Fix BUG_ON() reporting in real mode on powerpc

Fix BUG_ON() reporting in real mode on powerpc

[Balbir Singh]

From: Balbir Singh <bsingharora@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@samba.org>
Signed-off-by: Balbir Singh <bsingharora@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;
+
 		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

Re: 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@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@samba.org>
> Signed-off-by: Balbir Singh <bsingharora@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.

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

[Balbir Singh]

> 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 <paulus@samba.org>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
---
 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

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

[Anshuman Khandual]

On 02/17/2016 12:46 PM, Balbir Singh wrote:
>> > 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

Cant we use USER_REGION_ID directly ?

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

[Paul Mackerras]

On Wed, Feb 17, 2016 at 01:33:32PM +0530, Anshuman Khandual wrote:
> On 02/17/2016 12:46 PM, Balbir Singh wrote:
> >> > 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
> 
> Cant we use USER_REGION_ID directly ?

If we use USER_REGION_ID then the reader needs to know that the user
region is region 0 to understand the code.  Thus I think it is clearer
to use REGION_ID(bugaddr) == 0.  Whether or not the address is a user
region address is not really relevant to the question of whether it's
a physical address being accessed directly in real mode vs. a kernel
virtual address, which is what we're trying to determine.

Paul.

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

[Aneesh Kumar K.V]

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

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

[Balbir Singh]

>> 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>

<snip>

> 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.
>
>
I've tried and covered it in the changelog, I thought a code comment
would make sense for the very non  obvious cases and not repeat what
the code does as comment

Balbir Singh.

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

[Aneesh Kumar K.V]

Balbir Singh <bsingharora@gmail.com> writes:

>>> 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>
>
> <snip>
>
>> 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.
>>
>>
> I've tried and covered it in the changelog, I thought a code comment
> would make sense for the very non  obvious cases and not repeat what
> the code does as comment
>

The use of REGION_ID indicate that you are checking for region. Hence
the suggestion. Looking at this again, I suggest we should add a new
macro or will have to open code it. Because in the radix series we make
REGION_ID a hash config thing and this is generic stuff. 

-aneesh