powerpc/bpf: Fix BPF_SUB when imm == 0x80000000

We aren't handling subtraction involving an immediate value of
0x80000000 properly. Fix the same.

Fixes: 156d0e29 ("powerpc/ebpf/jit: Implement JIT compiler for extended BPF")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Fold in fix from Naveen to use imm <= 32768]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/fc4b1276eb10761fd7ce0814c8dd089da2815251.1633464148.git.naveen.n.rao@linux.vnet.ibm.com

arch/powerpc/net/bpf_jit_comp64.c

@@ -330,18 +330,25 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 			EMIT(PPC_RAW_SUB(dst_reg, dst_reg, src_reg));
 			goto bpf_alu32_trunc;
 		case BPF_ALU | BPF_ADD | BPF_K: /* (u32) dst += (u32) imm */
-		case BPF_ALU | BPF_SUB | BPF_K: /* (u32) dst -= (u32) imm */
 		case BPF_ALU64 | BPF_ADD | BPF_K: /* dst += imm */
+			if (!imm) {
+				goto bpf_alu32_trunc;
+			} else if (imm >= -32768 && imm < 32768) {
+				EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(imm)));
+			} else {
+				PPC_LI32(b2p[TMP_REG_1], imm);
+				EMIT(PPC_RAW_ADD(dst_reg, dst_reg, b2p[TMP_REG_1]));
+			}
+			goto bpf_alu32_trunc;
+		case BPF_ALU | BPF_SUB | BPF_K: /* (u32) dst -= (u32) imm */
 		case BPF_ALU64 | BPF_SUB | BPF_K: /* dst -= imm */
-			if (BPF_OP(code) == BPF_SUB)
-				imm = -imm;
-			if (imm) {
-				if (imm >= -32768 && imm < 32768)
-					EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(imm)));
-				else {
-					PPC_LI32(b2p[TMP_REG_1], imm);
-					EMIT(PPC_RAW_ADD(dst_reg, dst_reg, b2p[TMP_REG_1]));
-				}
+			if (!imm) {
+				goto bpf_alu32_trunc;
+			} else if (imm > -32768 && imm <= 32768) {
+				EMIT(PPC_RAW_ADDI(dst_reg, dst_reg, IMM_L(-imm)));
+			} else {
+				PPC_LI32(b2p[TMP_REG_1], imm);
+				EMIT(PPC_RAW_SUB(dst_reg, dst_reg, b2p[TMP_REG_1]));
 			}
 			goto bpf_alu32_trunc;
 		case BPF_ALU | BPF_MUL | BPF_X: /* (u32) dst *= (u32) src */