From: Yonghong Song <yhs@fb.com>
To: <bpf@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>, <kernel-team@fb.com>,
Ian Rogers <irogers@google.com>
Subject: [PATCH bpf] libbpf: do not use __builtin_offsetof for offsetof
Date: Mon, 10 Aug 2020 20:08:52 -0700 [thread overview]
Message-ID: <20200811030852.3396929-1-yhs@fb.com> (raw)
Commit 5fbc220862fc ("tools/libpf: Add offsetof/container_of macro
in bpf_helpers.h") added a macro offsetof() to get the offset of a
structure member:
#define offsetof(TYPE, MEMBER) ((size_t)&((TYPE *)0)->MEMBER)
In certain use cases, size_t type may not be available so
Commit da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof
for offsetof") changed to use __builtin_offsetof which removed
the dependency on type size_t, which I suggested.
But using __builtin_offsetof will prevent CO-RE relocation
generation in case that, e.g., TYPE is annotated with "preserve_access_info"
where a relocation is desirable in case the member offset is changed
in a different kernel version. So this patch reverted back to
the original macro but using "unsigned long" instead of "site_t".
Cc: Ian Rogers <irogers@google.com>
Fixes: da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
Signed-off-by: Yonghong Song <yhs@fb.com>
---
tools/lib/bpf/bpf_helpers.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index bc14db706b88..e9a4ecddb7a5 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -40,7 +40,7 @@
* Helper macro to manipulate data structures
*/
#ifndef offsetof
-#define offsetof(TYPE, MEMBER) __builtin_offsetof(TYPE, MEMBER)
+#define offsetof(TYPE, MEMBER) ((unsigned long)&((TYPE *)0)->MEMBER)
#endif
#ifndef container_of
#define container_of(ptr, type, member) \
--
2.24.1
next reply other threads:[~2020-08-11 3:08 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-11 3:08 Yonghong Song [this message]
2020-08-11 4:32 ` [PATCH bpf] libbpf: do not use __builtin_offsetof for offsetof Andrii Nakryiko
2020-08-11 4:34 ` Ian Rogers
2020-08-11 13:19 ` Daniel Borkmann
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