netdev - Re: [PATCH net] bpf: do not use reciprocal divide

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Date:	Wed, 15 Jan 2014 09:00:07 +0100
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	netdev@...r.kernel.org, dborkman@...hat.com,
	darkjames-ws@...kjames.pl, Mircea Gherzan <mgherzan@...il.com>,
	Russell King <rmk+kernel@....linux.org.uk>,
	Matt Evans <matt@...abs.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH net] bpf: do not use reciprocal divide

On Tue, Jan 14, 2014 at 11:02:41PM -0800, Eric Dumazet wrote:
> diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
> index 16871da37371..e349dc7d0992 100644
> --- a/arch/s390/net/bpf_jit_comp.c
> +++ b/arch/s390/net/bpf_jit_comp.c
> @@ -371,11 +371,11 @@ static int bpf_jit_insn(struct bpf_jit *jit, struct sock_filter *filter,
>  		/* dr %r4,%r12 */
>  		EMIT2(0x1d4c);
>  		break;
> -	case BPF_S_ALU_DIV_K: /* A = reciprocal_divide(A, K) */
> -		/* m %r4,<d(K)>(%r13) */
> -		EMIT4_DISP(0x5c40d000, EMIT_CONST(K));
> -		/* lr %r5,%r4 */
> -		EMIT2(0x1854);
> +	case BPF_S_ALU_DIV_K: /* A /= K */
> +		/* lhi %r4,0 */
> +		EMIT4(0xa7480000);
> +		/* d %r4,<d(K)>(%r13) */
> +		EMIT4_DISP(0x5d40d000, EMIT_CONST(K));
>  		break;

The s390 part looks good.

> diff --git a/net/core/filter.c b/net/core/filter.c
> index 01b780856db2..ad30d626a5bd 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -166,7 +165,7 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
>  			A /= X;
>  			continue;
>  		case BPF_S_ALU_DIV_K:
> -			A = reciprocal_divide(A, K);
> +			A /= K;
>  			continue;
>  		case BPF_S_ALU_MOD_X:
>  			if (X == 0)
> @@ -553,11 +552,6 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>  		/* Some instructions need special checks */
>  		switch (code) {
>  		case BPF_S_ALU_DIV_K:
> -			/* check for division by zero */
> -			if (ftest->k == 0)
> -				return -EINVAL;
> -			ftest->k = reciprocal_value(ftest->k);
> -			break;

Are you sure you want to remove the k == 0 check? Is there something
else that would prevent a division by zero?

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