On Tue, Jul 30, 2019 at 2:24 PM Song Liu <songliubraving@xxxxxx> wrote: > > > > > On Jul 30, 2019, at 12:53 PM, Andrii Nakryiko <andriin@xxxxxx> 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@xxxxxx> > > --- > > 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.
