libbpf: Do not use __builtin_offsetof for offsetof

Commit 5fbc220 ("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 da7a350 ("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".

Fixes: da7a350 ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
Signed-off-by: Yonghong Song <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Ian Rogers <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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)				\