From: Jan Glauber <jglauber@redhat.com>
Subject: [RHEL5.1 PATCH] kprobes breaks BUG_ON on s390
Date: Wed, 07 Mar 2007 13:57:05 +0100
Bugzilla: 231155
Message-Id: <1173272225.5242.31.camel@localhost.localdomain>
Changelog: [s390] kprobes breaks BUG_ON


BUG_ON is not working correctly on s390. Due to the kprobes
notifier only the printk but not the following Oops output is shown
if we hit a BUG_ON. The presence of this bug in the GA version of 
RHEL5 makes me not feel very comfortable, we should fix this early
in the 5.1 cycle.

More details:
The illegal operation handler calls the die notifier with DIE_BPT to
let kprobes pick up its breakpoint. If kprobes does not find its
breakpoint it returns NOTIFY_STOP instead of NOTIFY_DONE.
Since we use stop_machine_run on s390 to arm/disarm the kprobes
breakpoints the race that kprobe_handler tries to solve by checking
for the kprobes breakpoints does not exist. Removing the check makes
BUG_ON working again.


Jan

jglauber@redhat.com
jang@de.ibm.com

---

Index: linux-rhel5/arch/s390/kernel/kprobes.c
===================================================================
--- linux-rhel5.orig/arch/s390/kernel/kprobes.c	2007-03-05 18:23:29.000000000 +0100
+++ linux-rhel5/arch/s390/kernel/kprobes.c	2007-03-06 14:29:12.000000000 +0100
@@ -343,21 +343,14 @@ static int __kprobes kprobe_handler(stru
 	}
 
 	p = get_kprobe(addr);
-	if (!p) {
-		if (*addr != BREAKPOINT_INSTRUCTION) {
-			/*
-			 * The breakpoint instruction was removed right
-			 * after we hit it.  Another cpu has removed
-			 * either a probepoint or a debugger breakpoint
-			 * at this address.  In either case, no further
-			 * handling of this interrupt is appropriate.
-			 *
-			 */
-			ret = 1;
-		}
-		/* Not one of ours: let kernel handle it */
+	if (!p)
+		/*
+		 * No kprobe at this address. The fault has not been
+		 * caused by a kprobe breakpoint. The race of breakpoint
+		 * vs. kprobe remove does not exist because on s390 we
+		 * use stop_machine_run to arm/disarm the breakpoints.
+		 */
 		goto no_kprobe;
-	}
 
 	kcb->kprobe_status = KPROBE_HIT_ACTIVE;
 	set_current_kprobe(p, regs, kcb);