Re: Re：Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
>> >In generic version in lib/math/div64.c, there is no checking of 'base'
>> >either.
>> >Do we really want to add this check in the powerpc version only ?
>>
>> >The only user of __div64_32() is do_div() in
>> >include/asm-generic/div64.h. Wouldn't it be better to do the check there ?
>>
>> >Christophe
>>
>> Yet, I have noticed that there is no checking of 'base' in these functions.
>> But I am not sure how to check is better.As we know that the result is
>> undefined when divisor is zero. It maybe good to print error and dump stack.
>> Let the process to know that the divisor is zero by sending SIGFPE.
>>
>> diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
>> index a3b98c86f077..161c656ee3ee 100644
>> --- a/include/asm-generic/div64.h
>> +++ b/include/asm-generic/div64.h
>> @@ -43,6 +43,11 @@
>> # define do_div(n,base) ({ \
>> uint32_t __base = (base); \
>> uint32_t __rem; \
>> + if (unlikely(base == 0)) { \
>> + pr_err("do_div base=%d\n",base);\
>> + dump_stack(); \
>> + force_sig(SIGFPE); \
>> + }
>>
> I suspect this will generate a strong reaction. SIGFPE is for user space
> instruction attempting a division by zero. A division by zero in the
> kernel is a kernel bug, period, and you don't want to kill a user
> process for this reason.
> If it happens in an interrupt, the context of the kernel may not even be
> related to the current process.
> Many other architectures (x86 for example) already trigger an exception
> on a division by zero but the handler will find that the exception
> happened in kernel context and generate an Oops, not raise a signal in a
> (possibly innocent) userland process.
OK. So just don't touch do_div functions in include/asm-generic/div64.h
But for powerpc it can not trigger an exception when divisor is 0 in __div64_32.
So the patch as below is still useful for powerpc. If this patch looks good for
you, please help to review. I will send the new patch later.
Thanks for your reply.
diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S
index 4354928ed62e..1d3561cf16fa 100644
--- a/arch/powerpc/boot/div64.S
+++ b/arch/powerpc/boot/div64.S
@@ -13,8 +13,10 @@
.globl __div64_32
.globl __div64_32
__div64_32:
+ cmplwi r4,0# check if divisor r4 is zero
lwz r5,0(r3)# get the dividend into r5/r6
lwz r6,4(r3)
+ beq 5f # jump to label 5 if r4(divisor) is zero
cmplw r5,r4
li r7,0
li r8,0
@@ -52,7 +54,7 @@ __div64_32:
4: stw r7,0(r3)# return the quotient in *r3
stw r8,4(r3)
mr r3,r6 # return the remainder in r3
- blr
+5: blr # return if divisor r4 is zero
/*
* Extended precision shifts.
diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
index 3d5426e7dcc4..570774d9782d 100644
--- a/arch/powerpc/lib/div64.S
+++ b/arch/powerpc/lib/div64.S
@@ -13,8 +13,10 @@
#include
_GLOBAL(__div64_32)
+ cmplwi r4,0# check if divisor r4 is zero
lwz r5,0(r3)# get the dividend into r5/r6
lwz r6,4(r3)
+ beq 5f # jump to label 5 if r4(divisor) is zero
cmplw r5,r4
li r7,0
li r8,0
@@ -52,4 +54,4 @@ _GLOBAL(__div64_32)
4: stw r7,0(r3)# return the quotient in *r3
stw r8,4(r3)
mr r3,r6 # return the remainder in r3
- blr
+5: blr # return if divisor r4 is zero
Guohua
Re: Re：Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
>> Yet, I have noticed that there is no checking of 'base' in these functions. >> But I am not sure how to check is better.As we know that the result is >> undefined when divisor is zero. It maybe good to print error and dump stack. >> Let the process to know that the divisor is zero by sending SIGFPE. > That is now what the PowerPC integer divide insns do: they just leave > the result undefined (and they can set the overflow flag then, but no > one uses that). OK ,So just keep the patch as below. If this patch looks good for you, please help to review. I will send the new patch later. Thanks for your reply. diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S index 4354928ed62e..1d3561cf16fa 100644 --- a/arch/powerpc/boot/div64.S +++ b/arch/powerpc/boot/div64.S @@ -13,8 +13,10 @@ .globl __div64_32 .globl __div64_32 __div64_32: + cmplwi r4,0# check if divisor r4 is zero lwz r5,0(r3)# get the dividend into r5/r6 lwz r6,4(r3) + beq 5f # jump to label 5 if r4(divisor) is zero cmplw r5,r4 li r7,0 li r8,0 @@ -52,7 +54,7 @@ __div64_32: 4: stw r7,0(r3)# return the quotient in *r3 stw r8,4(r3) mr r3,r6 # return the remainder in r3 - blr +5: blr # return if divisor r4 is zero /* * Extended precision shifts. diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S index 3d5426e7dcc4..570774d9782d 100644 --- a/arch/powerpc/lib/div64.S +++ b/arch/powerpc/lib/div64.S @@ -13,8 +13,10 @@ #include _GLOBAL(__div64_32) + cmplwi r4,0# check if divisor r4 is zero lwz r5,0(r3)# get the dividend into r5/r6 lwz r6,4(r3) + beq 5f # jump to label 5 if r4(divisor) is zero cmplw r5,r4 li r7,0 li r8,0 @@ -52,4 +54,4 @@ _GLOBAL(__div64_32) 4: stw r7,0(r3)# return the quotient in *r3 stw r8,4(r3) mr r3,r6 # return the remainder in r3 - blr +5: blr # return if divisor r4 is zero Guohua
Re：Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
>In generic version in lib/math/div64.c, there is no checking of 'base'
>either.
>Do we really want to add this check in the powerpc version only ?
>The only user of __div64_32() is do_div() in
>include/asm-generic/div64.h. Wouldn't it be better to do the check there ?
>Christophe
Yet, I have noticed that there is no checking of 'base' in these functions.
But I am not sure how to check is better.As we know that the result is
undefined when divisor is zero. It maybe good to print error and dump stack.
Let the process to know that the divisor is zero by sending SIGFPE.
diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
index a3b98c86f077..161c656ee3ee 100644
--- a/include/asm-generic/div64.h
+++ b/include/asm-generic/div64.h
@@ -43,6 +43,11 @@
# define do_div(n,base) ({ \
uint32_t __base = (base); \
uint32_t __rem; \
+ if (unlikely(base == 0)) { \
+ pr_err("do_div base=%d\n",base);\
+ dump_stack(); \
+ force_sig(SIGFPE); \
+ }
Then it also needto add this checking in functions of
div64_s64(), div64_u64(), div64_u64_rem(), div_s64_rem and div_u64_rem ()
in include/linux/math64.h
+ if (unlikely(divisor == 0)) {
+ pr_err("%s divisor=0\n",__func__);
+ dump_stack();
+ force_sig(SIGFPE);
+ }
Guohua
>> lwz r5,0(r3)# get the dividend into r5/r6
>> lwz r6,4(r3)
>> cmplw r5,r4
>>@@ -52,6 +55,7 @@ __div64_32:
>> 4: stw r7,0(r3)# return the quotient in *r3
>> stw r8,4(r3)
>> mr r3,r6 # return the remainder in r3
>>+5: # return if divisor r4 is zero
>> blr
>>
>> /*
>>diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
>>index 3d5426e7dcc4..1cc9bcabf678 100644
>>--- a/arch/powerpc/lib/div64.S
>>+++ b/arch/powerpc/lib/div64.S
>>@@ -13,6 +13,9 @@
>> #include
>>
>> _GLOBAL(__div64_32)
>>+ li r9,0
>>+ cmplw r4,r9 # check if divisor r4 is zero
>>+ beq 5f # jump to label 5 if r4(divisor) is zero
>> lwz r5,0(r3)# get the dividend into r5/r6
>> lwz r6,4(r3)
>> cmplw r5,r4
>>@@ -52,4 +55,5 @@ _GLOBAL(__div64_32)
>> 4: stw r7,0(r3)# return the quotient in *r3
>> stw r8,4(r3)
>> mr r3,r6 # return the remainder in r3
>>+5: # return if divisor r4 is zero
>> blr
>>
Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
>> When cat /proc/pid/stat, do_task_stat will call into cputime_adjust,
>> which call stack is like this:
>>
>> [17179954.674326]BookE Watchdog detected hard LOCKUP on cpu 0
>> [17179954.674331]dCPU: 0 PID: 1262 Comm: TICK Tainted: PW O
>> 4.4.176 #1
>> [17179954.674339]dtask: dc9d7040 task.stack: d3cb4000
>> [17179954.674344]NIP: c001b1a8 LR: c006a7ac CTR:
>> [17179954.674349]REGS: e6fe1f10 TRAP: 3202 Tainted: PW O
>> (4.4.176)
>> [17179954.674355]MSR: 00021002 CR: 28002224 XER:
>> [17179954.674364]
>> GPR00: 0016 d3cb5cb0 dc9d7040 d3cb5cc0 025d ffe15b24
>>
>> GPR08: de86aead 03ff 2800 0084d1c0
>>
>> GPR16: b5929ca0 b4bb7a48 c0863c08 048d 0062 0062
>> 000f
>> GPR24: d3cb5d08 d3cb5d60 d3cb5d64 00029002 d3e9c214 f30e
>> d3e9c20c
>> [17179954.674410]NIP [c001b1a8] __div64_32+0x60/0xa0
>> [17179954.674422]LR [c006a7ac] cputime_adjust+0x124/0x138
>> [17179954.674434]Call Trace:
>> [17179961.832693]Call Trace:
>> [17179961.832695][d3cb5cb0] [c006a6dc] cputime_adjust+0x54/0x138 (unreliable)
>> [17179961.832705][d3cb5cf0] [c006a818] task_cputime_adjusted+0x58/0x80
>> [17179961.832713][d3cb5d20] [c01dab44] do_task_stat+0x298/0x870
>> [17179961.832720][d3cb5de0] [c01d4948] proc_single_show+0x60/0xa4
>> [17179961.832728][d3cb5e10] [c01963d8] seq_read+0x2d8/0x52c
>> [17179961.832736][d3cb5e80] [c01702fc] __vfs_read+0x40/0x114
>> [17179961.832744][d3cb5ef0] [c0170b1c] vfs_read+0x9c/0x10c
>> [17179961.832751][d3cb5f10] [c0171440] SyS_read+0x68/0xc4
>> [17179961.832759][d3cb5f40] [c0010a40] ret_from_syscall+0x0/0x3c
>>
>> do_task_stat->task_cputime_adjusted->cputime_adjust->scale_stime->div_u64
>> ->div_u64_rem->do_div->__div64_32
>>
>> In some corner case, stime + utime = 0 if overflow. Even in v5.8.2 kernel
>> the cputime has changed from unsigned long to u64 data type. About 200
>> days, the lowwer 32 bit will be 0x. Because divisor for __div64_32
>> is unsigned long data type,which is 32 bit for powepc 32, the bug still
>> exists.
>>
>> So it is also a bug in the cputime_adjust which does not check if
>> stime + utime = 0
>>
>> time = scale_stime((__force u64)stime, (__force u64)rtime,
>> (__force u64)(stime + utime));
>>
>> The commit 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()") in
>> mainline kernel may has fixed this case. But it is also better to check
>> if divisor is 0 in __div64_32 for other situation.
>>
>> Signed-off-by: Guohua Zhong
>> Fixes:14cf11af6cf6 "( powerpc: Merge enough to start building in
>> arch/powerpc.)"
>> Fixes:94b212c29f68 "( powerpc: Move ppc64 boot wrapper code over to
>> arch/powerpc)"
>> Cc: sta...@vger.kernel.org # v2.6.15+
>> ---
>> arch/powerpc/boot/div64.S | 4
>> arch/powerpc/lib/div64.S | 4
>> 2 files changed, 8 insertions(+)
>>
>> diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S
>> index 4354928ed62e..39a25b9712d1 100644
>> --- a/arch/powerpc/boot/div64.S
>> +++ b/arch/powerpc/boot/div64.S
>> @@ -13,6 +13,9 @@
>>
>> .globl __div64_32
>> __div64_32:
>> +li r9,0
>> +cmplw r4,r9 # check if divisor r4 is zero
>> +beq 5f # jump to label 5 if r4(divisor) is zero
>> lwz r5,0(r3)# get the dividend into r5/r6
>> lwz r6,4(r3)
>> cmplw r5,r4
>> @@ -52,6 +55,7 @@ __div64_32:
>> 4: stw r7,0(r3)# return the quotient in *r3
>> stw r8,4(r3)
>> mr r3,r6 # return the remainder in r3
>> +5: # return if divisor r4 is zero
>> blr
>>
>> /*
>> diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
>> index 3d5426e7dcc4..1cc9bcabf678 100644
>> --- a/arch/powerpc/lib/div64.S
>> +++ b/arch/powerpc/lib/div64.S
>> @@ -13,6 +13,9 @@
>> #include
>>
>> _GLOBAL(__div64_32)
>> +li r9,0
>You don't need to load r9 with 0, use cmplwi instead.
I will change cmplw to cmplwi in next patch as your suggestion. Thanks
>> +cmplw r4,r9 # check if divisor r4 is zero
>> +beq 5f # jump to label 5 if r4(divisor) is zero
>You should leave space between the compare and the branch (i.e. have
>other instructions
Re: [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
>> When cat /proc/pid/stat, do_task_stat will call into cputime_adjust,
>> which call stack is like this:
>>
>> [17179954.674326]BookE Watchdog detected hard LOCKUP on cpu 0
>> [17179954.674331]dCPU: 0 PID: 1262 Comm: TICK Tainted: PW O
>> 4.4.176 #1
>> [17179954.674339]dtask: dc9d7040 task.stack: d3cb4000
>> [17179954.674344]NIP: c001b1a8 LR: c006a7ac CTR:
>> [17179954.674349]REGS: e6fe1f10 TRAP: 3202 Tainted: PW O
>> (4.4.176)
>> [17179954.674355]MSR: 00021002 CR: 28002224 XER:
>> [17179954.674364]
>> GPR00: 0016 d3cb5cb0 dc9d7040 d3cb5cc0 025d ffe15b24
>>
>> GPR08: de86aead 03ff 2800 0084d1c0
>>
>> GPR16: b5929ca0 b4bb7a48 c0863c08 048d 0062 0062
>> 000f
>> GPR24: d3cb5d08 d3cb5d60 d3cb5d64 00029002 d3e9c214 f30e
>> d3e9c20c
>> [17179954.674410]NIP [c001b1a8] __div64_32+0x60/0xa0
>> [17179954.674422]LR [c006a7ac] cputime_adjust+0x124/0x138
>> [17179954.674434]Call Trace:
>> [17179961.832693]Call Trace:
>> [17179961.832695][d3cb5cb0] [c006a6dc] cputime_adjust+0x54/0x138 (unreliable)
>> [17179961.832705][d3cb5cf0] [c006a818] task_cputime_adjusted+0x58/0x80
>> [17179961.832713][d3cb5d20] [c01dab44] do_task_stat+0x298/0x870
>> [17179961.832720][d3cb5de0] [c01d4948] proc_single_show+0x60/0xa4
>> [17179961.832728][d3cb5e10] [c01963d8] seq_read+0x2d8/0x52c
>> [17179961.832736][d3cb5e80] [c01702fc] __vfs_read+0x40/0x114
>> [17179961.832744][d3cb5ef0] [c0170b1c] vfs_read+0x9c/0x10c
>> [17179961.832751][d3cb5f10] [c0171440] SyS_read+0x68/0xc4
>> [17179961.832759][d3cb5f40] [c0010a40] ret_from_syscall+0x0/0x3c
>>
>> do_task_stat->task_cputime_adjusted->cputime_adjust->scale_stime->div_u64
>> ->div_u64_rem->do_div->__div64_32
>>
>> In some corner case, stime + utime = 0 if overflow. Even in v5.8.2 kernel
>> the cputime has changed from unsigned long to u64 data type. About 200
>> days, the lowwer 32 bit will be 0x. Because divisor for __div64_32
>> is unsigned long data type,which is 32 bit for powepc 32, the bug still
>> exists.
>>
>> So it is also a bug in the cputime_adjust which does not check if
>> stime + utime = 0
>>
>> time = scale_stime((__force u64)stime, (__force u64)rtime,
>> (__force u64)(stime + utime));
>>
>> The commit 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()") in
>> mainline kernel may has fixed this case. But it is also better to check
>> if divisor is 0 in __div64_32 for other situation.
>>
>> Signed-off-by: Guohua Zhong
>> Fixes:14cf11af6cf6 "( powerpc: Merge enough to start building in
>> arch/powerpc.)"
>> Fixes:94b212c29f68 "( powerpc: Move ppc64 boot wrapper code over to
>> arch/powerpc)"
>> Cc: sta...@vger.kernel.org # v2.6.15+
>> ---
>> arch/powerpc/boot/div64.S | 4
>> arch/powerpc/lib/div64.S | 4
>> 2 files changed, 8 insertions(+)
>>
>> diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S
>> index 4354928ed62e..39a25b9712d1 100644
>> --- a/arch/powerpc/boot/div64.S
>> +++ b/arch/powerpc/boot/div64.S
>> @@ -13,6 +13,9 @@
>>
>> .globl __div64_32
>> __div64_32:
>> +li r9,0
>> +cmplw r4,r9 # check if divisor r4 is zero
>> +beq 5f # jump to label 5 if r4(divisor) is zero
>> lwz r5,0(r3)# get the dividend into r5/r6
>> lwz r6,4(r3)
>> cmplw r5,r4
>> @@ -52,6 +55,7 @@ __div64_32:
>> 4: stw r7,0(r3)# return the quotient in *r3
>> stw r8,4(r3)
>> mr r3,r6 # return the remainder in r3
>> +5: # return if divisor r4 is zero
>> blr
>>
>> /*
>> diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
>> index 3d5426e7dcc4..1cc9bcabf678 100644
>> --- a/arch/powerpc/lib/div64.S
>> +++ b/arch/powerpc/lib/div64.S
>> @@ -13,6 +13,9 @@
>> #include
>>
>> _GLOBAL(__div64_32)
>> +li r9,0
>You don't need to load r9 with 0, use cmplwi instead.
I will change cmplw to cmplwi in next patch as your suggestion. Thanks
>> +cmplw r4,r9 # check if divisor r4 is zero
>> +beq 5f # jump to label 5 if r4(divisor) is zero
>You should leave space between the compare and the branch (i.e. have
>other instructions
[PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
When cat /proc/pid/stat, do_task_stat will call into cputime_adjust,
which call stack is like this:
[17179954.674326]BookE Watchdog detected hard LOCKUP on cpu 0
[17179954.674331]dCPU: 0 PID: 1262 Comm: TICK Tainted: PW O4.4.176
#1
[17179954.674339]dtask: dc9d7040 task.stack: d3cb4000
[17179954.674344]NIP: c001b1a8 LR: c006a7ac CTR:
[17179954.674349]REGS: e6fe1f10 TRAP: 3202 Tainted: PW O
(4.4.176)
[17179954.674355]MSR: 00021002 CR: 28002224 XER:
[17179954.674364]
GPR00: 0016 d3cb5cb0 dc9d7040 d3cb5cc0 025d ffe15b24
GPR08: de86aead 03ff 2800 0084d1c0
GPR16: b5929ca0 b4bb7a48 c0863c08 048d 0062 0062 000f
GPR24: d3cb5d08 d3cb5d60 d3cb5d64 00029002 d3e9c214 f30e d3e9c20c
[17179954.674410]NIP [c001b1a8] __div64_32+0x60/0xa0
[17179954.674422]LR [c006a7ac] cputime_adjust+0x124/0x138
[17179954.674434]Call Trace:
[17179961.832693]Call Trace:
[17179961.832695][d3cb5cb0] [c006a6dc] cputime_adjust+0x54/0x138 (unreliable)
[17179961.832705][d3cb5cf0] [c006a818] task_cputime_adjusted+0x58/0x80
[17179961.832713][d3cb5d20] [c01dab44] do_task_stat+0x298/0x870
[17179961.832720][d3cb5de0] [c01d4948] proc_single_show+0x60/0xa4
[17179961.832728][d3cb5e10] [c01963d8] seq_read+0x2d8/0x52c
[17179961.832736][d3cb5e80] [c01702fc] __vfs_read+0x40/0x114
[17179961.832744][d3cb5ef0] [c0170b1c] vfs_read+0x9c/0x10c
[17179961.832751][d3cb5f10] [c0171440] SyS_read+0x68/0xc4
[17179961.832759][d3cb5f40] [c0010a40] ret_from_syscall+0x0/0x3c
do_task_stat->task_cputime_adjusted->cputime_adjust->scale_stime->div_u64
->div_u64_rem->do_div->__div64_32
In some corner case, stime + utime = 0 if overflow. Even in v5.8.2 kernel
the cputime has changed from unsigned long to u64 data type. About 200
days, the lowwer 32 bit will be 0x. Because divisor for __div64_32
is unsigned long data type,which is 32 bit for powepc 32, the bug still
exists.
So it is also a bug in the cputime_adjust which does not check if
stime + utime = 0
time = scale_stime((__force u64)stime, (__force u64)rtime,
(__force u64)(stime + utime));
The commit 3dc167ba5729 ("sched/cputime: Improve cputime_adjust()") in
mainline kernel may has fixed this case. But it is also better to check
if divisor is 0 in __div64_32 for other situation.
Signed-off-by: Guohua Zhong
Fixes:14cf11af6cf6 "( powerpc: Merge enough to start building in arch/powerpc.)"
Fixes:94b212c29f68 "( powerpc: Move ppc64 boot wrapper code over to
arch/powerpc)"
Cc: sta...@vger.kernel.org # v2.6.15+
---
arch/powerpc/boot/div64.S | 4
arch/powerpc/lib/div64.S | 4
2 files changed, 8 insertions(+)
diff --git a/arch/powerpc/boot/div64.S b/arch/powerpc/boot/div64.S
index 4354928ed62e..39a25b9712d1 100644
--- a/arch/powerpc/boot/div64.S
+++ b/arch/powerpc/boot/div64.S
@@ -13,6 +13,9 @@
.globl __div64_32
__div64_32:
+ li r9,0
+ cmplw r4,r9 # check if divisor r4 is zero
+ beq 5f # jump to label 5 if r4(divisor) is zero
lwz r5,0(r3)# get the dividend into r5/r6
lwz r6,4(r3)
cmplw r5,r4
@@ -52,6 +55,7 @@ __div64_32:
4: stw r7,0(r3)# return the quotient in *r3
stw r8,4(r3)
mr r3,r6 # return the remainder in r3
+5: # return if divisor r4 is zero
blr
/*
diff --git a/arch/powerpc/lib/div64.S b/arch/powerpc/lib/div64.S
index 3d5426e7dcc4..1cc9bcabf678 100644
--- a/arch/powerpc/lib/div64.S
+++ b/arch/powerpc/lib/div64.S
@@ -13,6 +13,9 @@
#include
_GLOBAL(__div64_32)
+ li r9,0
+ cmplw r4,r9 # check if divisor r4 is zero
+ beq 5f # jump to label 5 if r4(divisor) is zero
lwz r5,0(r3)# get the dividend into r5/r6
lwz r6,4(r3)
cmplw r5,r4
@@ -52,4 +55,5 @@ _GLOBAL(__div64_32)
4: stw r7,0(r3)# return the quotient in *r3
stw r8,4(r3)
mr r3,r6 # return the remainder in r3
+5: # return if divisor r4 is zero
blr
--
2.12.3
