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Date: Tue, 14 Jan 2014 23:02:41 -0800
From: Eric Dumazet <eric.dumazet@...il.com>
To: Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc: 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>,
Heiko Carstens <heiko.carstens@...ibm.com>
Subject: [PATCH net] bpf: do not use reciprocal divide
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>
---
Please review arch code to make sure I made no mistake, thanks !
arch/arm/net/bpf_jit_32.c | 4 +---
arch/powerpc/net/bpf_jit_comp.c | 5 ++---
arch/s390/net/bpf_jit_comp.c | 10 +++++-----
arch/sparc/net/bpf_jit_comp.c | 3 +--
arch/x86/net/bpf_jit_comp.c | 8 ++++----
net/core/filter.c | 30 ++----------------------------
6 files changed, 15 insertions(+), 45 deletions(-)
diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
index 9ed155ad0f97..81f2b6d37b06 100644
--- a/arch/arm/net/bpf_jit_32.c
+++ b/arch/arm/net/bpf_jit_32.c
@@ -641,10 +641,8 @@ 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) */
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..8044ddc6b320 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -223,10 +223,9 @@ 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 */
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..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;
case BPF_S_ALU_MOD_X: /* A %= X */
jit->seen |= SEEN_XREG | SEEN_RET0;
diff --git a/arch/sparc/net/bpf_jit_comp.c b/arch/sparc/net/bpf_jit_comp.c
index 218b6b23c378..125045063b91 100644
--- a/arch/sparc/net/bpf_jit_comp.c
+++ b/arch/sparc/net/bpf_jit_comp.c
@@ -498,8 +498,7 @@ void bpf_jit_compile(struct sk_filter *fp)
emit_alu_K(MUL, K);
break;
case BPF_S_ALU_DIV_K: /* A /= K */
- emit_alu_K(MUL, K);
- emit_read_y(r_A);
+ 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..f3d1f54de012 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -364,10 +364,10 @@ void bpf_jit_compile(struct sk_filter *fp)
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 */
+ 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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