libbpf: Streamline error reporting for high-level APIs

Implement changes to error reporting for high-level libbpf APIs to make them
less surprising and less error-prone to users:
  - in all the cases when error happens, errno is set to an appropriate error
    value;
  - in libbpf 1.0 mode, all pointer-returning APIs return NULL on error and
    error code is communicated through errno; this applies both to APIs that
    already returned NULL before (so now they communicate more detailed error
    codes), as well as for many APIs that used ERR_PTR() macro and encoded
    error numbers as fake pointers.
  - in legacy (default) mode, those APIs that were returning ERR_PTR(err),
    continue doing so, but still set errno.

With these changes, errno can be always used to extract actual error,
regardless of legacy or libbpf 1.0 modes. This is utilized internally in
libbpf in places where libbpf uses it's own high-level APIs.
libbpf_get_error() is adapted to handle both cases completely transparently to
end-users (and is used by libbpf consistently as well).

More context, justification, and discussion can be found in "Libbpf: the road
to v1.0" document ([0]).

  [0] https://docs.google.com/document/d/1UyjTZuPFWiPFyKk1tV5an11_iaRuec6U-ZESZ54nNTY

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210525035935.1461796-5-andrii@kernel.org

tools/lib/bpf/bpf_prog_linfo.c

@@ -106,19 +106,19 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
 	nr_linfo = info->nr_line_info;
 
 	if (!nr_linfo)
-		return NULL;
+		return errno = EINVAL, NULL;
 
 	/*
 	 * The min size that bpf_prog_linfo has to access for
 	 * searching purpose.
 	 */
 	if (info->line_info_rec_size <
 	    offsetof(struct bpf_line_info, file_name_off))
-		return NULL;
+		return errno = EINVAL, NULL;
 
 	prog_linfo = calloc(1, sizeof(*prog_linfo));
 	if (!prog_linfo)
-		return NULL;
+		return errno = ENOMEM, NULL;
 
 	/* Copy xlated line_info */
 	prog_linfo->nr_linfo = nr_linfo;
@@ -174,7 +174,7 @@ struct bpf_prog_linfo *bpf_prog_linfo__new(const struct bpf_prog_info *info)
 
 err_free:
 	bpf_prog_linfo__free(prog_linfo);
-	return NULL;
+	return errno = EINVAL, NULL;
 }
 
 const struct bpf_line_info *
@@ -186,19 +186,19 @@ bpf_prog_linfo__lfind_addr_func(const struct bpf_prog_linfo *prog_linfo,
 	const __u64 *jited_linfo;
 
 	if (func_idx >= prog_linfo->nr_jited_func)
-		return NULL;
+		return errno = ENOENT, NULL;
 
 	nr_linfo = prog_linfo->nr_jited_linfo_per_func[func_idx];
 	if (nr_skip >= nr_linfo)
-		return NULL;
+		return errno = ENOENT, NULL;
 
 	start = prog_linfo->jited_linfo_func_idx[func_idx] + nr_skip;
 	jited_rec_size = prog_linfo->jited_rec_size;
 	raw_jited_linfo = prog_linfo->raw_jited_linfo +
 		(start * jited_rec_size);
 	jited_linfo = raw_jited_linfo;
 	if (addr < *jited_linfo)
-		return NULL;
+		return errno = ENOENT, NULL;
 
 	nr_linfo -= nr_skip;
 	rec_size = prog_linfo->rec_size;
@@ -225,13 +225,13 @@ bpf_prog_linfo__lfind(const struct bpf_prog_linfo *prog_linfo,
 
 	nr_linfo = prog_linfo->nr_linfo;
 	if (nr_skip >= nr_linfo)
-		return NULL;
+		return errno = ENOENT, NULL;
 
 	rec_size = prog_linfo->rec_size;
 	raw_linfo = prog_linfo->raw_linfo + (nr_skip * rec_size);
 	linfo = raw_linfo;
 	if (insn_off < linfo->insn_off)
-		return NULL;
+		return errno = ENOENT, NULL;
 
 	nr_linfo -= nr_skip;
 	for (i = 0; i < nr_linfo; i++) {