Fix BUG_ON() reporting in real mode on powerpc

[Paul Mackerras]

On Wed, Feb 17, 2016 at 03:43:11PM +1100, Balbir Singh wrote:
> From: Balbir Singh <bsingharora at gmail.com>
> 
> 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 <paulus at samba.org>
> Signed-off-by: Balbir Singh <bsingharora at gmail.com>
> ---
>  arch/powerpc/kernel/traps.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> index b6becc7..8f28120 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 +1159,13 @@ void __kprobes program_check_exception(struct pt_regs *regs)
>  				== NOTIFY_STOP)
>  			goto bail;
>  
> +		if (!(regs->msr & MSR_IR))
> +			bugaddr = regs->nip + PAGE_OFFSET;
> +		else
> +			bugaddr = regs->nip;

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.

Paul.