On Mon, Aug 10, 2020 at 9:33 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> 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>
Acked-by: Ian Rogers <irogers@xxxxxxxxxx>
Thanks,
Ian
> > 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
> >
