libbpf: Streamline error reporting for low-level APIs

Ensure that low-level APIs behave uniformly across the libbpf as follows:
  - in case of an error, errno is always set to the correct error code;
  - when libbpf 1.0 mode is enabled with LIBBPF_STRICT_DIRECT_ERRS option to
    libbpf_set_strict_mode(), return -Exxx error value directly, instead of -1;
  - by default, until libbpf 1.0 is released, keep returning -1 directly.

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/20210525035935.1461796-4-andrii@kernel.org

tools/lib/bpf/libbpf_internal.h

@@ -11,6 +11,9 @@
 
 #include <stdlib.h>
 #include <limits.h>
+#include <errno.h>
+#include <linux/err.h>
+#include "libbpf_legacy.h"
 
 /* make sure libbpf doesn't use kernel-only integer typedefs */
 #pragma GCC poison u8 u16 u32 u64 s8 s16 s32 s64
@@ -436,4 +439,27 @@ int btf_type_visit_str_offs(struct btf_type *t, str_off_visit_fn visit, void *ct
 int btf_ext_visit_type_ids(struct btf_ext *btf_ext, type_id_visit_fn visit, void *ctx);
 int btf_ext_visit_str_offs(struct btf_ext *btf_ext, str_off_visit_fn visit, void *ctx);
 
+extern enum libbpf_strict_mode libbpf_mode;
+
+/* handle direct returned errors */
+static inline int libbpf_err(int ret)
+{
+	if (ret < 0)
+		errno = -ret;
+	return ret;
+}
+
+/* handle errno-based (e.g., syscall or libc) errors according to libbpf's
+ * strict mode settings
+ */
+static inline int libbpf_err_errno(int ret)
+{
+	if (libbpf_mode & LIBBPF_STRICT_DIRECT_ERRS)
+		/* errno is already assumed to be set on error */
+		return ret < 0 ? -errno : ret;
+
+	/* legacy: on error return -1 directly and don't touch errno */
+	return ret;
+}
+
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */