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selftests/bpf: add BPF_CORE_READ relocatable read macro

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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>
This commit is contained in:
Andrii Nakryiko 2019-08-07 14:39:52 -07:00 committed by Alexei Starovoitov
parent ddc7c30426
commit 2dc26d5a4f
1 changed files with 20 additions and 0 deletions
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20
tools/testing/selftests/bpf/bpf_helpers.h
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@ -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