bpftool: Use a local copy of BPF_LINK_TYPE_PERF_EVENT in pid_iter.bpf.c

In order to allow the BPF program in bpftool's pid_iter.bpf.c to compile
correctly on hosts where vmlinux.h does not define
BPF_LINK_TYPE_PERF_EVENT (running kernel versions lower than 5.15, for
example), define and use a local copy of the enum value. This requires
LLVM 12 or newer to build the BPF program.

Fixes: cbdaf71f ("bpftool: Add bpf_cookie to link output")
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230707095425.168126-4-quentin@isovalent.com

tools/bpf/bpftool/skeleton/pid_iter.bpf.c

@@ -24,6 +24,10 @@ struct perf_event___local {
 	u64 bpf_cookie;
 } __attribute__((preserve_access_index));
 
+enum bpf_link_type___local {
+	BPF_LINK_TYPE_PERF_EVENT___local = 7,
+};
+
 extern const void bpf_link_fops __ksym;
 extern const void bpf_map_fops __ksym;
 extern const void bpf_prog_fops __ksym;
@@ -93,10 +97,13 @@ int iter(struct bpf_iter__task_file *ctx)
 	e.pid = task->tgid;
 	e.id = get_obj_id(file->private_data, obj_type);
 
-	if (obj_type == BPF_OBJ_LINK) {
+	if (obj_type == BPF_OBJ_LINK &&
+	    bpf_core_enum_value_exists(enum bpf_link_type___local,
+				       BPF_LINK_TYPE_PERF_EVENT___local)) {
 		struct bpf_link *link = (struct bpf_link *) file->private_data;
 
-		if (BPF_CORE_READ(link, type) == BPF_LINK_TYPE_PERF_EVENT) {
+		if (link->type == bpf_core_enum_value(enum bpf_link_type___local,
+						      BPF_LINK_TYPE_PERF_EVENT___local)) {
 			e.has_bpf_cookie = true;
 			e.bpf_cookie = get_bpf_cookie(link);
 		}