Merge branch 'libbpf-field-existence'

Andrii Nakryiko says:

====================
This patch set generalizes libbpf's CO-RE relocation support. In addition to
existing field's byte offset relocation, libbpf now supports field existence
relocations, which are emitted by Clang when using
__builtin_preserve_field_info(<field>, BPF_FIELD_EXISTS). A convenience
bpf_core_field_exists() macro is added to bpf_core_read.h BPF-side header,
along the bpf_field_info_kind enum containing currently supported types of
field information libbpf supports. This list will grow as libbpf gains support
for other relo kinds.

This patch set upgrades the format of .BTF.ext's relocation record to match
latest Clang's format (12 -> 16 bytes). This is not a breaking change, as the
previous format hasn't been released yet as part of official Clang version
release.

v1->v2:
- unify bpf_field_info_kind enum and naming changes (Alexei);
- added bpf_core_field_exists() to bpf_core_read.h.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 23 insertions, 1 deletions

diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
index 4daf04c254935..a273df3784f4f 100644
--- a/tools/lib/bpf/bpf_core_read.h
+++ b/tools/lib/bpf/bpf_core_read.h
@@ -3,6 +3,28 @@
 #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()
  * built-in, provided by Clang.
@@ -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