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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Wed, 17 Feb 2016 20:46:24 +0530

Balbir Singh <[email-masked]> writes:

>> 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;
> +

Can we add some comments around this. When i looked at this first, i was
wondering how nip can be in user region. But then realized that what we
are checking here is kernel address used in real mode. The use of
REGION_ID eventhough simpler is confusing. Hence adding the comment with
details Paul mentioned in email will help.

>  		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;
>  		}
> -- 

-aneesh