s390/bpf: Fix sign extension in branch_ku

Both signed and unsigned variants of BPF_JMP | BPF_K require
sign-extending the immediate. JIT emits cgfi for the signed case,
which is correct, and clgfi for the unsigned case, which is not
correct: clgfi zero-extends the immediate.

s390 does not provide an instruction that does sign-extension and
unsigned comparison at the same time. Therefore, fix by first loading
the sign-extended immediate into work register REG_1 and proceeding
as if it's BPF_X.

Fixes: 4e9b4a6883 ("s390/bpf: Use relative long branches")
Reported-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Link: https://lore.kernel.org/bpf/20200717165326.6786-3-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -1417,21 +1417,10 @@ branch_ks:
 		}
 		break;
 branch_ku:
-		is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
-		/* clfi or clgfi %dst,imm */
-		EMIT6_IMM(is_jmp32 ? 0xc20f0000 : 0xc20e0000,
-			  dst_reg, imm);
-		if (!is_first_pass(jit) &&
-		    can_use_rel(jit, addrs[i + off + 1])) {
-			/* brc mask,off */
-			EMIT4_PCREL_RIC(0xa7040000,
-					mask >> 12, addrs[i + off + 1]);
-		} else {
-			/* brcl mask,off */
-			EMIT6_PCREL_RILC(0xc0040000,
-					 mask >> 12, addrs[i + off + 1]);
-		}
-		break;
+		/* lgfi %w1,imm (load sign extend imm) */
+		src_reg = REG_1;
+		EMIT6_IMM(0xc0010000, src_reg, imm);
+		goto branch_xu;
 branch_xs:
 		is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
 		if (!is_first_pass(jit) &&