[bpf] libbpf: do not use __builtin_offsetof for offsetof

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(-)

On Mon, Aug 10, 2020 at 8:09 PM Yonghong Song <yhs@fb.com> 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@google.com>
> Fixes: da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
> Signed-off-by: Yonghong Song <yhs@fb.com>
> ---

LGTM.

Acked-by: Andrii Nakryiko <andriin@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
>

On Mon, Aug 10, 2020 at 9:33 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Mon, Aug 10, 2020 at 8:09 PM Yonghong Song <yhs@fb.com> 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@google.com>
> > Fixes: da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
> > Signed-off-by: Yonghong Song <yhs@fb.com>
> > ---
>
> LGTM.
>
> Acked-by: Andrii Nakryiko <andriin@fb.com>

Acked-by: Ian Rogers <irogers@google.com>

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
> >

On 8/11/20 5:08 AM, Yonghong Song 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@google.com>
> Fixes: da7a35062bcc ("libbpf bpf_helpers: Use __builtin_offsetof for offsetof")
> Signed-off-by: Yonghong Song <yhs@fb.com>

Applied, thanks!