From: Andrii Nakryiko <andriin@fb.com>
Date: Wed, 7 Aug 2019 14:39:52 -0700
Subject: selftests/bpf: add BPF_CORE_READ relocatable read macro
Patch-mainline: v5.4-rc1
Git-commit: 2dc26d5a4f2e97364542030a4a51e4a50c14bae3
References: bsc#1155518

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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Gary Lin <glin@suse.com>
---
 tools/testing/selftests/bpf/bpf_helpers.h |   20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

--- 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