Date:   Tue, 27 Jul 2021 12:39:27 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 0/5] libbpf: rename btf__get_from_id() and
 btf__load() APIs, support split BTF

2021-07-23 08:51 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
> On Fri, Jul 23, 2021 at 2:58 AM Quentin Monnet <quentin@...valent.com> wrote:
>>
>> 2021-07-22 19:45 UTC-0700 ~ Andrii Nakryiko <andrii.nakryiko@...il.com>
>>> On Thu, Jul 22, 2021 at 5:58 PM Andrii Nakryiko
>>> <andrii.nakryiko@...il.com> wrote:
>>>>
>>>> On Wed, Jul 21, 2021 at 8:38 AM Quentin Monnet <quentin@...valent.com> wrote:
>>>>>
>>>>> As part of the effort to move towards a v1.0 for libbpf [0], this set
>>>>> improves some confusing function names related to BTF loading from and to
>>>>> the kernel:
>>>>>
>>>>> - btf__load() becomes btf__load_into_kernel().
>>>>> - btf__get_from_id becomes btf__load_from_kernel_by_id().
>>>>> - A new version btf__load_from_kernel_by_id_split() extends the former to
>>>>>   add support for split BTF.
>>>>>
>>>>> The old functions are not removed or marked as deprecated yet, there
>>>>> should be in a future libbpf version.
>>>>
>>>> Oh, and I was thinking about this whole deprecation having to be done
>>>> in two steps. It's super annoying to keep track of that. Ideally, we'd
>>>> have some macro that can mark API deprecated "in the future", when
>>>> actual libbpf version is >= to defined version. So something like
>>>> this:
>>>>
>>>> LIBBPF_DEPRECATED_AFTER(V(0,5), "API that will be marked deprecated in v0.6")
>>>
>>> Better:
>>>
>>> LIBBPF_DEPRECATED_SINCE(0, 6, "API that will be marked deprecated in v0.6")

So I've been looking into this, and it's not _that_ simple to do. Unless
I missed something about preprocessing macros, I cannot bake a "#if" in
a "#define", to have the attribute printed if and only if the current
version is >= 0.6 in this example.

I've come up with something, but it is not optimal because I have to
write a check and macros for each version number used with the
LIBBPF_DEPRECATED_SINCE macro. If we really wanted to automate that part
I guess we could generate a header with those macros from the Makefile
and include it in libbpf_common.h, but that does not really look much
cleaner to me.

Here's my current code, below - does it correspond to what you had in
mind? Or did you think of something else?

------

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index ec14aa725bb0..095d5dc30d50 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -8,6 +8,7 @@ LIBBPF_VERSION := $(shell \
 	grep -oE '^LIBBPF_([0-9.]+)' libbpf.map | \
 	sort -rV | head -n1 | cut -d'_' -f2)
 LIBBPF_MAJOR_VERSION := $(firstword $(subst ., ,$(LIBBPF_VERSION)))
+LIBBPF_MINOR_VERSION := $(firstword $(subst ., ,$(subst $(LIBBPF_MAJOR_VERSION)., ,$(LIBBPF_VERSION))))
 
 MAKEFLAGS += --no-print-directory
 
@@ -86,6 +87,8 @@ override CFLAGS += -Werror -Wall
 override CFLAGS += $(INCLUDES)
 override CFLAGS += -fvisibility=hidden
 override CFLAGS += -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
+override CFLAGS += -DLIBBPF_MAJOR_VERSION=$(LIBBPF_MAJOR_VERSION)
+override CFLAGS += -DLIBBPF_MINOR_VERSION=$(LIBBPF_MINOR_VERSION)
 
 # flags specific for shared library
 SHLIB_FLAGS := -DSHARED -fPIC
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index cf8490f95641..8b6b5442dbd8 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -45,7 +45,8 @@ LIBBPF_API struct btf *btf__parse_raw(const char *path);
 LIBBPF_API struct btf *btf__parse_raw_split(const char *path, struct btf *base_btf);
 LIBBPF_API struct btf *btf__load_from_kernel_by_id(__u32 id);
 LIBBPF_API struct btf *btf__load_from_kernel_by_id_split(__u32 id, struct btf *base_btf);
-LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
+LIBBPF_API LIBBPF_DEPRECATED_SINCE(0, 6, "use btf__load_from_kernel_by_id instead")
+int btf__get_from_id(__u32 id, struct btf **btf);
 
 LIBBPF_API int btf__finalize_data(struct bpf_object *obj, struct btf *btf);
 LIBBPF_API int btf__load(struct btf *btf);
diff --git a/tools/lib/bpf/libbpf_common.h b/tools/lib/bpf/libbpf_common.h
index 947d8bd8a7bb..9ba9f8135dc8 100644
--- a/tools/lib/bpf/libbpf_common.h
+++ b/tools/lib/bpf/libbpf_common.h
@@ -17,6 +17,28 @@
 
 #define LIBBPF_DEPRECATED(msg) __attribute__((deprecated(msg)))
 
+#ifndef LIBBPF_DEPRECATED_SINCE
+#define __LIBBPF_VERSION_CHECK(major, minor) \
+	LIBBPF_MAJOR_VERSION > major || \
+		(LIBBPF_MAJOR_VERSION == major && LIBBPF_MINOR_VERSION >= minor)
+
+/* Add checks for other versions below when planning deprecation of API symbols
+ * with the LIBBPF_DEPRECATED_SINCE macro.
+ */
+#if __LIBBPF_VERSION_CHECK(0, 6)
+#define __LIBBPF_MARK_DEPRECATED_0_6(X) X
+#else
+#define __LIBBPF_MARK_DEPRECATED_0_6(X)
+#endif
+
+#define __LIBBPF_DEPRECATED_SINCE(major, minor, msg) \
+	__LIBBPF_MARK_DEPRECATED_ ## major ## _ ## minor (LIBBPF_DEPRECATED("v" # major "." # minor "+, " msg))
+
+/* Mark a symbol as deprecated when libbpf version is >= {major}.{minor} */
+#define LIBBPF_DEPRECATED_SINCE(major, minor, msg) \
+	__LIBBPF_DEPRECATED_SINCE(major, minor, msg)
+#endif /* LIBBPF_DEPRECATED_SINCE */
+
 /* Helper macro to declare and initialize libbpf options struct
  *
  * This dance with uninitialized declaration, followed by memset to zero,

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