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 <edumazet@google.com>
Reported-by: Jakub Zawadzki <darkjames-ws@darkjames.pl>
Cc: Mircea Gherzan <mgherzan@gmail.com>
Cc: Daniel Borkmann <dxchgb@gmail.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Matt Evans <matt@ozlabs.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 14 insertions, 3 deletions

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);