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libbpf: Add BPF-side definitions of supported field relocation kinds

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Add enum definition for Clang's __builtin_preserve_field_info()
second argument (info_kind). Currently only byte offset and existence
are supported. Corresponding Clang changes introducing this built-in can
be found at [0]

  [0] https://reviews.llvm.org/D67980

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191015182849.3922287-5-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2019-10-15 11:28:48 -07:00 committed by Alexei Starovoitov
parent 62561eb442
commit 01340e3191
1 changed files with 23 additions and 1 deletions
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24
tools/lib/bpf/bpf_core_read.h
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@ -2,6 +2,28 @@
#ifndef __BPF_CORE_READ_H__
#define __BPF_CORE_READ_H__
/*
* enum bpf_field_info_kind is passed as a second argument into
* __builtin_preserve_field_info() built-in to get a specific aspect of
* a field, captured as a first argument. __builtin_preserve_field_info(field,
* info_kind) returns __u32 integer and produces BTF field relocation, which
* is understood and processed by libbpf during BPF object loading. See
* selftests/bpf for examples.
*/
enum bpf_field_info_kind {
BPF_FIELD_BYTE_OFFSET = 0, /* field byte offset */
BPF_FIELD_EXISTS = 2, /* field existence in target kernel */
};
/*
* Convenience macro to check that field actually exists in target kernel's.
* Returns:
* 1, if matching field is present in target kernel;
* 0, if no matching field found.
*/
#define bpf_core_field_exists(field) \
__builtin_preserve_field_info(field, BPF_FIELD_EXISTS)
/*
* bpf_core_read() abstracts away bpf_probe_read() call and captures offset
* relocation for source address using __builtin_preserve_access_index()
@ -12,7 +34,7 @@
* 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.
* semantics in comments to struct bpf_field_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