Re: [PATCH v2 bpf-next 03/12] selftests/bpf: add BPF_CORE_READ relocatable read macro

Andrii Nakryiko
2019-07-30T21:26:23Z

On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@fb.com> wrote:
>
>
>
> > On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@fb.com> wrote:
> >
> > Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which
> > automatically captures offset relocation.
> >
> > Signed-off-by: Andrii Nakryiko <andriin@fb.com>
> > ---
> > tools/testing/selftests/bpf/bpf_helpers.h | 19 +++++++++++++++++++
> > 1 file changed, 19 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h
> > index f804f210244e..81bc51293d11 100644
> > --- a/tools/testing/selftests/bpf/bpf_helpers.h
> > +++ b/tools/testing/selftests/bpf/bpf_helpers.h
> > @@ -501,4 +501,23 @@ struct pt_regs;
> > (void *)(PT_REGS_FP(ctx) + sizeof(ip))); })
> > #endif
> >
> > +/*
> > + * BPF_CORE_READ abstracts away bpf_probe_read() call and captures offset
> > + * relocation for source address using __builtin_preserve_access_index()
> > + * built-in, provided by Clang.
> > + *
> > + * __builtin_preserve_access_index() takes as an argument an expression of
> > + * taking an address of a field within struct/union. It makes compiler emit
> > + * a relocation, which records BTF type ID describing root struct/union and an
> > + * accessor string which describes exact embedded field that was used to take
> > + * an address. See detailed description of this relocation format and
> > + * semantics in comments to struct bpf_offset_reloc in libbpf_internal.h.
> > + *
> > + * This relocation allows libbpf to adjust BPF instruction to use correct
> > + * actual field offset, based on target kernel BTF type that matches original
> > + * (local) BTF, used to record relocation.
> > + */
> > +#define BPF_CORE_READ(dst, src) \
> > + bpf_probe_read(dst, sizeof(*src), __builtin_preserve_access_index(src))
>
> We should use "sizeof(*(src))"
>

Good point. Also (dst) instead of just (dst). Will update.