libbpf: Fix detection of BPF helper call instruction

BPF_CALL | BPF_JMP32 is explicitly not allowed by verifier for BPF helper
calls, so don't detect it as a valid call. Also drop the check on func_id
pointer, as it's currently always non-null.

Fixes: 109cea5a ("libbpf: Sanitize BPF program code for bpf_probe_read_{kernel, user}[_str]")
Reported-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200820061411.1755905-1-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -5840,14 +5840,12 @@ static int bpf_object__collect_reloc(struct bpf_object *obj)
 
 static bool insn_is_helper_call(struct bpf_insn *insn, enum bpf_func_id *func_id)
 {
-	__u8 class = BPF_CLASS(insn->code);
-
-	if ((class == BPF_JMP || class == BPF_JMP32) &&
+	if (BPF_CLASS(insn->code) == BPF_JMP &&
 	    BPF_OP(insn->code) == BPF_CALL &&
 	    BPF_SRC(insn->code) == BPF_K &&
-	    insn->src_reg == 0 && insn->dst_reg == 0) {
-		    if (func_id)
-			    *func_id = insn->imm;
+	    insn->src_reg == 0 &&
+	    insn->dst_reg == 0) {
+		    *func_id = insn->imm;
 		    return true;
 	}
 	return false;