libbpf: fix offsetof() and container_of() to work with CO-RE

It seems like __builtin_offset() doesn't preserve CO-RE field
relocations properly. So if offsetof() macro is defined through
__builtin_offset(), CO-RE-enabled BPF code using container_of() will be
subtly and silently broken.

To avoid this problem, redefine offsetof() and container_of() in the
form that works with CO-RE relocations more reliably.

Fixes: 5fbc220862 ("tools/libpf: Add offsetof/container_of macro in bpf_helpers.h")
Reported-by: Lennart Poettering <lennart@poettering.net>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20230509065502.2306180-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/bpf_helpers.h

@@ -77,16 +77,21 @@
 /*
  * Helper macros to manipulate data structures
  */
-#ifndef offsetof
-#define offsetof(TYPE, MEMBER)	((unsigned long)&((TYPE *)0)->MEMBER)
-#endif
-#ifndef container_of
+
+/* offsetof() definition that uses __builtin_offset() might not preserve field
+ * offset CO-RE relocation properly, so force-redefine offsetof() using
+ * old-school approach which works with CO-RE correctly
+ */
+#undef offsetof
+#define offsetof(type, member)	((unsigned long)&((type *)0)->member)
+
+/* redefined container_of() to ensure we use the above offsetof() macro */
+#undef container_of
 #define container_of(ptr, type, member)				\
 	({							\
 		void *__mptr = (void *)(ptr);			\
 		((type *)(__mptr - offsetof(type, member)));	\
 	})
-#endif
 
 /*
  * Compiler (optimization) barrier.