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

src/ringbuf.c

@@ -69,23 +69,23 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
 		err = -errno;
 		pr_warn("ringbuf: failed to get map info for fd=%d: %d\n",
 			map_fd, err);
-		return err;
+		return libbpf_err(err);
 	}
 
 	if (info.type != BPF_MAP_TYPE_RINGBUF) {
 		pr_warn("ringbuf: map fd=%d is not BPF_MAP_TYPE_RINGBUF\n",
 			map_fd);
-		return -EINVAL;
+		return libbpf_err(-EINVAL);
 	}
 
 	tmp = libbpf_reallocarray(rb->rings, rb->ring_cnt + 1, sizeof(*rb->rings));
 	if (!tmp)
-		return -ENOMEM;
+		return libbpf_err(-ENOMEM);
 	rb->rings = tmp;
 
 	tmp = libbpf_reallocarray(rb->events, rb->ring_cnt + 1, sizeof(*rb->events));
 	if (!tmp)
-		return -ENOMEM;
+		return libbpf_err(-ENOMEM);
 	rb->events = tmp;
 
 	r = &rb->rings[rb->ring_cnt];
@@ -103,7 +103,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
 		err = -errno;
 		pr_warn("ringbuf: failed to mmap consumer page for map fd=%d: %d\n",
 			map_fd, err);
-		return err;
+		return libbpf_err(err);
 	}
 	r->consumer_pos = tmp;
 
@@ -118,7 +118,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
 		ringbuf_unmap_ring(rb, r);
 		pr_warn("ringbuf: failed to mmap data pages for map fd=%d: %d\n",
 			map_fd, err);
-		return err;
+		return libbpf_err(err);
 	}
 	r->producer_pos = tmp;
 	r->data = tmp + rb->page_size;
@@ -133,7 +133,7 @@ int ring_buffer__add(struct ring_buffer *rb, int map_fd,
 		ringbuf_unmap_ring(rb, r);
 		pr_warn("ringbuf: failed to epoll add map fd=%d: %d\n",
 			map_fd, err);
-		return err;
+		return libbpf_err(err);
 	}
 
 	rb->ring_cnt++;
@@ -165,11 +165,11 @@ ring_buffer__new(int map_fd, ring_buffer_sample_fn sample_cb, void *ctx,
 	int err;
 
 	if (!OPTS_VALID(opts, ring_buffer_opts))
-		return NULL;
+		return errno = EINVAL, NULL;
 
 	rb = calloc(1, sizeof(*rb));
 	if (!rb)
-		return NULL;
+		return errno = ENOMEM, NULL;
 
 	rb->page_size = getpagesize();
 
@@ -188,7 +188,7 @@ ring_buffer__new(int map_fd, ring_buffer_sample_fn sample_cb, void *ctx,
 
 err_out:
 	ring_buffer__free(rb);
-	return NULL;
+	return errno = -err, NULL;
 }
 
 static inline int roundup_len(__u32 len)
@@ -260,7 +260,7 @@ int ring_buffer__consume(struct ring_buffer *rb)
 
 		err = ringbuf_process_ring(ring);
 		if (err < 0)
-			return err;
+			return libbpf_err(err);
 		res += err;
 	}
 	if (res > INT_MAX)
@@ -279,7 +279,7 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 
 	cnt = epoll_wait(rb->epoll_fd, rb->events, rb->ring_cnt, timeout_ms);
 	if (cnt < 0)
-		return -errno;
+		return libbpf_err(-errno);
 
 	for (i = 0; i < cnt; i++) {
 		__u32 ring_id = rb->events[i].data.fd;
@@ -287,7 +287,7 @@ int ring_buffer__poll(struct ring_buffer *rb, int timeout_ms)
 
 		err = ringbuf_process_ring(ring);
 		if (err < 0)
-			return err;
+			return libbpf_err(err);
 		res += err;
 	}
 	if (res > INT_MAX)