On Mon, Aug 10, 2020 at 8:09 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> 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@xxxxxxxxxx>
> Fixes: da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
LGTM.
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
> 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
>
