From aee636c4809fa54848ff07a899b326eb1f9987a2 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 15 Jan 2014 06:50:07 -0800 Subject: bpf: do not use reciprocal divide 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 Reported-by: Jakub Zawadzki Cc: Mircea Gherzan Cc: Daniel Borkmann Cc: Hannes Frederic Sowa Cc: Matt Evans Cc: Martin Schwidefsky Cc: Heiko Carstens Cc: David S. Miller Signed-off-by: David S. Miller --- arch/s390/net/bpf_jit_comp.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-) (limited to 'arch/s390') 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,(%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,(%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,(%r13) */ -- cgit