filter: add MOD operation

Add a new ALU opcode, to compute a modulus.

Commit ffe06c17afbbb used an ancillary to implement XOR_X,
but here we reserve one of the available ALU opcode to implement both
MOD_X and MOD_K

Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: George Bakos <gbakos@alpinista.org>
Cc: Jay Schulist <jschlst@samba.org>
Cc: Jiri Pirko <jpirko@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/filter.h

@@ -74,6 +74,8 @@ struct sock_fprog {	/* Required for SO_ATTACH_FILTER. */
 #define         BPF_LSH         0x60
 #define         BPF_RSH         0x70
 #define         BPF_NEG         0x80
+#define		BPF_MOD		0x90
+
 #define         BPF_JA          0x00
 #define         BPF_JEQ         0x10
 #define         BPF_JGT         0x20
@@ -196,6 +198,8 @@ enum {
 	BPF_S_ALU_MUL_K,
 	BPF_S_ALU_MUL_X,
 	BPF_S_ALU_DIV_X,
+	BPF_S_ALU_MOD_K,
+	BPF_S_ALU_MOD_X,
 	BPF_S_ALU_AND_K,
 	BPF_S_ALU_AND_X,
 	BPF_S_ALU_OR_K,

net/core/filter.c

@@ -167,6 +167,14 @@ unsigned int sk_run_filter(const struct sk_buff *skb,
 		case BPF_S_ALU_DIV_K:
 			A = reciprocal_divide(A, K);
 			continue;
+		case BPF_S_ALU_MOD_X:
+			if (X == 0)
+				return 0;
+			A %= X;
+			continue;
+		case BPF_S_ALU_MOD_K:
+			A %= K;
+			continue;
 		case BPF_S_ALU_AND_X:
 			A &= X;
 			continue;
@@ -469,6 +477,8 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
 		[BPF_ALU|BPF_MUL|BPF_K]  = BPF_S_ALU_MUL_K,
 		[BPF_ALU|BPF_MUL|BPF_X]  = BPF_S_ALU_MUL_X,
 		[BPF_ALU|BPF_DIV|BPF_X]  = BPF_S_ALU_DIV_X,
+		[BPF_ALU|BPF_MOD|BPF_K]  = BPF_S_ALU_MOD_K,
+		[BPF_ALU|BPF_MOD|BPF_X]  = BPF_S_ALU_MOD_X,
 		[BPF_ALU|BPF_AND|BPF_K]  = BPF_S_ALU_AND_K,
 		[BPF_ALU|BPF_AND|BPF_X]  = BPF_S_ALU_AND_X,
 		[BPF_ALU|BPF_OR|BPF_K]   = BPF_S_ALU_OR_K,
@@ -531,6 +541,11 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
 				return -EINVAL;
 			ftest->k = reciprocal_value(ftest->k);
 			break;
+		case BPF_S_ALU_MOD_K:
+			/* check for division by zero */
+			if (ftest->k == 0)
+				return -EINVAL;
+			break;
 		case BPF_S_LD_MEM:
 		case BPF_S_LDX_MEM:
 		case BPF_S_ST: