linux-kernel - Re: Re：Re: [PATCH] powerpc: Fix a bug in __div64

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Message-ID: <20200824115407.55896-1-zhongguohua1@huawei.com>
Date:   Mon, 24 Aug 2020 19:54:07 +0800
From:   Guohua Zhong <zhongguohua1@...wei.com>
To:     <segher@...nel.crashing.org>
CC:     <christophe.leroy@...roup.eu>, <gregkh@...uxfoundation.org>,
        <linux-kernel@...r.kernel.org>, <linuxppc-dev@...ts.ozlabs.org>,
        <nixiaoming@...wei.com>, <paulus@...ba.org>,
        <stable@...r.kernel.org>, <wangle6@...wei.com>,
        <zhongguohua1@...wei.com>
Subject: 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 <asm/processor.h>

 _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