libbpf: add .BTF.ext offset relocation section loading

Add support for BPF CO-RE offset relocations. Add section/record
iteration macros for .BTF.ext. These macro are useful for iterating over
each .BTF.ext record, either for dumping out contents or later for BPF
CO-RE relocation handling.

To enable other parts of libbpf to work with .BTF.ext contents, moved
a bunch of type definitions into libbpf_internal.h.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/btf.c

@@ -35,47 +35,6 @@ struct btf {
 	int fd;
 };
 
-struct btf_ext_info {
-	/*
-	 * info points to the individual info section (e.g. func_info and
-	 * line_info) from the .BTF.ext. It does not include the __u32 rec_size.
-	 */
-	void *info;
-	__u32 rec_size;
-	__u32 len;
-};
-
-struct btf_ext {
-	union {
-		struct btf_ext_header *hdr;
-		void *data;
-	};
-	struct btf_ext_info func_info;
-	struct btf_ext_info line_info;
-	__u32 data_size;
-};
-
-struct btf_ext_info_sec {
-	__u32	sec_name_off;
-	__u32	num_info;
-	/* Followed by num_info * record_size number of bytes */
-	__u8	data[0];
-};
-
-/* The minimum bpf_func_info checked by the loader */
-struct bpf_func_info_min {
-	__u32   insn_off;
-	__u32   type_id;
-};
-
-/* The minimum bpf_line_info checked by the loader */
-struct bpf_line_info_min {
-	__u32	insn_off;
-	__u32	file_name_off;
-	__u32	line_off;
-	__u32	line_col;
-};
-
 static inline __u64 ptr_to_u64(const void *ptr)
 {
 	return (__u64) (unsigned long) ptr;
@@ -822,6 +781,9 @@ static int btf_ext_setup_info(struct btf_ext *btf_ext,
 	/* The start of the info sec (including the __u32 record_size). */
 	void *info;
 
+	if (ext_sec->len == 0)
+		return 0;
+
 	if (ext_sec->off & 0x03) {
 		pr_debug(".BTF.ext %s section is not aligned to 4 bytes\n",
 		     ext_sec->desc);
@@ -925,11 +887,24 @@ static int btf_ext_setup_line_info(struct btf_ext *btf_ext)
 	return btf_ext_setup_info(btf_ext, &param);
 }
 
+static int btf_ext_setup_offset_reloc(struct btf_ext *btf_ext)
+{
+	struct btf_ext_sec_setup_param param = {
+		.off = btf_ext->hdr->offset_reloc_off,
+		.len = btf_ext->hdr->offset_reloc_len,
+		.min_rec_size = sizeof(struct bpf_offset_reloc),
+		.ext_info = &btf_ext->offset_reloc_info,
+		.desc = "offset_reloc",
+	};
+
+	return btf_ext_setup_info(btf_ext, &param);
+}
+
 static int btf_ext_parse_hdr(__u8 *data, __u32 data_size)
 {
 	const struct btf_ext_header *hdr = (struct btf_ext_header *)data;
 
-	if (data_size < offsetof(struct btf_ext_header, func_info_off) ||
+	if (data_size < offsetofend(struct btf_ext_header, hdr_len) ||
 	    data_size < hdr->hdr_len) {
 		pr_debug("BTF.ext header not found");
 		return -EINVAL;
@@ -987,6 +962,9 @@ struct btf_ext *btf_ext__new(__u8 *data, __u32 size)
 	}
 	memcpy(btf_ext->data, data, size);
 
+	if (btf_ext->hdr->hdr_len <
+	    offsetofend(struct btf_ext_header, line_info_len))
+		goto done;
 	err = btf_ext_setup_func_info(btf_ext);
 	if (err)
 		goto done;
@@ -995,6 +973,13 @@ struct btf_ext *btf_ext__new(__u8 *data, __u32 size)
 	if (err)
 		goto done;
 
+	if (btf_ext->hdr->hdr_len <
+	    offsetofend(struct btf_ext_header, offset_reloc_len))
+		goto done;
+	err = btf_ext_setup_offset_reloc(btf_ext);
+	if (err)
+		goto done;
+
 done:
 	if (err) {
 		btf_ext__free(btf_ext);

tools/lib/bpf/btf.h

@@ -58,6 +58,10 @@ struct btf_ext_header {
 	__u32	func_info_len;
 	__u32	line_info_off;
 	__u32	line_info_len;
+
+	/* optional part of .BTF.ext header */
+	__u32	offset_reloc_off;
+	__u32	offset_reloc_len;
 };
 
 LIBBPF_API void btf__free(struct btf *btf);

tools/lib/bpf/libbpf_internal.h

@@ -29,6 +29,10 @@
 #ifndef max
 # define max(x, y) ((x) < (y) ? (y) : (x))
 #endif
+#ifndef offsetofend
+# define offsetofend(TYPE, FIELD) \
+	(offsetof(TYPE, FIELD) + sizeof(((TYPE *)0)->FIELD))
+#endif
 
 extern void libbpf_print(enum libbpf_print_level level,
 			 const char *format, ...)
@@ -46,4 +50,105 @@ do {				\
 int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
 			 const char *str_sec, size_t str_len);
 
+struct btf_ext_info {
+	/*
+	 * info points to the individual info section (e.g. func_info and
+	 * line_info) from the .BTF.ext. It does not include the __u32 rec_size.
+	 */
+	void *info;
+	__u32 rec_size;
+	__u32 len;
+};
+
+#define for_each_btf_ext_sec(seg, sec)					\
+	for (sec = (seg)->info;						\
+	     (void *)sec < (seg)->info + (seg)->len;			\
+	     sec = (void *)sec + sizeof(struct btf_ext_info_sec) +	\
+		   (seg)->rec_size * sec->num_info)
+
+#define for_each_btf_ext_rec(seg, sec, i, rec)				\
+	for (i = 0, rec = (void *)&(sec)->data;				\
+	     i < (sec)->num_info;					\
+	     i++, rec = (void *)rec + (seg)->rec_size)
+
+struct btf_ext {
+	union {
+		struct btf_ext_header *hdr;
+		void *data;
+	};
+	struct btf_ext_info func_info;
+	struct btf_ext_info line_info;
+	struct btf_ext_info offset_reloc_info;
+	__u32 data_size;
+};
+
+struct btf_ext_info_sec {
+	__u32	sec_name_off;
+	__u32	num_info;
+	/* Followed by num_info * record_size number of bytes */
+	__u8	data[0];
+};
+
+/* The minimum bpf_func_info checked by the loader */
+struct bpf_func_info_min {
+	__u32   insn_off;
+	__u32   type_id;
+};
+
+/* The minimum bpf_line_info checked by the loader */
+struct bpf_line_info_min {
+	__u32	insn_off;
+	__u32	file_name_off;
+	__u32	line_off;
+	__u32	line_col;
+};
+
+/* The minimum bpf_offset_reloc checked by the loader
+ *
+ * Offset 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;
+ * - type_id - BTF type ID of the "root" (containing) entity of a relocatable
+ *   offset;
+ * - 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
+ *   getelementptr ([0]) instruction's arguments for identifying offset to 
+ *   a field.
+ *
+ * Example to provide a better feel.
+ *
+ *   struct sample {
+ *       int a;
+ *       struct {
+ *           int b[10];
+ *       };
+ *   };
+ *
+ *   struct sample *s = ...;
+ *   int x = &s->a;     // encoded as "0:0" (a is field #0)
+ *   int y = &s->b[5];  // encoded as "0:1:0:5" (anon struct is field #1, 
+ *                      // b is field #0 inside anon struct, accessing elem #5)
+ *   int z = &s[10]->b; // encoded as "10:1" (ptr is used as an array)
+ *
+ * type_id for all relocs in this example  will capture BTF type id of
+ * `struct sample`.
+ *
+ * Such relocation is emitted when using __builtin_preserve_access_index()
+ * Clang built-in, passing expression that captures field address, e.g.:
+ *
+ * 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
+ * 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 {
+	__u32   insn_off;
+	__u32   type_id;
+	__u32   access_str_off;
+};
+
 #endif /* __LIBBPF_LIBBPF_INTERNAL_H */