struct pt_regs *regs)
+{
+	const struct exception_table_entry *fixup;
+
+	if (user_mode(regs))
+		return;
+
+	switch (report_bug(regs->psw.addr - (regs->int_code >> 16), regs)) {
+	case BUG_TRAP_TYPE_NONE:
+		fixup = s390_search_extables(regs->psw.addr);
+		if (fixup)
+			regs->psw.addr = extable_fixup(fixup);
+		break;
+	case BUG_TRAP_TYPE_WARN:
+		break;
+	case BUG_TRAP_TYPE_BUG:
+		die(regs, "monitor event");
+		break;
+	}
+}
+
 void kernel_stack_overflow(struct pt_regs *regs)
 {
 	bust_spinlocks(1);
@@ -255,8 +271,23 @@ void kernel_stack_overflow(struct pt_regs *regs)
 }
 NOKPROBE_SYMBOL(kernel_stack_overflow);
 
+static void test_monitor_call(void)
+{
+	int val = 1;
+
+	asm volatile(
+		"	mc	0,0\n"
+		"0:	xgr	%0,%0\n"
+		"1:\n"
+		EX_TABLE(0b,1b)
+		: "+d" (val));
+	if (!val)
+		panic("Monitor call doesn't work!\n");
+}
+
 void __init trap_init(void)
 {
 	sort_extable(__start_dma_ex_table, __stop_dma_ex_table);
 	local_mcck_enable();
+	test_monitor_call();
 }