bpf: allow any program to use the bpf_map_sum_elem_count kfunc

Register the bpf_map_sum_elem_count func for all programs, and update the
map_ptr subtest of the test_progs test to test the new functionality.

The usage is allowed as long as the pointer to the map is trusted (when
using tracing programs) or is a const pointer to map, as in the following
example:

    struct {
            __uint(type, BPF_MAP_TYPE_HASH);
            ...
    } hash SEC(".maps");

    ...

    static inline int some_bpf_prog(void)
    {
            struct bpf_map *map = (struct bpf_map *)&hash;
            __s64 count;

            count = bpf_map_sum_elem_count(map);

            ...
    }

Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
Link: https://lore.kernel.org/r/20230719092952.41202-5-aspsk@isovalent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/map_iter.c

@@ -226,6 +226,6 @@ static const struct btf_kfunc_id_set bpf_map_iter_kfunc_set = {
 
 static int init_subsystem(void)
 {
-	return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_map_iter_kfunc_set);
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_UNSPEC, &bpf_map_iter_kfunc_set);
 }
 late_initcall(init_subsystem);

tools/testing/selftests/bpf/progs/map_ptr_kern.c

@@ -103,6 +103,8 @@ struct {
 	__type(value, __u32);
 } m_hash SEC(".maps");
 
+__s64 bpf_map_sum_elem_count(struct bpf_map *map) __ksym;
+
 static inline int check_hash(void)
 {
 	struct bpf_htab *hash = (struct bpf_htab *)&m_hash;
@@ -115,6 +117,8 @@ static inline int check_hash(void)
 	VERIFY(hash->elem_size == 64);
 
 	VERIFY(hash->count.counter == 0);
+	VERIFY(bpf_map_sum_elem_count(map) == 0);
+
 	for (i = 0; i < HALF_ENTRIES; ++i) {
 		const __u32 key = i;
 		const __u32 val = 1;
@@ -123,6 +127,7 @@ static inline int check_hash(void)
 			return 0;
 	}
 	VERIFY(hash->count.counter == HALF_ENTRIES);
+	VERIFY(bpf_map_sum_elem_count(map) == HALF_ENTRIES);
 
 	return 1;
 }