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/x86/net/bpf_jit_comp.c | 14 ++++++++++---- 1 file changed, 10 insertions(+), 4 deletions(-) (limited to 'arch/x86/net') 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; -- cgit v1.2.3