linux-kernel - [PATCH] libbpf bpf_helpers: Use __builtin

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Date:   Fri, 17 Jul 2020 00:23:19 -0700
From:   Ian Rogers <irogers@...gle.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Stanislav Fomichev <sdf@...gle.com>,
        Ian Rogers <irogers@...gle.com>
Subject: [PATCH] libbpf bpf_helpers: Use __builtin_offsetof for offsetof if available

The non-builtin route for offsetof has a dependency on size_t from
stdlib.h/stdint.h that is undeclared and may break targets.
The offsetof macro in bpf_helpers may disable the same macro in other
headers that have a #ifdef offsetof guard. Rather than add additional
dependencies improve the offsetof macro declared here to use the
builtin if available.

Signed-off-by: Ian Rogers <irogers@...gle.com>
---
 tools/lib/bpf/bpf_helpers.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index a510d8ed716f..ed2ac74fc515 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -40,8 +40,12 @@
  * Helper macro to manipulate data structures
  */
 #ifndef offsetof
+#if __has_builtin(__builtin_offsetof)
+#define offsetof(TYPE, MEMBER)  __builtin_offsetof(TYPE, MEMBER)
+#else
 #define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)
 #endif
+#endif
 #ifndef container_of
 #define container_of(ptr, type, member)				\
 	({							\
-- 
2.28.0.rc0.105.gf9edc3c819-goog