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SubjectRe: Re:Re : [PATCH] powerpc: Fix a bug in __div64_32 if divisor is zero
On Mon, Aug 24, 2020 at 07:54:07PM +0800, Guohua Zhong wrote:
> >> 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

Just "beqlr".

This instruction scheduling hurts all CPUs that aren't 8xx, fwiw (but
likely only in the case where r4 *is* zero, so who cares :-) )

So... What is the *goal* of this patch? It looks like the routine
would not get into a loop if r4 is 0, just return the wrong result?
But, it *always* will, there *is* no right result?

No caller should call it with zero as divisor ever, so in that sense,
checking for it in the division routine is just pure wasted work.


Segher

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