[PATCH v2 bpf-next 1/5] libbpf: add libbpf_set_strict_mode() API to turn on libbpf 1.0 behaviors

[Andrii Nakryiko <andrii@kernel.org>]

Add libbpf_set_strict_mode() API that allows application to simulate libbpf
1.0 breaking changes before libbpf 1.0 is released. This will help users
migrate gradually and with confidence.

For now only ALL or NONE options are available, subsequent patches will add
more flags. This patch is preliminary for selftests/bpf changes.

Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/Makefile        |  1 +
 tools/lib/bpf/libbpf.c        | 17 +++++++++++++
 tools/lib/bpf/libbpf.h        |  1 +
 tools/lib/bpf/libbpf.map      |  5 ++++
 tools/lib/bpf/libbpf_legacy.h | 47 +++++++++++++++++++++++++++++++++++
 5 files changed, 71 insertions(+)
 create mode 100644 tools/lib/bpf/libbpf_legacy.h

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index e43e1896cb4b..15420303cf06 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -229,6 +229,7 @@ install_headers: $(BPF_HELPER_DEFS)
 		$(call do_install,libbpf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,btf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,libbpf_common.h,$(prefix)/include/bpf,644); \
+		$(call do_install,libbpf_legacy.h,$(prefix)/include/bpf,644); \
 		$(call do_install,xsk.h,$(prefix)/include/bpf,644); \
 		$(call do_install,bpf_helpers.h,$(prefix)/include/bpf,644); \
 		$(call do_install,$(BPF_HELPER_DEFS),$(prefix)/include/bpf,644); \
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index b396e45b17ea..1425d7ed0f2f 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -151,6 +151,23 @@ static inline __u64 ptr_to_u64(const void *ptr)
 	return (__u64) (unsigned long) ptr;
 }
 
+/* this goes away in libbpf 1.0 */
+enum libbpf_strict_mode libbpf_mode = LIBBPF_STRICT_NONE;
+
+int libbpf_set_strict_mode(enum libbpf_strict_mode mode)
+{
+	/* __LIBBPF_STRICT_LAST is the last power-of-2 value used + 1, so to
+	 * get all possible values we compensate last +1, and then (2*x - 1)
+	 * to get the bit mask
+	 */
+	if (mode != LIBBPF_STRICT_ALL
+	    && (mode & ~((__LIBBPF_STRICT_LAST - 1) * 2 - 1)))
+		return errno = EINVAL, -EINVAL;
+
+	libbpf_mode = mode;
+	return 0;
+}
+
 enum kern_feature_id {
 	/* v4.14: kernel support for program & map names. */
 	FEAT_PROG_NAME,
diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index d98523558f39..6e61342ba56c 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -18,6 +18,7 @@
 #include <linux/bpf.h>
 
 #include "libbpf_common.h"
+#include "libbpf_legacy.h"
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 0229e01e8ccc..bbe99b1db1a9 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -370,3 +370,8 @@ LIBBPF_0.4.0 {
 		bpf_tc_hook_destroy;
 		bpf_tc_query;
 } LIBBPF_0.3.0;
+
+LIBBPF_0.5.0 {
+	global:
+		libbpf_set_strict_mode;
+} LIBBPF_0.4.0;
diff --git a/tools/lib/bpf/libbpf_legacy.h b/tools/lib/bpf/libbpf_legacy.h
new file mode 100644
index 000000000000..7482cfe22ab2
--- /dev/null
+++ b/tools/lib/bpf/libbpf_legacy.h
@@ -0,0 +1,47 @@
+/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+
+/*
+ * Libbpf legacy APIs (either discouraged or deprecated, as mentioned in [0])
+ *
+ *   [0] https://docs.google.com/document/d/1UyjTZuPFWiPFyKk1tV5an11_iaRuec6U-ZESZ54nNTY
+ *
+ * Copyright (C) 2021 Facebook
+ */
+#ifndef __LIBBPF_LEGACY_BPF_H
+#define __LIBBPF_LEGACY_BPF_H
+
+#include <linux/bpf.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include "libbpf_common.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+enum libbpf_strict_mode {
+	/* Turn on all supported strict features of libbpf to simulate libbpf
+	 * v1.0 behavior.
+	 * This will be the default behavior in libbpf v1.0.
+	 */
+	LIBBPF_STRICT_ALL = 0xffffffff,
+
+	/*
+	 * Disable any libbpf 1.0 behaviors. This is the default before libbpf
+	 * v1.0. It won't be supported anymore in v1.0, please update your
+	 * code so that it handles LIBBPF_STRICT_ALL mode before libbpf v1.0.
+	 */
+	LIBBPF_STRICT_NONE = 0x00,
+
+	__LIBBPF_STRICT_LAST,
+};
+
+LIBBPF_API int libbpf_set_strict_mode(enum libbpf_strict_mode mode);
+
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#endif /* __LIBBPF_LEGACY_BPF_H */
-- 
2.30.2