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Date: Wed, 15 Jan 2014 15:10:36 +0000
From: Matt Evans <matt@...abs.org>
To: Eric Dumazet <eric.dumazet@...il.com>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>,
Martin Schwidefsky <schwidefsky@...ibm.com>,
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>
Subject: Re: [PATCH v2 net] bpf: do not use reciprocal divide
Hi Eric,
On 2014-01-15 14:50, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
>
> At first Jakub Zawadzki noticed that some divisions by
> reciprocal_divide
> were not correct. (off by one in some cases)
> http://www.wireshark.org/~darkjames/reciprocal-buggy.c
>
> He could also show this with BPF:
> http://www.wireshark.org/~darkjames/set-and-dump-filter-k-bug.c
>
> The reciprocal divide in linux kernel is not generic enough,
> lets remove its use in BPF, as it is not worth the pain with
> current cpus.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Jakub Zawadzki <darkjames-ws@...kjames.pl>
> Cc: Mircea Gherzan <mgherzan@...il.com>
> Cc: Daniel Borkmann <dxchgb@...il.com>
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> Cc: Matt Evans <matt@...abs.org>
> Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
> Cc: Heiko Carstens <heiko.carstens@...ibm.com>
> Cc: David S. Miller <davem@...emloft.net>
> ---
> v2: Fixed sparc code as David kindly suggested
> Added tests on K being 1 (divide by 1 is a nop
> modulo by 1 clears A),
> as Martin Schwidefsky seems concerned by this case.
>
> Please review arch code to make sure I made no mistake, thanks !
PPC looks fine; I had a look at the core/ARM parts which also look good.
I'd forgotten where the DIV0 checking occurred, so I also benefited from
your hint to Heiko. :)
Cheers,
Matt
>
> arch/arm/net/bpf_jit_32.c | 6 +++---
> arch/powerpc/net/bpf_jit_comp.c | 7 ++++---
> arch/s390/net/bpf_jit_comp.c | 17 ++++++++++++-----
> arch/sparc/net/bpf_jit_comp.c | 17 ++++++++++++++---
> arch/x86/net/bpf_jit_comp.c | 14 ++++++++++----
> net/core/filter.c | 30 ++----------------------------
> 6 files changed, 45 insertions(+), 46 deletions(-)
>
> diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
> index 9ed155ad0f97..271b5e971568 100644
> --- a/arch/arm/net/bpf_jit_32.c
> +++ b/arch/arm/net/bpf_jit_32.c
> @@ -641,10 +641,10 @@ load_ind:
> emit(ARM_MUL(r_A, r_A, r_X), ctx);
> break;
> case BPF_S_ALU_DIV_K:
> - /* current k == reciprocal_value(userspace k) */
> + if (k == 1)
> + break;
> emit_mov_i(r_scratch, k, ctx);
> - /* A = top 32 bits of the product */
> - emit(ARM_UMULL(r_scratch, r_A, r_A, r_scratch), ctx);
> + emit_udiv(r_A, r_A, r_scratch, ctx);
> break;
> case BPF_S_ALU_DIV_X:
> update_on_xread(ctx);
> diff --git a/arch/powerpc/net/bpf_jit_comp.c
> b/arch/powerpc/net/bpf_jit_comp.c
> index ac3c2a10dafd..555034f8505e 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -223,10 +223,11 @@ static int bpf_jit_build_body(struct sk_filter
> *fp, u32 *image,
> }
> PPC_DIVWU(r_A, r_A, r_X);
> break;
> - case BPF_S_ALU_DIV_K: /* A = reciprocal_divide(A, K); */
> + case BPF_S_ALU_DIV_K: /* A /= K */
> + if (K == 1)
> + break;
> PPC_LI32(r_scratch1, K);
> - /* Top 32 bits of 64bit result -> A */
> - PPC_MULHWU(r_A, r_A, r_scratch1);
> + PPC_DIVWU(r_A, r_A, r_scratch1);
> break;
> case BPF_S_ALU_AND_X:
> ctx->seen |= SEEN_XREG;
> diff --git a/arch/s390/net/bpf_jit_comp.c
> b/arch/s390/net/bpf_jit_comp.c
> index 16871da37371..fc0fa77728e1 100644
> --- a/arch/s390/net/bpf_jit_comp.c
> +++ b/arch/s390/net/bpf_jit_comp.c
> @@ -371,11 +371,13 @@ 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 */
> + if (K == 1)
> + break;
> + /* lhi %r4,0 */
> + EMIT4(0xa7480000);
> + /* d %r4,<d(K)>(%r13) */
> + EMIT4_DISP(0x5d40d000, EMIT_CONST(K));
> break;
> case BPF_S_ALU_MOD_X: /* A %= X */
> jit->seen |= SEEN_XREG | SEEN_RET0;
> @@ -391,6 +393,11 @@ static int bpf_jit_insn(struct bpf_jit *jit,
> struct sock_filter *filter,
> EMIT2(0x1854);
> break;
> case BPF_S_ALU_MOD_K: /* A %= K */
> + if (K == 1) {
> + /* lhi %r5,0 */
> + EMIT4(0xa7580000);
> + break;
> + }
> /* lhi %r4,0 */
> EMIT4(0xa7480000);
> /* d %r4,<d(K)>(%r13) */
> diff --git a/arch/sparc/net/bpf_jit_comp.c
> b/arch/sparc/net/bpf_jit_comp.c
> index 218b6b23c378..01fe9946d388 100644
> --- a/arch/sparc/net/bpf_jit_comp.c
> +++ b/arch/sparc/net/bpf_jit_comp.c
> @@ -497,9 +497,20 @@ void bpf_jit_compile(struct sk_filter *fp)
> case BPF_S_ALU_MUL_K: /* A *= K */
> emit_alu_K(MUL, K);
> break;
> - case BPF_S_ALU_DIV_K: /* A /= K */
> - emit_alu_K(MUL, K);
> - emit_read_y(r_A);
> + case BPF_S_ALU_DIV_K: /* A /= K with K != 0*/
> + if (K == 1)
> + break;
> + emit_write_y(G0);
> +#ifdef CONFIG_SPARC32
> + /* The Sparc v8 architecture requires
> + * three instructions between a %y
> + * register write and the first use.
> + */
> + emit_nop();
> + emit_nop();
> + emit_nop();
> +#endif
> + emit_alu_K(DIV, K);
> break;
> case BPF_S_ALU_DIV_X: /* A /= X; */
> emit_cmpi(r_X, 0);
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 26328e800869..4ed75dd81d05 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -359,15 +359,21 @@ void bpf_jit_compile(struct sk_filter *fp)
> EMIT2(0x89, 0xd0); /* mov %edx,%eax */
> break;
> case BPF_S_ALU_MOD_K: /* A %= K; */
> + if (K == 1) {
> + CLEAR_A();
> + break;
> + }
> EMIT2(0x31, 0xd2); /* xor %edx,%edx */
> EMIT1(0xb9);EMIT(K, 4); /* mov imm32,%ecx */
> EMIT2(0xf7, 0xf1); /* div %ecx */
> EMIT2(0x89, 0xd0); /* mov %edx,%eax */
> break;
> - case BPF_S_ALU_DIV_K: /* A = reciprocal_divide(A, K); */
> - EMIT3(0x48, 0x69, 0xc0); /* imul imm32,%rax,%rax */
> - EMIT(K, 4);
> - EMIT4(0x48, 0xc1, 0xe8, 0x20); /* shr $0x20,%rax */
> + case BPF_S_ALU_DIV_K: /* A /= K */
> + if (K == 1)
> + break;
> + EMIT2(0x31, 0xd2); /* xor %edx,%edx */
> + EMIT1(0xb9);EMIT(K, 4); /* mov imm32,%ecx */
> + EMIT2(0xf7, 0xf1); /* div %ecx */
> break;
> case BPF_S_ALU_AND_X:
> seen |= SEEN_XREG;
> 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
> @@ -36,7 +36,6 @@
> #include <asm/uaccess.h>
> #include <asm/unaligned.h>
> #include <linux/filter.h>
> -#include <linux/reciprocal_div.h>
> #include <linux/ratelimit.h>
> #include <linux/seccomp.h>
> #include <linux/if_vlan.h>
> @@ -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;
> case BPF_S_ALU_MOD_K:
> /* check for division by zero */
> if (ftest->k == 0)
> @@ -853,27 +847,7 @@ void sk_decode_filter(struct sock_filter *filt,
> struct sock_filter *to)
> to->code = decodes[code];
> to->jt = filt->jt;
> to->jf = filt->jf;
> -
> - if (code == BPF_S_ALU_DIV_K) {
> - /*
> - * When loaded this rule user gave us X, which was
> - * translated into R = r(X). Now we calculate the
> - * RR = r(R) and report it back. If next time this
> - * value is loaded and RRR = r(RR) is calculated
> - * then the R == RRR will be true.
> - *
> - * One exception. X == 1 translates into R == 0 and
> - * we can't calculate RR out of it with r().
> - */
> -
> - if (filt->k == 0)
> - to->k = 1;
> - else
> - to->k = reciprocal_value(filt->k);
> -
> - BUG_ON(reciprocal_value(to->k) != filt->k);
> - } else
> - to->k = filt->k;
> + to->k = filt->k;
> }
>
> int sk_get_filter(struct sock *sk, struct sock_filter __user *ubuf,
> unsigned int len)
--
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