From 2dc26d5a4f2e97364542030a4a51e4a50c14bae3 Mon Sep 17 00:00:00 2001 From: Andrii Nakryiko Date: Wed, 7 Aug 2019 14:39:52 -0700 Subject: selftests/bpf: add BPF_CORE_READ relocatable read macro Add BPF_CORE_READ macro used in tests to do bpf_core_read(), which automatically captures offset relocation. Signed-off-by: Andrii Nakryiko Signed-off-by: Alexei Starovoitov --- tools/testing/selftests/bpf/bpf_helpers.h | 20 ++++++++++++++++++++ 1 file changed, 20 insertions(+) (limited to 'tools/testing/selftests/bpf/bpf_helpers.h') diff --git a/tools/testing/selftests/bpf/bpf_helpers.h b/tools/testing/selftests/bpf/bpf_helpers.h index 120aa86c58d3..8b503ea142f0 100644 --- a/tools/testing/selftests/bpf/bpf_helpers.h +++ b/tools/testing/selftests/bpf/bpf_helpers.h @@ -504,4 +504,24 @@ 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)) + #endif -- cgit v1.2.3-59-g8ed1b