> On Jul 30, 2019, at 2:26 PM, Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > 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. I think dst as-is is fine. "," is the very last in precedence list.
