net: bpf_jit: fix an off-one bug in x86_64 cond jump target

x86 jump instruction size is 2 or 5 bytes (near/long jump), not 2 or 6
bytes.

In case a conditional jump is followed by a long jump, conditional jump
target is one byte past the start of target instruction.

Signed-off-by: Markus Kötter <nepenthesdev@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index bfab3fa..7b65f75 100644 (file)
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -568,8 +568,8 @@ cond_branch:                        f_offset = addrs[i + filter[i].jf] - addrs[i];
                                        break;
                                }
                                if (filter[i].jt != 0) {
-                                       if (filter[i].jf)
-                                               t_offset += is_near(f_offset) ? 2 : 6;
+                                       if (filter[i].jf && f_offset)
+                                               t_offset += is_near(f_offset) ? 2 : 5;
                                        EMIT_COND_JMP(t_op, t_offset);
                                        if (filter[i].jf)
                                                EMIT_JMP(f_offset);