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, 18 insertions, 7 deletions

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index f51444fc7eb7..5bfe85d4f8aa 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -110,7 +110,7 @@ struct btf_ext {
 	};
 	struct btf_ext_info func_info;
 	struct btf_ext_info line_info;
-	struct btf_ext_info offset_reloc_info;
+	struct btf_ext_info field_reloc_info;
 	__u32 data_size;
 };
 
@@ -135,13 +135,23 @@ struct bpf_line_info_min {
 	__u32	line_col;
 };
 
-/* The minimum bpf_offset_reloc checked by the loader
+/* bpf_field_info_kind encodes which aspect of captured field has to be
+ * adjusted by relocations. Currently supported values are:
+ *   - BPF_FIELD_BYTE_OFFSET: field offset (in bytes);
+ *   - BPF_FIELD_EXISTS: field existence (1, if field exists; 0, otherwise);
+ */
+enum bpf_field_info_kind {
+	BPF_FIELD_BYTE_OFFSET = 0,	/* field byte offset */
+	BPF_FIELD_EXISTS = 2,		/* field existence in target kernel */
+};
+
+/* The minimum bpf_field_reloc checked by the loader
  *
- * Offset relocation captures the following data:
+ * Field relocation captures the following data:
  * - insn_off - instruction offset (in bytes) within a BPF program that needs
- *   its insn->imm field to be relocated with actual offset;
+ *   its insn->imm field to be relocated with actual field info;
  * - type_id - BTF type ID of the "root" (containing) entity of a relocatable
- *   offset;
+ *   field;
  * - access_str_off - offset into corresponding .BTF string section. String
  *   itself encodes an accessed field using a sequence of field and array
  *   indicies, separated by colon (:). It's conceptually very close to LLVM's
@@ -172,15 +182,16 @@ struct bpf_line_info_min {
  * bpf_probe_read(&dst, sizeof(dst),
  *		  __builtin_preserve_access_index(&src->a.b.c));
  *
- * In this case Clang will emit offset relocation recording necessary data to
+ * In this case Clang will emit field relocation recording necessary data to
  * be able to find offset of embedded `a.b.c` field within `src` struct.
  *
  *   [0] https://llvm.org/docs/LangRef.html#getelementptr-instruction
  */
-struct bpf_offset_reloc {
+struct bpf_field_reloc {
 	__u32   insn_off;
 	__u32   type_id;
 	__u32   access_str_off;
+	enum bpf_field_info_kind kind;
 };
 
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */