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

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Balbir Singh <bsingharora@gmail.com> writes:

>> It might be a little better to do this:
>>=20
>> 		bugaddr =3D regs->nip;
>> 		if (REGION_ID(bugaddr) =3D=3D 0 && !(regs->msr & MSR_IR))
>> 			bugaddr +=3D PAGE_OFFSET;
>>=20
>> 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 <paulus@samba.org>
> Signed-off-by: Balbir Singh <bsingharora@gmail.com>
> ---
> =C2=A0arch/powerpc/kernel/traps.c | 7 ++++++-
> =C2=A01 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_re=
gs *regs)
> =C2=A0		goto bail;
> =C2=A0	}
> =C2=A0	if (reason & REASON_TRAP) {
> +		unsigned long bugaddr;
> =C2=A0		/* Debugger is first in line to stop recursive faults in
> =C2=A0		=C2=A0* rcu_lock, notify_die, or atomic_notifier_call_chain */
> =C2=A0		if (debugger_bpt(regs))
> @@ -1158,8 +1159,12 @@ void __kprobes program_check_exception(struct pt_r=
egs *regs)
> =C2=A0				=3D=3D NOTIFY_STOP)
> =C2=A0			goto bail;
> =C2=A0
> +		bugaddr =3D regs->nip;
> +		if ((REGION_ID(bugaddr) =3D=3D 0) && !(regs->msr & MSR_IR))
> +			bugaddr +=3D 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.


> =C2=A0		if (!(regs->msr & MSR_PR) &&=C2=A0=C2=A0/* not user-mode */
> -		=C2=A0=C2=A0=C2=A0=C2=A0report_bug(regs->nip, regs) =3D=3D BUG_TRAP_TY=
PE_WARN) {
> +		=C2=A0=C2=A0=C2=A0=C2=A0report_bug(bugaddr, regs) =3D=3D BUG_TRAP_TYPE=
_WARN) {
> =C2=A0			regs->nip +=3D 4;
> =C2=A0			goto bail;
> =C2=A0		}
> --=C2=A0

-aneesh