Subject: Re: Fix BUG_ON() reporting in real mode on powerpc

From: Balbir Singh <[email-masked]>
Date: Wed, 17 Feb 2016 18:16:05 +1100

> It might be a little better to do this:
> 
> 		bugaddr = regs->nip;
> 		if (REGION_ID(bugaddr) == 0 && !(regs->msr & MSR_IR))
> 			bugaddr += PAGE_OFFSET;
> 
> It is possible to execute from addresses with the 0xc000... on top in
> real mode, because the CPU ignores the top 4 address bits in real
> mode.

Good catch! Thank you

Changelog:
     Don't add PAGE_OFFSET blindly, check if REGION_ID is 0

I ran into this issue while debugging an early boot problem.
The system hit a BUG_ON() but report bug failed to print the
line number and file name. The reason being that the system
was running in real mode and report_bug() searches for
addresses in the PAGE_OFFSET+ region

Suggested-by: Paul Mackerras <[email-masked]>
Signed-off-by: Balbir Singh <[email-masked]>
---
 arch/powerpc/kernel/traps.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index b6becc7..4de4fe7 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -1148,6 +1148,7 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 		goto bail;
 	}
 	if (reason & REASON_TRAP) {
+		unsigned long bugaddr;
 		/* Debugger is first in line to stop recursive faults in
 		 * rcu_lock, notify_die, or atomic_notifier_call_chain */
 		if (debugger_bpt(regs))
@@ -1158,8 +1159,12 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 				== NOTIFY_STOP)
 			goto bail;
 
+		bugaddr = regs->nip;
+		if ((REGION_ID(bugaddr) == 0) && !(regs->msr & MSR_IR))
+			bugaddr += PAGE_OFFSET;
+
 		if (!(regs->msr & MSR_PR) &&  /* not user-mode */
-		    report_bug(regs->nip, regs) == BUG_TRAP_TYPE_WARN) {
+		    report_bug(bugaddr, regs) == BUG_TRAP_TYPE_WARN) {
 			regs->nip += 4;
 			goto bail;
 		}
-- 
2.5.0