s390/bpf: Fix optimizing out zero-extensions

Currently the JIT completely removes things like `reg32 += 0`,
however, the BPF_ALU semantics requires the target register to be
zero-extended in such cases.

Fix by optimizing out only the arithmetic operation, but not the
subsequent zero-extension.
Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Fixes: 05462310 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/net/bpf_jit_comp.c

@@ -760,10 +760,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT4(0xb9080000, dst_reg, src_reg);
 		break;
 	case BPF_ALU | BPF_ADD | BPF_K: /* dst = (u32) dst + (u32) imm */
-		if (!imm)
-			break;
-		/* alfi %dst,imm */
-		EMIT6_IMM(0xc20b0000, dst_reg, imm);
+		if (imm != 0) {
+			/* alfi %dst,imm */
+			EMIT6_IMM(0xc20b0000, dst_reg, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_ADD | BPF_K: /* dst = dst + imm */
@@ -785,10 +785,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT4(0xb9090000, dst_reg, src_reg);
 		break;
 	case BPF_ALU | BPF_SUB | BPF_K: /* dst = (u32) dst - (u32) imm */
-		if (!imm)
-			break;
-		/* alfi %dst,-imm */
-		EMIT6_IMM(0xc20b0000, dst_reg, -imm);
+		if (imm != 0) {
+			/* alfi %dst,-imm */
+			EMIT6_IMM(0xc20b0000, dst_reg, -imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_SUB | BPF_K: /* dst = dst - imm */
@@ -815,10 +815,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT4(0xb90c0000, dst_reg, src_reg);
 		break;
 	case BPF_ALU | BPF_MUL | BPF_K: /* dst = (u32) dst * (u32) imm */
-		if (imm == 1)
-			break;
-		/* msfi %r5,imm */
-		EMIT6_IMM(0xc2010000, dst_reg, imm);
+		if (imm != 1) {
+			/* msfi %r5,imm */
+			EMIT6_IMM(0xc2010000, dst_reg, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_MUL | BPF_K: /* dst = dst * imm */
@@ -871,6 +871,8 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 			if (BPF_OP(insn->code) == BPF_MOD)
 				/* lhgi %dst,0 */
 				EMIT4_IMM(0xa7090000, dst_reg, 0);
+			else
+				EMIT_ZERO(dst_reg);
 			break;
 		}
 		/* lhi %w0,0 */
@@ -1003,10 +1005,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT4(0xb9820000, dst_reg, src_reg);
 		break;
 	case BPF_ALU | BPF_XOR | BPF_K: /* dst = (u32) dst ^ (u32) imm */
-		if (!imm)
-			break;
-		/* xilf %dst,imm */
-		EMIT6_IMM(0xc0070000, dst_reg, imm);
+		if (imm != 0) {
+			/* xilf %dst,imm */
+			EMIT6_IMM(0xc0070000, dst_reg, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_XOR | BPF_K: /* dst = dst ^ imm */
@@ -1037,10 +1039,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT6_DISP_LH(0xeb000000, 0x000d, dst_reg, dst_reg, src_reg, 0);
 		break;
 	case BPF_ALU | BPF_LSH | BPF_K: /* dst = (u32) dst << (u32) imm */
-		if (imm == 0)
-			break;
-		/* sll %dst,imm(%r0) */
-		EMIT4_DISP(0x89000000, dst_reg, REG_0, imm);
+		if (imm != 0) {
+			/* sll %dst,imm(%r0) */
+			EMIT4_DISP(0x89000000, dst_reg, REG_0, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_LSH | BPF_K: /* dst = dst << imm */
@@ -1062,10 +1064,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT6_DISP_LH(0xeb000000, 0x000c, dst_reg, dst_reg, src_reg, 0);
 		break;
 	case BPF_ALU | BPF_RSH | BPF_K: /* dst = (u32) dst >> (u32) imm */
-		if (imm == 0)
-			break;
-		/* srl %dst,imm(%r0) */
-		EMIT4_DISP(0x88000000, dst_reg, REG_0, imm);
+		if (imm != 0) {
+			/* srl %dst,imm(%r0) */
+			EMIT4_DISP(0x88000000, dst_reg, REG_0, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_RSH | BPF_K: /* dst = dst >> imm */
@@ -1087,10 +1089,10 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
 		EMIT6_DISP_LH(0xeb000000, 0x000a, dst_reg, dst_reg, src_reg, 0);
 		break;
 	case BPF_ALU | BPF_ARSH | BPF_K: /* ((s32) dst >> imm */
-		if (imm == 0)
-			break;
-		/* sra %dst,imm(%r0) */
-		EMIT4_DISP(0x8a000000, dst_reg, REG_0, imm);
+		if (imm != 0) {
+			/* sra %dst,imm(%r0) */
+			EMIT4_DISP(0x8a000000, dst_reg, REG_0, imm);
+		}
 		EMIT_ZERO(dst_reg);
 		break;
 	case BPF_ALU64 | BPF_ARSH | BPF_K: /* ((s64) dst) >>= imm */