arm64: bpf: fix mod-by-zero case

Turns out in the case of modulo by zero in a BPF program:
	A = A % X;  (X == 0)
the expected behavior is to terminate with return value 0.

The bug in JIT is exposed by a new test case [1].

[1] https://lkml.org/lkml/2015/11/4/499

Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
Reported-by: Yang Shi <yang.shi@linaro.org>
Reported-by: Xi Wang <xi.wang@gmail.com>
CC: Alexei Starovoitov <ast@plumgrid.com>
Fixes: e54bcde3d6 ("arm64: eBPF JIT compiler")
Cc: <stable@vger.kernel.org> # 3.18+
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/net/bpf_jit_comp.c

@@ -269,6 +269,8 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		break;
 	case BPF_ALU | BPF_DIV | BPF_X:
 	case BPF_ALU64 | BPF_DIV | BPF_X:
+	case BPF_ALU | BPF_MOD | BPF_X:
+	case BPF_ALU64 | BPF_MOD | BPF_X:
 	{
 		const u8 r0 = bpf2a64[BPF_REG_0];
 
@@ -281,16 +283,19 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx)
 		check_imm26(jmp_offset);
 		emit(A64_B(jmp_offset), ctx);
 		/* else */
-		emit(A64_UDIV(is64, dst, dst, src), ctx);
+		switch (BPF_OP(code)) {
+		case BPF_DIV:
+			emit(A64_UDIV(is64, dst, dst, src), ctx);
+			break;
+		case BPF_MOD:
+			ctx->tmp_used = 1;
+			emit(A64_UDIV(is64, tmp, dst, src), ctx);
+			emit(A64_MUL(is64, tmp, tmp, src), ctx);
+			emit(A64_SUB(is64, dst, dst, tmp), ctx);
+			break;
+		}
 		break;
 	}
-	case BPF_ALU | BPF_MOD | BPF_X:
-	case BPF_ALU64 | BPF_MOD | BPF_X:
-		ctx->tmp_used = 1;
-		emit(A64_UDIV(is64, tmp, dst, src), ctx);
-		emit(A64_MUL(is64, tmp, tmp, src), ctx);
-		emit(A64_SUB(is64, dst, dst, tmp), ctx);
-		break;
 	case BPF_ALU | BPF_LSH | BPF_X:
 	case BPF_ALU64 | BPF_LSH | BPF_X:
 		emit(A64_LSLV(is64, dst, dst, src), ctx);