[PATCH v5 bpf-next 0/9] Add kind layout to BTF

[PATCH v5 bpf-next 0/9] Add kind layout to BTF

[Alan Maguire]

Update struct btf_header to add a new "kind_layout" section containing
a description of how to parse the BTF kinds known about at BTF
encoding time.  This provides the opportunity for tools that might
not know all of these kinds - as is the case when older tools run
on more newly-generated BTF - to still parse the BTF provided,
even if it cannot all be used.

Also add support to bpftool to dump metadata about BTF; its size,
header information and kind layout section.

The ideas here were discussed at [1], with further discussion
at [2].

Patches for pahole will enable the kind layout addition during
BTF generation are at [3], but even absent these the addition of the
kind_layout feature in the final patch in this series should not break
anything since such unknown features are simply ignored during pahole
BTF generation.

Changes since v4: [4]:

- removed CRC generation since it is not needed to handle modules
  built at different time than kernel; distilled base BTF supports
  this now
- fixed up bpftool display of empty kind names, comment/documentation
  indentation (Quentin, patches 8, 9)

Changes since v3 [5]:

- fixed mismerge issues with kbuild changes for BTF generation
  (patches 9, 14)
- fixed a few small issues in libbpf with kind layout representation
  (patches 2, 4)

Changes since v2 [6]:

- drop "optional" kind flag (Andrii, patch 1)
- allocate "struct btf_header" for struct btf to ensure
  we can always access new fields (Andrii, patch 2)
- use an internal BTF kind array in btf.c to simplify
  kind encoding (Andrii, patch 2)
- drop use of kind layout information for in-kernel parsing,
  since the kernel needs to be strict in what it accepts
  (Andrii, patch 6)
- added CRC verification for BTF objects and for matching
  with base object (Alexei, patches 7,8)
- fixed bpftool json output (Quentin, patch 10)
- added standalone module BTF support, tests (patches 13-17)

Changes since RFC

- Terminology change from meta -> kind_layout
  (Alexei and Andrii)
- Simplify representation, removing meta header
  and just having kind layout section (Alexei)
- Fixed bpftool to have JSON support, support
  prefix match, documented changes (Quentin)
- Separated metadata opts into add_kind_layout
  and add_crc
- Added additional positive/negative tests
  to cover basic unknown kind, one with an
  info_sz object following it and one with
  N elem_sz elements following it.
- Updated pahole-flags to use help output
  rather than version to see if features
  are present


[1] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=Nuc1g8JPFC34MA@mail.gmail.com/
[2] https://lore.kernel.org/bpf/20230531201936.1992188-1-alan.maguire@oracle.com/
[3] https://lore.kernel.org/dwarves/20250528095349.788793-1-alan.maguire@oracle.com/
[4] https://lore.kernel.org/bpf/20231112124834.388735-1-alan.maguire@oracle.com/
[5] https://lore.kernel.org/bpf/20231110110304.63910-1-alan.maguire@oracle.com/
[6] https://lore.kernel.org/bpf/20230616171728.530116-1-alan.maguire@oracle.com/


Alan Maguire (9):
  btf: add kind layout encoding to UAPI
  libbpf: Support kind layout section handling in BTF
  libbpf: use kind layout to compute an unknown kind size
  libbpf: Add kind layout encoding support
  libbpf: BTF validation can use kind layout for unknown kinds
  btf: support kernel parsing of BTF with kind layout
  selftests/bpf: test kind encoding/decoding
  bpftool: add BTF dump "format meta" to dump header/metadata
  kbuild, bpf: Specify "kind_layout" optional feature

 include/uapi/linux/btf.h                      |  11 +
 kernel/bpf/btf.c                              |  96 ++++--
 scripts/Makefile.btf                          |   2 +-
 tools/bpf/bpftool/bash-completion/bpftool     |   2 +-
 tools/bpf/bpftool/btf.c                       |  90 ++++-
 tools/include/uapi/linux/btf.h                |  11 +
 tools/lib/bpf/btf.c                           | 316 ++++++++++++++----
 tools/lib/bpf/btf.h                           |  20 ++
 tools/lib/bpf/libbpf.map                      |   1 +
 .../selftests/bpf/prog_tests/btf_kind.c       | 176 ++++++++++
 10 files changed, 627 insertions(+), 98 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_kind.c

-- 
2.39.3

[PATCH v5 bpf-next 1/9] btf: add kind layout encoding to UAPI

[Alan Maguire]

BTF kind layouts provide information to parse BTF kinds. By separating
parsing BTF from using all the information it provides, we allow BTF
to encode new features even if they cannot be used by readers. This
will be helpful in particular for cases where older tools are used
to parse newer BTF with kinds the older tools do not recognize;
the BTF can still be parsed in such cases using kind layout.

The intent is to support encoding of kind layouts optionally so that
tools like pahole can add this information. For each kind, we record

- kind-related flags
- length of singular element following struct btf_type
- length of each of the btf_vlen() elements following

The ideas here were discussed at [1], [2]; hence

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>

[1] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=Nuc1g8JPFC34MA@mail.gmail.com/
[2] https://lore.kernel.org/bpf/20230531201936.1992188-1-alan.maguire@oracle.com/
---
 include/uapi/linux/btf.h       | 11 +++++++++++
 tools/include/uapi/linux/btf.h | 11 +++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
index 266d4ffa6c07..0b156178e664 100644
--- a/include/uapi/linux/btf.h
+++ b/include/uapi/linux/btf.h
@@ -8,6 +8,15 @@
 #define BTF_MAGIC	0xeB9F
 #define BTF_VERSION	1
 
+/* kind layout section consists of a struct btf_kind_layout for each known
+ * kind at BTF encoding time.
+ */
+struct btf_kind_layout {
+	__u16 flags;		/* currently unused */
+	__u8 info_sz;		/* size of singular element after btf_type */
+	__u8 elem_sz;		/* size of each of btf_vlen(t) elements */
+};
+
 struct btf_header {
 	__u16	magic;
 	__u8	version;
@@ -19,6 +28,8 @@ struct btf_header {
 	__u32	type_len;	/* length of type section	*/
 	__u32	str_off;	/* offset of string section	*/
 	__u32	str_len;	/* length of string section	*/
+	__u32	kind_layout_off;/* offset of kind layout section */
+	__u32	kind_layout_len;/* length of kind layout section */
 };
 
 /* Max # of type identifier */
diff --git a/tools/include/uapi/linux/btf.h b/tools/include/uapi/linux/btf.h
index 266d4ffa6c07..0b156178e664 100644
--- a/tools/include/uapi/linux/btf.h
+++ b/tools/include/uapi/linux/btf.h
@@ -8,6 +8,15 @@
 #define BTF_MAGIC	0xeB9F
 #define BTF_VERSION	1
 
+/* kind layout section consists of a struct btf_kind_layout for each known
+ * kind at BTF encoding time.
+ */
+struct btf_kind_layout {
+	__u16 flags;		/* currently unused */
+	__u8 info_sz;		/* size of singular element after btf_type */
+	__u8 elem_sz;		/* size of each of btf_vlen(t) elements */
+};
+
 struct btf_header {
 	__u16	magic;
 	__u8	version;
@@ -19,6 +28,8 @@ struct btf_header {
 	__u32	type_len;	/* length of type section	*/
 	__u32	str_off;	/* offset of string section	*/
 	__u32	str_len;	/* length of string section	*/
+	__u32	kind_layout_off;/* offset of kind layout section */
+	__u32	kind_layout_len;/* length of kind layout section */
 };
 
 /* Max # of type identifier */
-- 
2.39.3

[PATCH v5 bpf-next 2/9] libbpf: Support kind layout section handling in BTF

[Alan Maguire]

Support reading in kind layout fixing endian issues on reading;
also support writing kind layout section to raw BTF object.
There is not yet an API to populate the kind layout with meaningful
information.

As part of this, we need to consider multiple valid BTF header
sizes; the original or the kind layout-extended headers.
So to support this, the "struct btf" representation is modified
to always allocate a "struct btf_header" and copy the valid
portion from the raw data to it; this means we can always safely
check fields like btf->hdr->kind_layout_len.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/btf.c | 212 +++++++++++++++++++++++++++++++-------------
 1 file changed, 151 insertions(+), 61 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index f1d495dc66bb..43d1fce8977c 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -41,42 +41,53 @@ struct btf {
 
 	/*
 	 * When BTF is loaded from an ELF or raw memory it is stored
-	 * in a contiguous memory block. The hdr, type_data, and, strs_data
+	 * in a contiguous memory block. The  type_data, and, strs_data
 	 * point inside that memory region to their respective parts of BTF
 	 * representation:
 	 *
-	 * +--------------------------------+
-	 * |  Header  |  Types  |  Strings  |
-	 * +--------------------------------+
-	 * ^          ^         ^
-	 * |          |         |
-	 * hdr        |         |
-	 * types_data-+         |
-	 * strs_data------------+
+	 * +--------------------------------+---------------------+
+	 * |  Header  |  Types  |  Strings  |Optional kind layout |
+	 * +--------------------------------+---------------------+
+	 * ^          ^         ^           ^
+	 * |          |         |           |
+	 * raw_data   |         |           |
+	 * types_data-+         |           |
+	 * strs_data------------+           |
+	 * kind_layout----------------------+
+	 *
+	 * A separate struct btf_header is allocated for btf->hdr,
+	 * and header information is copied into it.  This allows us
+	 * to handle header data for various header formats; the original,
+	 * the extended header with kind layout, etc.
 	 *
 	 * If BTF data is later modified, e.g., due to types added or
 	 * removed, BTF deduplication performed, etc, this contiguous
-	 * representation is broken up into three independently allocated
-	 * memory regions to be able to modify them independently.
+	 * representation is broken up into four independent memory
+	 * regions.
+	 *
 	 * raw_data is nulled out at that point, but can be later allocated
 	 * and cached again if user calls btf__raw_data(), at which point
-	 * raw_data will contain a contiguous copy of header, types, and
-	 * strings:
+	 * raw_data will contain a contiguous copy of header, types, strings
+	 * and optionally kind_layout.  kind_layout optionally points to a
+	 * kind_layout array - this allows us to encode information about
+	 * the kinds known at encoding time.  If kind_layout is NULL no
+	 * kind information is encoded.
 	 *
-	 * +----------+  +---------+  +-----------+
-	 * |  Header  |  |  Types  |  |  Strings  |
-	 * +----------+  +---------+  +-----------+
-	 * ^             ^            ^
-	 * |             |            |
-	 * hdr           |            |
-	 * types_data----+            |
-	 * strset__data(strs_set)-----+
+	 * +----------+  +---------+  +-----------+   +-----------+
+	 * |  Header  |  |  Types  |  |  Strings  |   |kind_layout|
+	 * +----------+  +---------+  +-----------+   +-----------+
+	 * ^             ^            ^               ^
+	 * |             |            |               |
+	 * hdr           |            |               |
+	 * types_data----+            |               |
+	 * strset__data(strs_set)-----+               |
+	 * kind_layout--------------------------------+
 	 *
-	 *               +----------+---------+-----------+
-	 *               |  Header  |  Types  |  Strings  |
-	 * raw_data----->+----------+---------+-----------+
+	 *               +----------+---------+-----------+---------------------+
+	 *               |  Header  |  Types  |  Strings  | Optional kind layout|
+	 * raw_data----->+----------+---------+-----------+---------------------+
 	 */
-	struct btf_header *hdr;
+	struct btf_header *hdr; /* separately-allocated header data */
 
 	void *types_data;
 	size_t types_data_cap; /* used size stored in hdr->type_len */
@@ -124,6 +135,13 @@ struct btf {
 	/* whether raw_data is a (read-only) mmap */
 	bool raw_data_is_mmap;
 
+	/* is BTF modifiable? i.e. is it split into separate sections as described above? */
+	bool modifiable;
+	/* Points either at raw kind layout data in parsed BTF (if present), or
+	 * at an allocated kind layout array when BTF is modifiable.
+	 */
+	void *kind_layout;
+
 	/* BTF object FD, if loaded into kernel */
 	int fd;
 
@@ -215,7 +233,7 @@ static int btf_add_type_idx_entry(struct btf *btf, __u32 type_off)
 	return 0;
 }
 
-static void btf_bswap_hdr(struct btf_header *h)
+static void btf_bswap_hdr(struct btf_header *h, __u32 hdr_len)
 {
 	h->magic = bswap_16(h->magic);
 	h->hdr_len = bswap_32(h->hdr_len);
@@ -223,50 +241,68 @@ static void btf_bswap_hdr(struct btf_header *h)
 	h->type_len = bswap_32(h->type_len);
 	h->str_off = bswap_32(h->str_off);
 	h->str_len = bswap_32(h->str_len);
+	/* May be operating on raw data with hdr_len that does not include below fields */
+	if (hdr_len >= sizeof(struct btf_header)) {
+		h->kind_layout_off = bswap_32(h->kind_layout_off);
+		h->kind_layout_len = bswap_32(h->kind_layout_len);
+	}
 }
 
 static int btf_parse_hdr(struct btf *btf)
 {
-	struct btf_header *hdr = btf->hdr;
+	struct btf_header *hdr = btf->raw_data;
+	__u32 hdr_len = hdr->hdr_len;
 	__u32 meta_left;
 
-	if (btf->raw_size < sizeof(struct btf_header)) {
+	if (btf->raw_size < offsetofend(struct btf_header, str_len)) {
 		pr_debug("BTF header not found\n");
 		return -EINVAL;
 	}
 
 	if (hdr->magic == bswap_16(BTF_MAGIC)) {
 		btf->swapped_endian = true;
-		if (bswap_32(hdr->hdr_len) != sizeof(struct btf_header)) {
+		hdr_len = bswap_32(hdr->hdr_len);
+		if (hdr_len < offsetofend(struct btf_header, str_len)) {
 			pr_warn("Can't load BTF with non-native endianness due to unsupported header length %u\n",
-				bswap_32(hdr->hdr_len));
+				hdr_len);
 			return -ENOTSUP;
 		}
-		btf_bswap_hdr(hdr);
 	} else if (hdr->magic != BTF_MAGIC) {
 		pr_debug("Invalid BTF magic: %x\n", hdr->magic);
 		return -EINVAL;
 	}
 
-	if (btf->raw_size < hdr->hdr_len) {
+	if (btf->raw_size < hdr_len) {
 		pr_debug("BTF header len %u larger than data size %u\n",
-			 hdr->hdr_len, btf->raw_size);
+			 hdr_len, btf->raw_size);
 		return -EINVAL;
 	}
 
-	meta_left = btf->raw_size - hdr->hdr_len;
-	if (meta_left < (long long)hdr->str_off + hdr->str_len) {
+	/* At this point, we have basic header information, so allocate btf->hdr */
+	btf->hdr = calloc(1, sizeof(struct btf_header));
+	if (!btf->hdr) {
+		pr_debug("BTF header allocation failed\n");
+		return -ENOMEM;
+	}
+	if (btf->swapped_endian)
+		btf_bswap_hdr(hdr, hdr_len);
+	memcpy(btf->hdr, hdr, hdr_len < sizeof(struct btf_header) ? hdr_len :
+								    sizeof(struct btf_header));
+
+	meta_left = btf->raw_size - hdr_len;
+	if (meta_left < (long long)btf->hdr->str_off + btf->hdr->str_len) {
 		pr_debug("Invalid BTF total size: %u\n", btf->raw_size);
 		return -EINVAL;
 	}
 
-	if ((long long)hdr->type_off + hdr->type_len > hdr->str_off) {
+	if ((long long)btf->hdr->type_off + btf->hdr->type_len > btf->hdr->str_off) {
 		pr_debug("Invalid BTF data sections layout: type data at %u + %u, strings data at %u + %u\n",
-			 hdr->type_off, hdr->type_len, hdr->str_off, hdr->str_len);
+			 btf->hdr->type_off, btf->hdr->type_len, btf->hdr->str_off,
+			 btf->hdr->str_len);
 		return -EINVAL;
 	}
 
-	if (hdr->type_off % 4) {
+	if (btf->hdr->type_off % 4) {
 		pr_debug("BTF type section is not aligned to 4 bytes\n");
 		return -EINVAL;
 	}
@@ -293,6 +329,32 @@ static int btf_parse_str_sec(struct btf *btf)
 	return 0;
 }
 
+static void btf_bswap_kind_layout_sec(struct btf_kind_layout *k, int len)
+{
+	struct btf_kind_layout *end = (void *)k + len;
+
+	while (k < end) {
+		k->flags = bswap_16(k->flags);
+		k++;
+	}
+}
+
+static int btf_parse_kind_layout_sec(struct btf *btf)
+{
+	const struct btf_header *hdr = btf->hdr;
+
+	if (!hdr->kind_layout_off || !hdr->kind_layout_len)
+		return 0;
+
+	if (hdr->kind_layout_len % sizeof(struct btf_kind_layout) != 0) {
+		pr_debug("Invalid BTF kind layout section\n");
+		return -EINVAL;
+	}
+	btf->kind_layout = btf->raw_data + btf->hdr->hdr_len + btf->hdr->kind_layout_off;
+
+	return 0;
+}
+
 static int btf_type_size(const struct btf_type *t)
 {
 	const int base_size = sizeof(struct btf_type);
@@ -952,7 +1014,8 @@ __s32 btf__find_by_name_kind(const struct btf *btf, const char *type_name,
 
 static bool btf_is_modifiable(const struct btf *btf)
 {
-	return (void *)btf->hdr != btf->raw_data;
+	/* BTF is modifiable if split into multiple sections */
+	return btf->modifiable;
 }
 
 static void btf_free_raw_data(struct btf *btf)
@@ -981,10 +1044,11 @@ void btf__free(struct btf *btf)
 		 * might still have a cached contiguous raw data present,
 		 * which will be unconditionally freed below.
 		 */
-		free(btf->hdr);
 		free(btf->types_data);
 		strset__free(btf->strs_set);
+		free(btf->kind_layout);
 	}
+	free(btf->hdr);
 	btf_free_raw_data(btf);
 	free(btf->raw_data_swapped);
 	free(btf->type_offs);
@@ -995,6 +1059,7 @@ void btf__free(struct btf *btf)
 
 static struct btf *btf_new_empty(struct btf *base_btf)
 {
+	struct btf_header *hdr;
 	struct btf *btf;
 
 	btf = calloc(1, sizeof(*btf));
@@ -1023,14 +1088,20 @@ static struct btf *btf_new_empty(struct btf *base_btf)
 		return ERR_PTR(-ENOMEM);
 	}
 
-	btf->hdr = btf->raw_data;
-	btf->hdr->hdr_len = sizeof(struct btf_header);
-	btf->hdr->magic = BTF_MAGIC;
-	btf->hdr->version = BTF_VERSION;
+	hdr = btf->raw_data;
+	hdr->hdr_len = sizeof(struct btf_header);
+	hdr->magic = BTF_MAGIC;
+	hdr->version = BTF_VERSION;
 
-	btf->types_data = btf->raw_data + btf->hdr->hdr_len;
-	btf->strs_data = btf->raw_data + btf->hdr->hdr_len;
-	btf->hdr->str_len = base_btf ? 0 : 1; /* empty string at offset 0 */
+	btf->types_data = btf->raw_data + hdr->hdr_len;
+	btf->strs_data = btf->raw_data + hdr->hdr_len;
+	hdr->str_len = base_btf ? 0 : 1; /* empty string at offset 0 */
+	btf->hdr = calloc(1, sizeof(struct btf_header));
+	if (!btf->hdr) {
+		free(btf);
+		return ERR_PTR(-ENOMEM);
+	}
+	memcpy(btf->hdr, hdr, sizeof(*hdr));
 
 	return btf;
 }
@@ -1079,7 +1150,6 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf, b
 
 	btf->raw_size = size;
 
-	btf->hdr = btf->raw_data;
 	err = btf_parse_hdr(btf);
 	if (err)
 		goto done;
@@ -1088,6 +1158,7 @@ static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf, b
 	btf->types_data = btf->raw_data + btf->hdr->hdr_len + btf->hdr->type_off;
 
 	err = btf_parse_str_sec(btf);
+	err = err ?: btf_parse_kind_layout_sec(btf);
 	err = err ?: btf_parse_type_sec(btf);
 	err = err ?: btf_sanity_check(btf);
 	if (err)
@@ -1551,6 +1622,11 @@ static void *btf_get_raw_data(const struct btf *btf, __u32 *size, bool swap_endi
 	}
 
 	data_sz = hdr->hdr_len + hdr->type_len + hdr->str_len;
+	if (btf->kind_layout) {
+		data_sz = roundup(data_sz, 4);
+		data_sz += hdr->kind_layout_len;
+		hdr->kind_layout_off = roundup(hdr->type_len + hdr->str_len, 4);
+	}
 	data = calloc(1, data_sz);
 	if (!data)
 		return NULL;
@@ -1558,7 +1634,7 @@ static void *btf_get_raw_data(const struct btf *btf, __u32 *size, bool swap_endi
 
 	memcpy(p, hdr, hdr->hdr_len);
 	if (swap_endian)
-		btf_bswap_hdr(p);
+		btf_bswap_hdr(p, hdr->hdr_len);
 	p += hdr->hdr_len;
 
 	memcpy(p, btf->types_data, hdr->type_len);
@@ -1577,7 +1653,13 @@ static void *btf_get_raw_data(const struct btf *btf, __u32 *size, bool swap_endi
 	p += hdr->type_len;
 
 	memcpy(p, btf_strs_data(btf), hdr->str_len);
-	p += hdr->str_len;
+
+	if (btf->kind_layout) {
+		p = data + hdr->hdr_len + hdr->kind_layout_off;
+		memcpy(p, btf->kind_layout, hdr->kind_layout_len);
+		if (swap_endian)
+			btf_bswap_kind_layout_sec(p, hdr->kind_layout_len);
+	}
 
 	*size = data_sz;
 	return data;
@@ -1718,13 +1800,13 @@ static void btf_invalidate_raw_data(struct btf *btf)
 	}
 }
 
-/* Ensure BTF is ready to be modified (by splitting into a three memory
- * regions for header, types, and strings). Also invalidate cached
- * raw_data, if any.
+/* Ensure BTF is ready to be modified (by splitting into memory regions
+ * for types and strings, with kind layout section if needed (btf->hdr
+ * is already a separate region).  Also invalidate cached raw_data, if any.
  */
 static int btf_ensure_modifiable(struct btf *btf)
 {
-	void *hdr, *types;
+	void *types, *kind_layout = NULL;
 	struct strset *set = NULL;
 	int err = -ENOMEM;
 
@@ -1734,15 +1816,20 @@ static int btf_ensure_modifiable(struct btf *btf)
 		return 0;
 	}
 
-	/* split raw data into three memory regions */
-	hdr = malloc(btf->hdr->hdr_len);
+	/* split raw data into memory regions; btf->hdr is done already. */
 	types = malloc(btf->hdr->type_len);
-	if (!hdr || !types)
+	if (!types)
 		goto err_out;
-
-	memcpy(hdr, btf->hdr, btf->hdr->hdr_len);
 	memcpy(types, btf->types_data, btf->hdr->type_len);
 
+	if (btf->hdr->kind_layout_len && btf->hdr->kind_layout_off) {
+		kind_layout = malloc(btf->hdr->kind_layout_len);
+		if (!kind_layout)
+			goto err_out;
+		memcpy(kind_layout, btf->raw_data + btf->hdr->hdr_len + btf->hdr->kind_layout_off,
+		       btf->hdr->kind_layout_len);
+	}
+
 	/* build lookup index for all strings */
 	set = strset__new(BTF_MAX_STR_OFFSET, btf->strs_data, btf->hdr->str_len);
 	if (IS_ERR(set)) {
@@ -1751,11 +1838,12 @@ static int btf_ensure_modifiable(struct btf *btf)
 	}
 
 	/* only when everything was successful, update internal state */
-	btf->hdr = hdr;
 	btf->types_data = types;
 	btf->types_data_cap = btf->hdr->type_len;
 	btf->strs_data = NULL;
 	btf->strs_set = set;
+	if (kind_layout)
+		btf->kind_layout = kind_layout;
 	/* if BTF was created from scratch, all strings are guaranteed to be
 	 * unique and deduplicated
 	 */
@@ -1767,12 +1855,14 @@ static int btf_ensure_modifiable(struct btf *btf)
 	/* invalidate raw_data representation */
 	btf_invalidate_raw_data(btf);
 
+	btf->modifiable = true;
+
 	return 0;
 
 err_out:
 	strset__free(set);
-	free(hdr);
 	free(types);
+	free(kind_layout);
 	return err;
 }
 
-- 
2.39.3

[PATCH v5 bpf-next 3/9] libbpf: use kind layout to compute an unknown kind size

[Alan Maguire]

This allows BTF parsing to proceed even if we do not know the
kind.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/btf.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 43d1fce8977c..7a197dbfc689 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -355,7 +355,29 @@ static int btf_parse_kind_layout_sec(struct btf *btf)
 	return 0;
 }
 
-static int btf_type_size(const struct btf_type *t)
+/* for unknown kinds, consult kind layout. */
+static int btf_type_size_unknown(const struct btf *btf, const struct btf_type *t)
+{
+	int size = sizeof(struct btf_type);
+	struct btf_kind_layout *k = NULL;
+	__u16 vlen = btf_vlen(t);
+	__u8 kind = btf_kind(t);
+
+	if (btf->kind_layout)
+		k = &((struct btf_kind_layout *)btf->kind_layout)[kind];
+
+	if (!k || (void *)k > ((void *)btf->kind_layout + btf->hdr->kind_layout_len)) {
+		pr_debug("Unsupported BTF_KIND: %u\n", btf_kind(t));
+		return -EINVAL;
+	}
+
+	size += k->info_sz;
+	size += vlen * k->elem_sz;
+
+	return size;
+}
+
+static int btf_type_size(const struct btf *btf, const struct btf_type *t)
 {
 	const int base_size = sizeof(struct btf_type);
 	__u16 vlen = btf_vlen(t);
@@ -391,8 +413,7 @@ static int btf_type_size(const struct btf_type *t)
 	case BTF_KIND_DECL_TAG:
 		return base_size + sizeof(struct btf_decl_tag);
 	default:
-		pr_debug("Unsupported BTF_KIND:%u\n", btf_kind(t));
-		return -EINVAL;
+		return btf_type_size_unknown(btf, t);
 	}
 }
 
@@ -491,7 +512,7 @@ static int btf_parse_type_sec(struct btf *btf)
 		if (btf->swapped_endian)
 			btf_bswap_type_base(next_type);
 
-		type_size = btf_type_size(next_type);
+		type_size = btf_type_size(btf, next_type);
 		if (type_size < 0)
 			return type_size;
 		if (next_type + type_size > end_type) {
@@ -1989,7 +2010,7 @@ static int btf_add_type(struct btf_pipe *p, const struct btf_type *src_type)
 	__u32 *str_off;
 	int sz, err;
 
-	sz = btf_type_size(src_type);
+	sz = btf_type_size(p->src, src_type);
 	if (sz < 0)
 		return libbpf_err(sz);
 
@@ -2071,7 +2092,7 @@ int btf__add_btf(struct btf *btf, const struct btf *src_btf)
 		struct btf_field_iter it;
 		__u32 *type_id, *str_off;
 
-		sz = btf_type_size(t);
+		sz = btf_type_size(src_btf, t);
 		if (sz < 0) {
 			/* unlikely, has to be corrupted src_btf */
 			err = sz;
@@ -5354,7 +5375,7 @@ static int btf_dedup_compact_types(struct btf_dedup *d)
 			continue;
 
 		t = btf__type_by_id(d->btf, id);
-		len = btf_type_size(t);
+		len = btf_type_size(d->btf, t);
 		if (len < 0)
 			return len;
 
-- 
2.39.3

[PATCH v5 bpf-next 4/9] libbpf: Add kind layout encoding support

[Alan Maguire]

Support encoding of BTF kind layout data via btf__new_empty_opts().

Current supported opts are base_btf and add_kind_layout.

Kind layout information is maintained in btf.c in the
kind_layouts[] array; when BTF is created with the
add_kind_layout option it represents the current view
of supported BTF kinds.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/btf.c      | 61 ++++++++++++++++++++++++++++++++++++++--
 tools/lib/bpf/btf.h      | 20 +++++++++++++
 tools/lib/bpf/libbpf.map |  1 +
 3 files changed, 79 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 7a197dbfc689..f3c4dc0c9007 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -30,6 +30,35 @@
 
 static struct btf_type btf_void;
 
+/* Describe how kinds are laid out; some have a singular element following the "struct btf_type",
+ * some have BTF_INFO_VLEN(t->info) elements.  Specify sizes for both.  Flags are currently unused.
+ * Kind layout can be optionally added to the BTF representation in a dedicated section to
+ * facilitate parsing.  New kinds must be added here.
+ */
+struct btf_kind_layout kind_layouts[NR_BTF_KINDS] = {
+/*	flags	singular element size		vlen element(s) size */
+{	0,	0,				0				}, /* _UNKN */
+{	0,	sizeof(__u32),			0				}, /* _INT */
+{	0,	0,				0				}, /* _PTR */
+{	0,	sizeof(struct btf_array),	0				}, /* _ARRAY */
+{	0,	0,				sizeof(struct btf_member)	}, /* _STRUCT */
+{	0,	0,				sizeof(struct btf_member)	}, /* _UNION */
+{	0,	0,				sizeof(struct btf_enum)		}, /* _ENUM */
+{	0,	0,				0				}, /* _FWD */
+{	0,	0,				0				}, /* _TYPEDEF */
+{	0,	0,				0				}, /* _VOLATILE */
+{	0,	0,				0				}, /* _CONST */
+{	0,	0,				0				}, /* _RESTRICT */
+{	0,	0,				0				}, /* _FUNC */
+{	0,	0,				sizeof(struct btf_param)	}, /* _FUNC_PROTO */
+{	0,	sizeof(struct btf_var),		0				}, /* _VAR */
+{	0,	0,				sizeof(struct btf_var_secinfo)	}, /* _DATASEC */
+{	0,	0,				0				}, /* _FLOAT */
+{	0,	sizeof(struct btf_decl_tag),	0				}, /* _DECL_TAG */
+{	0,	0,				0				}, /* _TYPE_TAG */
+{	0,	0,				sizeof(struct btf_enum64)	}, /* _ENUM64 */
+};
+
 struct btf {
 	/* raw BTF data in native endianness */
 	void *raw_data;
@@ -1078,8 +1107,9 @@ void btf__free(struct btf *btf)
 	free(btf);
 }
 
-static struct btf *btf_new_empty(struct btf *base_btf)
+static struct btf *btf_new_empty(struct btf_new_opts *opts)
 {
+	struct btf *base_btf = OPTS_GET(opts, base_btf, NULL);
 	struct btf_header *hdr;
 	struct btf *btf;
 
@@ -1122,6 +1152,17 @@ static struct btf *btf_new_empty(struct btf *base_btf)
 		free(btf);
 		return ERR_PTR(-ENOMEM);
 	}
+
+	if (opts->add_kind_layout) {
+		hdr->kind_layout_len = sizeof(kind_layouts);
+		btf->kind_layout = malloc(hdr->kind_layout_len);
+		if (!btf->kind_layout) {
+			free(btf->hdr);
+			free(btf);
+			return ERR_PTR(-ENOMEM);
+		}
+		memcpy(btf->kind_layout, kind_layouts, sizeof(kind_layouts));
+	}
 	memcpy(btf->hdr, hdr, sizeof(*hdr));
 
 	return btf;
@@ -1129,12 +1170,26 @@ static struct btf *btf_new_empty(struct btf *base_btf)
 
 struct btf *btf__new_empty(void)
 {
-	return libbpf_ptr(btf_new_empty(NULL));
+	LIBBPF_OPTS(btf_new_opts, opts);
+
+	return libbpf_ptr(btf_new_empty(&opts));
 }
 
 struct btf *btf__new_empty_split(struct btf *base_btf)
 {
-	return libbpf_ptr(btf_new_empty(base_btf));
+	LIBBPF_OPTS(btf_new_opts, opts);
+
+	opts.base_btf = base_btf;
+
+	return libbpf_ptr(btf_new_empty(&opts));
+}
+
+struct btf *btf__new_empty_opts(struct btf_new_opts *opts)
+{
+	if (!OPTS_VALID(opts, btf_new_opts))
+		return libbpf_err_ptr(-EINVAL);
+
+	return libbpf_ptr(btf_new_empty(opts));
 }
 
 static struct btf *btf_new(const void *data, __u32 size, struct btf *base_btf, bool is_mmap)
diff --git a/tools/lib/bpf/btf.h b/tools/lib/bpf/btf.h
index 4392451d634b..829c9c47a97f 100644
--- a/tools/lib/bpf/btf.h
+++ b/tools/lib/bpf/btf.h
@@ -108,6 +108,26 @@ LIBBPF_API struct btf *btf__new_empty(void);
  */
 LIBBPF_API struct btf *btf__new_empty_split(struct btf *base_btf);
 
+struct btf_new_opts {
+	size_t sz;
+	struct btf *base_btf;	/* optional base BTF */
+	bool add_kind_layout;	/* add BTF kind layout information */
+	size_t:0;
+};
+#define btf_new_opts__last_field add_kind_layout
+
+/**
+ * @brief **btf__new_empty_opts()** creates an unpopulated BTF object with
+ * optional *base_btf* and BTF kind layout description if *add_kind_layout*
+ * is set
+ * @return new BTF object instance which has to be eventually freed with
+ * **btf__free()**
+ *
+ * On error, NULL is returned and the thread-local `errno` variable is
+ * set to the error code.
+ */
+LIBBPF_API struct btf *btf__new_empty_opts(struct btf_new_opts *opts);
+
 /**
  * @brief **btf__distill_base()** creates new versions of the split BTF
  * *src_btf* and its base BTF. The new base BTF will only contain the types
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index 1205f9a4fe04..2a854517baeb 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -443,4 +443,5 @@ LIBBPF_1.6.0 {
 		bpf_program__line_info_cnt;
 		btf__add_decl_attr;
 		btf__add_type_attr;
+		btf__new_empty_opts;
 } LIBBPF_1.5.0;
-- 
2.39.3

[PATCH v5 bpf-next 5/9] libbpf: BTF validation can use kind layout for unknown kinds

[Alan Maguire]

BTF parsing can use kind layout to navigate unknown kinds, so
btf_validate_type() should take kind layout information into
account to avoid failure when an unrecognized kind is met.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/lib/bpf/btf.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index f3c4dc0c9007..1116bc098d00 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -702,8 +702,12 @@ static int btf_validate_type(const struct btf *btf, const struct btf_type *t, __
 		break;
 	}
 	default:
-		pr_warn("btf: type [%u]: unrecognized kind %u\n", id, kind);
-		return -EINVAL;
+		/* Kind may be represented in kind layout information. */
+		if (btf_type_size_unknown(btf, t) < 0) {
+			pr_warn("btf: type [%u]: unrecognized kind %u\n", id, kind);
+			return -EINVAL;
+		}
+		break;
 	}
 	return 0;
 }
-- 
2.39.3

[PATCH v5 bpf-next 6/9] btf: support kernel parsing of BTF with kind layout

[Alan Maguire]

Validate kind layout if present, but because the kernel must be
strict in what it accepts, reject BTF with unsupported kinds,
even if they are in the kind layout information.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 kernel/bpf/btf.c | 96 ++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 76 insertions(+), 20 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 0f7828380895..a18d7b8ba14c 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -268,6 +268,7 @@ struct btf {
 	struct btf_id_dtor_kfunc_tab *dtor_kfunc_tab;
 	struct btf_struct_metas *struct_meta_tab;
 	struct btf_struct_ops_tab *struct_ops_tab;
+	struct btf_kind_layout *kind_layout;
 
 	/* split BTF support */
 	struct btf *base_btf;
@@ -5208,23 +5209,36 @@ static s32 btf_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (BTF_INFO_KIND(t->info) > BTF_KIND_MAX ||
-	    BTF_INFO_KIND(t->info) == BTF_KIND_UNKN) {
+	if (!btf_name_offset_valid(env->btf, t->name_off)) {
+		btf_verifier_log(env, "[%u] Invalid name_offset:%u",
+				 env->log_type_id, t->name_off);
+		return -EINVAL;
+	}
+
+	if (BTF_INFO_KIND(t->info) == BTF_KIND_UNKN) {
 		btf_verifier_log(env, "[%u] Invalid kind:%u",
 				 env->log_type_id, BTF_INFO_KIND(t->info));
 		return -EINVAL;
 	}
 
-	if (!btf_name_offset_valid(env->btf, t->name_off)) {
-		btf_verifier_log(env, "[%u] Invalid name_offset:%u",
-				 env->log_type_id, t->name_off);
+	if (BTF_INFO_KIND(t->info) > BTF_KIND_MAX && env->btf->kind_layout &&
+	    (BTF_INFO_KIND(t->info) * sizeof(struct btf_kind_layout)) <
+	     env->btf->hdr.kind_layout_len) {
+		btf_verifier_log(env, "[%u] unknown but required kind %u",
+				 env->log_type_id,
+				 BTF_INFO_KIND(t->info));
 		return -EINVAL;
+	} else {
+		if (BTF_INFO_KIND(t->info) > BTF_KIND_MAX) {
+			btf_verifier_log(env, "[%u] Invalid kind:%u",
+					 env->log_type_id, BTF_INFO_KIND(t->info));
+			return -EINVAL;
+		}
+		var_meta_size = btf_type_ops(t)->check_meta(env, t, meta_left);
+		if (var_meta_size < 0)
+			return var_meta_size;
 	}
 
-	var_meta_size = btf_type_ops(t)->check_meta(env, t, meta_left);
-	if (var_meta_size < 0)
-		return var_meta_size;
-
 	meta_left -= var_meta_size;
 
 	return saved_meta_left - meta_left;
@@ -5398,7 +5412,8 @@ static int btf_parse_str_sec(struct btf_verifier_env *env)
 	start = btf->nohdr_data + hdr->str_off;
 	end = start + hdr->str_len;
 
-	if (end != btf->data + btf->data_size) {
+	if (hdr->hdr_len < sizeof(struct btf_header) &&
+	    end != btf->data + btf->data_size) {
 		btf_verifier_log(env, "String section is not at the end");
 		return -EINVAL;
 	}
@@ -5419,9 +5434,41 @@ static int btf_parse_str_sec(struct btf_verifier_env *env)
 	return 0;
 }
 
+static int btf_parse_kind_layout_sec(struct btf_verifier_env *env)
+{
+	const struct btf_header *hdr = &env->btf->hdr;
+	struct btf *btf = env->btf;
+	void *start, *end;
+
+	if (hdr->hdr_len < sizeof(struct btf_header) ||
+	    hdr->kind_layout_len == 0)
+		return 0;
+
+	/* Kind layout section must align to 4 bytes */
+	if (hdr->kind_layout_off & (sizeof(u32) - 1)) {
+		btf_verifier_log(env, "Unaligned kind_layout_off");
+		return -EINVAL;
+	}
+	start = btf->nohdr_data + hdr->kind_layout_off;
+	end = start + hdr->kind_layout_len;
+
+	if (hdr->kind_layout_len < sizeof(struct btf_kind_layout)) {
+		btf_verifier_log(env, "Kind layout section is too small");
+		return -EINVAL;
+	}
+	if (end != btf->data + btf->data_size) {
+		btf_verifier_log(env, "Kind layout section is not at the end");
+		return -EINVAL;
+	}
+	btf->kind_layout = start;
+
+	return 0;
+}
+
 static const size_t btf_sec_info_offset[] = {
 	offsetof(struct btf_header, type_off),
 	offsetof(struct btf_header, str_off),
+	offsetof(struct btf_header, kind_layout_off),
 };
 
 static int btf_sec_info_cmp(const void *a, const void *b)
@@ -5436,44 +5483,49 @@ static int btf_check_sec_info(struct btf_verifier_env *env,
 			      u32 btf_data_size)
 {
 	struct btf_sec_info secs[ARRAY_SIZE(btf_sec_info_offset)];
-	u32 total, expected_total, i;
+	u32 nr_secs = ARRAY_SIZE(btf_sec_info_offset);
+	u32 total, expected_total, gap, i;
 	const struct btf_header *hdr;
 	const struct btf *btf;
 
 	btf = env->btf;
 	hdr = &btf->hdr;
 
+	if (hdr->hdr_len < sizeof(struct btf_header))
+		nr_secs--;
+
 	/* Populate the secs from hdr */
-	for (i = 0; i < ARRAY_SIZE(btf_sec_info_offset); i++)
+	for (i = 0; i < nr_secs; i++)
 		secs[i] = *(struct btf_sec_info *)((void *)hdr +
 						   btf_sec_info_offset[i]);
 
-	sort(secs, ARRAY_SIZE(btf_sec_info_offset),
+	sort(secs, nr_secs,
 	     sizeof(struct btf_sec_info), btf_sec_info_cmp, NULL);
 
 	/* Check for gaps and overlap among sections */
 	total = 0;
 	expected_total = btf_data_size - hdr->hdr_len;
-	for (i = 0; i < ARRAY_SIZE(btf_sec_info_offset); i++) {
+	for (i = 0; i < nr_secs; i++) {
 		if (expected_total < secs[i].off) {
 			btf_verifier_log(env, "Invalid section offset");
 			return -EINVAL;
 		}
-		if (total < secs[i].off) {
-			/* gap */
-			btf_verifier_log(env, "Unsupported section found");
-			return -EINVAL;
-		}
 		if (total > secs[i].off) {
 			btf_verifier_log(env, "Section overlap found");
 			return -EINVAL;
 		}
+		gap = secs[i].off - total;
+		if (gap >= 4) {
+			/* gap larger than alignment gap */
+			btf_verifier_log(env, "Unsupported section found");
+			return -EINVAL;
+		}
 		if (expected_total - total < secs[i].len) {
 			btf_verifier_log(env,
 					 "Total section length too long");
 			return -EINVAL;
 		}
-		total += secs[i].len;
+		total += secs[i].len + gap;
 	}
 
 	/* There is data other than hdr and known sections */
@@ -5809,6 +5861,10 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat
 	if (err)
 		goto errout;
 
+	err = btf_parse_kind_layout_sec(env);
+	if (err)
+		goto errout;
+
 	err = btf_parse_type_sec(env);
 	if (err)
 		goto errout;
-- 
2.39.3

[PATCH v5 bpf-next 7/9] selftests/bpf: test kind encoding/decoding

[Alan Maguire]

verify btf__new_empty_opts() adds kind layouts for all kinds supported,
and after adding kind-related types for an unknown kind, ensure that
parsing uses this info when that kind is encountered rather than
giving up.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 .../selftests/bpf/prog_tests/btf_kind.c       | 176 ++++++++++++++++++
 1 file changed, 176 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/btf_kind.c

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_kind.c b/tools/testing/selftests/bpf/prog_tests/btf_kind.c
new file mode 100644
index 000000000000..735ea0b588a5
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/btf_kind.c
@@ -0,0 +1,176 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2025, Oracle and/or its affiliates. */
+
+#include <test_progs.h>
+#include <bpf/btf.h>
+#include <bpf/libbpf.h>
+
+/* verify kind encoding exists for each kind */
+static void test_btf_kind_encoding(void)
+{
+	LIBBPF_OPTS(btf_new_opts, opts);
+	const struct btf_header *hdr;
+	const void *raw_btf;
+	struct btf *btf;
+	__u32 raw_size;
+
+	opts.add_kind_layout = true;
+	btf = btf__new_empty_opts(&opts);
+	if (!ASSERT_OK_PTR(btf, "btf_new"))
+		return;
+
+	raw_btf = btf__raw_data(btf, &raw_size);
+	if (!ASSERT_OK_PTR(raw_btf, "btf__raw_data"))
+		return;
+
+	hdr = raw_btf;
+
+	ASSERT_EQ(hdr->kind_layout_off % 4, 0, "kind_layout aligned");
+	ASSERT_EQ(hdr->kind_layout_len, sizeof(struct btf_kind_layout) * NR_BTF_KINDS,
+		  "kind_layout_len");
+	btf__free(btf);
+}
+
+static void write_raw_btf(const char *btf_path, void *raw_btf, size_t raw_size)
+{
+	int fd = open(btf_path, O_WRONLY | O_CREAT);
+
+	write(fd, raw_btf, raw_size);
+	close(fd);
+}
+
+/* fabricate an unrecognized kind at BTF_KIND_MAX + 1, and after adding
+ * the appropriate struct/typedefs to the BTF such that it recognizes
+ * this kind, ensure that parsing of BTF containing the unrecognized kind
+ * can succeed.
+ */
+void test_btf_kind_decoding(void)
+{
+	__s32 int_id, unrec_id, id, id2;
+	LIBBPF_OPTS(btf_new_opts, opts);
+	struct btf *btf, *new_btf;
+	struct btf_kind_layout *k;
+	struct btf_header *hdr;
+	const void *raw_btf;
+	struct btf_type *t;
+	char btf_path[64];
+	void *new_raw_btf;
+	__u32 raw_size;
+
+	opts.add_kind_layout = true;
+	btf = btf__new_empty_opts(&opts);
+	if (!ASSERT_OK_PTR(btf, "btf_new"))
+		return;
+
+	int_id = btf__add_int(btf, "test_char", 1, BTF_INT_CHAR);
+	if (!ASSERT_GT(int_id, 0, "add_int_id"))
+		return;
+
+	/* now create our type with unrecognized kind by adding a typedef kind
+	 * we will overwrite it with our unrecognized kind value.
+	 */
+	unrec_id = btf__add_typedef(btf, "unrec_kind", int_id);
+	if (!ASSERT_GT(unrec_id, 0, "add_unrec_id"))
+		return;
+
+	/* add an id after it that we will look up to verify we can parse
+	 * beyond unrecognized kinds.
+	 */
+	id = btf__add_typedef(btf, "test_lookup", int_id);
+	if (!ASSERT_GT(id, 0, "add_test_lookup_id"))
+		return;
+	id2 = btf__add_typedef(btf, "test_lookup2", int_id);
+	if (!ASSERT_GT(id2, 0, "add_test_lookup_id2"))
+		return;
+
+	raw_btf = (void *)btf__raw_data(btf, &raw_size);
+	if (!ASSERT_OK_PTR(raw_btf, "btf__raw_data"))
+		return;
+
+	new_raw_btf = calloc(1, raw_size + sizeof(*k));
+	memcpy(new_raw_btf, raw_btf, raw_size);
+
+	/* add new layout description */
+	hdr = new_raw_btf;
+	hdr->kind_layout_len += sizeof(*k);
+	k = new_raw_btf + hdr->hdr_len + hdr->kind_layout_off;
+	k[NR_BTF_KINDS].info_sz = 0;
+	k[NR_BTF_KINDS].elem_sz = 0;
+
+	/* now modify our typedef added above to be an unrecognized kind. */
+	t = (void *)hdr + hdr->hdr_len + hdr->type_off + sizeof(struct btf_type) +
+		sizeof(__u32);
+	t->info = (NR_BTF_KINDS << 24);
+
+	/* now write our BTF to a raw file, ready for parsing. */
+	snprintf(btf_path, sizeof(btf_path), "/tmp/btf_kind.%d", getpid());
+
+	write_raw_btf(btf_path, new_raw_btf, raw_size + sizeof(*k));
+
+	/* verify parsing succeeds, and that we can read type info past
+	 * the unrecognized kind.
+	 */
+	new_btf = btf__parse_raw(btf_path);
+	if (ASSERT_OK_PTR(new_btf, "btf__parse_raw")) {
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup",
+						 BTF_KIND_TYPEDEF), id,
+			  "verify_id_lookup");
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup2",
+						 BTF_KIND_TYPEDEF), id2,
+			  "verify_id2_lookup");
+	}
+	btf__free(new_btf);
+
+	/* next, change info_sz to equal sizeof(struct btf_type); this means the
+	 * "test_lookup" kind will be reinterpreted as a singular info element
+	 * following the unrecognized kind.
+	 */
+	k[NR_BTF_KINDS].info_sz = sizeof(struct btf_type);
+	write_raw_btf(btf_path, new_raw_btf, raw_size + sizeof(*k));
+
+	new_btf = btf__parse_raw(btf_path);
+	if (ASSERT_OK_PTR(new_btf, "btf__parse_raw")) {
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup",
+						 BTF_KIND_TYPEDEF), -ENOENT,
+			  "verify_id_not_found");
+		/* id of "test_lookup2" will be id2 -1 as we have removed one type */
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup2",
+						 BTF_KIND_TYPEDEF), id2 - 1,
+			  "verify_id_lookup2");
+
+	}
+	btf__free(new_btf);
+
+	/* next, change elem_sz to equal sizeof(struct btf_type)/2 and set
+	 * vlen associated with unrecognized type to 2; this allows us to verify
+	 * vlen-specified BTF can still be parsed.
+	 */
+	k[NR_BTF_KINDS].info_sz = 0;
+	k[NR_BTF_KINDS].elem_sz = sizeof(struct btf_type)/2;
+	t->info |= 2;
+	write_raw_btf(btf_path, new_raw_btf, raw_size + sizeof(*k));
+
+	new_btf = btf__parse_raw(btf_path);
+	if (ASSERT_OK_PTR(new_btf, "btf__parse_raw")) {
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup",
+						 BTF_KIND_TYPEDEF), -ENOENT,
+			  "verify_id_not_found");
+		/* id of "test_lookup2" will be id2 -1 as we have removed one type */
+		ASSERT_EQ(btf__find_by_name_kind(new_btf, "test_lookup2",
+						 BTF_KIND_TYPEDEF), id2 - 1,
+			  "verify_id_lookup2");
+
+	}
+	btf__free(new_btf);
+	free(new_raw_btf);
+	unlink(btf_path);
+	btf__free(btf);
+}
+
+void test_btf_kind(void)
+{
+	if (test__start_subtest("btf_kind_encoding"))
+		test_btf_kind_encoding();
+	if (test__start_subtest("btf_kind_decoding"))
+		test_btf_kind_decoding();
+}
-- 
2.39.3

[PATCH v5 bpf-next 8/9] bpftool: add BTF dump "format meta" to dump header/metadata

[Alan Maguire]

Provide a way to dump BTF metadata info via bpftool; this
consists of BTF size, header fields and kind layout info
(if available); for example

$ bpftool btf dump file vmlinux format meta
size 5169836
magic 0xeb9f
version 1
flags 0x1
hdr_len 40
type_len 3041436
type_off 0
str_len 2128279
str_off 3041436
kind_layout_len 80
kind_layout_off 5169716
kind 0    UNKNOWN    flags 0x0    info_sz 0    elem_sz 0
kind 1    INT        flags 0x0    info_sz 4    elem_sz 0
kind 2    PTR        flags 0x0    info_sz 0    elem_sz 0
kind 3    ARRAY      flags 0x0    info_sz 12   elem_sz 0
kind 4    STRUCT     flags 0x0    info_sz 0    elem_sz 12
...

JSON output is also supported:

$ bpftool -j btf dump file vmlinux format meta | jq
{
  "size": 5169836,
  "header": {
    "magic": 60319,
    "version": 1,
    "flags": 1,
    "hdr_len": 40,
    "type_len": 3041436,
    "type_off": 0,
    "str_len": 2128279,
    "str_off": 3041436,
    "kind_layout_len": 80,
    "kind_layout_offset": 5169716,
  },
  "kind_layouts": [
    {
      "kind": 0,
      "name": "UNKNOWN",
      "flags": 0,
      "info_sz": 0,
      "elem_sz": 0
    },
    {
      "kind": 1,
      "name": "INT",
      "flags": 0,
      "info_sz": 4,
      "elem_sz": 0
    },
...

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/bpf/bpftool/bash-completion/bpftool |  2 +-
 tools/bpf/bpftool/btf.c                   | 90 ++++++++++++++++++++++-
 2 files changed, 88 insertions(+), 4 deletions(-)

diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
index 1ce409a6cbd9..8accc9e153a7 100644
--- a/tools/bpf/bpftool/bash-completion/bpftool
+++ b/tools/bpf/bpftool/bash-completion/bpftool
@@ -928,7 +928,7 @@ _bpftool()
                             return 0
                             ;;
                         format)
-                            COMPREPLY=( $( compgen -W "c raw" -- "$cur" ) )
+                            COMPREPLY=( $( compgen -W "c raw meta" -- "$cur" ) )
                             ;;
                         root_id)
                             return 0;
diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index 6b14cbfa58aa..686608fb7b6c 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -835,6 +835,86 @@ static int dump_btf_c(const struct btf *btf,
 	return err;
 }
 
+static int dump_btf_meta(const struct btf *btf)
+{
+	const struct btf_header *hdr;
+	const struct btf_kind_layout *k;
+	__u8 i, nr_kinds = 0;
+	const void *data;
+	__u32 data_sz;
+
+	data = btf__raw_data(btf, &data_sz);
+	if (!data)
+		return -ENOMEM;
+	hdr = data;
+	if (json_output) {
+		jsonw_start_object(json_wtr);		/* metadata object */
+		jsonw_uint_field(json_wtr, "size", data_sz);
+		jsonw_name(json_wtr, "header");
+		jsonw_start_object(json_wtr);		/* header object */
+		jsonw_uint_field(json_wtr, "magic", hdr->magic);
+		jsonw_uint_field(json_wtr, "version", hdr->version);
+		jsonw_uint_field(json_wtr, "flags", hdr->flags);
+		jsonw_uint_field(json_wtr, "hdr_len", hdr->hdr_len);
+		jsonw_uint_field(json_wtr, "type_len", hdr->type_len);
+		jsonw_uint_field(json_wtr, "type_off", hdr->type_off);
+		jsonw_uint_field(json_wtr, "str_len", hdr->str_len);
+		jsonw_uint_field(json_wtr, "str_off", hdr->str_off);
+	} else {
+		printf("size %-10u\n", data_sz);
+		printf("magic 0x%-10x\nversion %-10d\nflags 0x%-10x\nhdr_len %-10u\n",
+		       hdr->magic, hdr->version, hdr->flags, hdr->hdr_len);
+		printf("type_len %-10u\ntype_off %-10u\n", hdr->type_len, hdr->type_off);
+		printf("str_len %-10u\nstr_off %-10u\n", hdr->str_len, hdr->str_off);
+	}
+
+	if (hdr->hdr_len < sizeof(struct btf_header)) {
+		if (json_output) {
+			jsonw_end_object(json_wtr);	/* end header object */
+			jsonw_end_object(json_wtr);	/* end metadata object */
+		}
+		return 0;
+	}
+	if (hdr->kind_layout_len > 0 && hdr->kind_layout_off > 0) {
+		k = (void *)hdr + hdr->hdr_len + hdr->kind_layout_off;
+		nr_kinds = hdr->kind_layout_len / sizeof(*k);
+	}
+	if (json_output) {
+		jsonw_uint_field(json_wtr, "kind_layout_len", hdr->kind_layout_len);
+		jsonw_uint_field(json_wtr, "kind_layout_offset", hdr->kind_layout_off);
+		jsonw_end_object(json_wtr);		/* end header object */
+
+		if (nr_kinds > 0) {
+			jsonw_name(json_wtr, "kind_layouts");
+			jsonw_start_array(json_wtr);
+			for (i = 0; i < nr_kinds; i++) {
+				jsonw_start_object(json_wtr);
+				jsonw_uint_field(json_wtr, "kind", i);
+				if (i < NR_BTF_KINDS)
+					jsonw_string_field(json_wtr, "name", btf_kind_str[i]);
+				else
+					jsonw_null_field(json_wtr, "name");
+				jsonw_uint_field(json_wtr, "flags", k[i].flags);
+				jsonw_uint_field(json_wtr, "info_sz", k[i].info_sz);
+				jsonw_uint_field(json_wtr, "elem_sz", k[i].elem_sz);
+				jsonw_end_object(json_wtr);
+			}
+			jsonw_end_array(json_wtr);
+		}
+		jsonw_end_object(json_wtr);		/* end metadata object */
+	} else {
+		printf("kind_layout_len %-10u\nkind_layout_off %-10u\n",
+		       hdr->kind_layout_len, hdr->kind_layout_off);
+		for (i = 0; i < nr_kinds; i++) {
+			printf("kind %-4d %-10s flags 0x%-4x info_sz %-4d elem_sz %-4d\n",
+			       i, i < NR_BTF_KINDS ? btf_kind_str[i] : "?",
+			       k[i].flags, k[i].info_sz, k[i].elem_sz);
+		}
+	}
+
+	return 0;
+}
+
 static const char sysfs_vmlinux[] = "/sys/kernel/btf/vmlinux";
 
 static struct btf *get_vmlinux_btf_from_sysfs(void)
@@ -880,7 +960,7 @@ static bool btf_is_kernel_module(__u32 btf_id)
 
 static int do_dump(int argc, char **argv)
 {
-	bool dump_c = false, sort_dump_c = true;
+	bool dump_c = false, sort_dump_c = true, dump_meta = false;
 	struct btf *btf = NULL, *base = NULL;
 	__u32 root_type_ids[MAX_ROOT_IDS];
 	bool have_id_filtering;
@@ -989,10 +1069,12 @@ static int do_dump(int argc, char **argv)
 			}
 			if (strcmp(*argv, "c") == 0) {
 				dump_c = true;
+			} else if (is_prefix(*argv, "meta")) {
+				dump_meta = true;
 			} else if (strcmp(*argv, "raw") == 0) {
 				dump_c = false;
 			} else {
-				p_err("unrecognized format specifier: '%s', possible values: raw, c",
+				p_err("unrecognized format specifier: '%s', possible values: raw, c, meta",
 				      *argv);
 				err = -EINVAL;
 				goto done;
@@ -1071,6 +1153,8 @@ static int do_dump(int argc, char **argv)
 			goto done;
 		}
 		err = dump_btf_c(btf, root_type_ids, root_type_cnt, sort_dump_c);
+	} else if (dump_meta) {
+		err = dump_btf_meta(btf);
 	} else {
 		err = dump_btf_raw(btf, root_type_ids, root_type_cnt);
 	}
@@ -1442,7 +1526,7 @@ static int do_help(int argc, char **argv)
 		"       %1$s %2$s help\n"
 		"\n"
 		"       BTF_SRC := { id BTF_ID | prog PROG | map MAP [{key | value | kv | all}] | file FILE }\n"
-		"       FORMAT  := { raw | c [unsorted] }\n"
+		"       FORMAT  := { raw | c [unsorted] | meta }\n"
 		"       " HELP_SPEC_MAP "\n"
 		"       " HELP_SPEC_PROGRAM "\n"
 		"       " HELP_SPEC_OPTIONS " |\n"
-- 
2.39.3

[PATCH v5 bpf-next 9/9] kbuild, bpf: Specify "kind_layout" optional feature

[Alan Maguire]

The "kind_layout" feature will add metadata about BTF kinds to the
generated BTF; its absence in pahole will not trigger an error so it
is safe to add unconditionally as it will simply be ignored if pahole
does not support it.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 scripts/Makefile.btf | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
index db76335dd917..c5a3526952ac 100644
--- a/scripts/Makefile.btf
+++ b/scripts/Makefile.btf
@@ -23,7 +23,7 @@ else
 # Switch to using --btf_features for v1.26 and later.
 pahole-flags-$(call test-ge, $(pahole-ver), 126)  = -j$(JOBS) --btf_features=encode_force,var,float,enum64,decl_tag,type_tag,optimized_func,consistent_func,decl_tag_kfuncs
 
-pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes
+pahole-flags-$(call test-ge, $(pahole-ver), 130) += --btf_features=attributes,kind_layout
 
 ifneq ($(KBUILD_EXTMOD),)
 module-pahole-flags-$(call test-ge, $(pahole-ver), 128) += --btf_features=distilled_base
-- 
2.39.3

Re: [PATCH v5 bpf-next 3/9] libbpf: use kind layout to compute an unknown kind size

[Alexei Starovoitov]

On Wed, May 28, 2025 at 2:58 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> This allows BTF parsing to proceed even if we do not know the
> kind.
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  tools/lib/bpf/btf.c | 35 ++++++++++++++++++++++++++++-------
>  1 file changed, 28 insertions(+), 7 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 43d1fce8977c..7a197dbfc689 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -355,7 +355,29 @@ static int btf_parse_kind_layout_sec(struct btf *btf)
>         return 0;
>  }
>
> -static int btf_type_size(const struct btf_type *t)
> +/* for unknown kinds, consult kind layout. */
> +static int btf_type_size_unknown(const struct btf *btf, const struct btf_type *t)
> +{
> +       int size = sizeof(struct btf_type);
> +       struct btf_kind_layout *k = NULL;
> +       __u16 vlen = btf_vlen(t);
> +       __u8 kind = btf_kind(t);
> +
> +       if (btf->kind_layout)
> +               k = &((struct btf_kind_layout *)btf->kind_layout)[kind];
> +
> +       if (!k || (void *)k > ((void *)btf->kind_layout + btf->hdr->kind_layout_len)) {
> +               pr_debug("Unsupported BTF_KIND: %u\n", btf_kind(t));
> +               return -EINVAL;

I'm missing the point around kind_layout->flags.
I was expecting that this helper and others at least
would check that flags == 0, but none of it is happening.
The patches say that flags is unused and do nothing.
Why add flags field at all?

> +       }
> +
> +       size += k->info_sz;
> +       size += vlen * k->elem_sz;
> +
> +       return size;
> +}

Re: [PATCH v5 bpf-next 3/9] libbpf: use kind layout to compute an unknown kind size

[Alan Maguire]

On 29/05/2025 06:35, Alexei Starovoitov wrote:
> On Wed, May 28, 2025 at 2:58 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> This allows BTF parsing to proceed even if we do not know the
>> kind.
>>
>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
>> ---
>>  tools/lib/bpf/btf.c | 35 ++++++++++++++++++++++++++++-------
>>  1 file changed, 28 insertions(+), 7 deletions(-)
>>
>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>> index 43d1fce8977c..7a197dbfc689 100644
>> --- a/tools/lib/bpf/btf.c
>> +++ b/tools/lib/bpf/btf.c
>> @@ -355,7 +355,29 @@ static int btf_parse_kind_layout_sec(struct btf *btf)
>>         return 0;
>>  }
>>
>> -static int btf_type_size(const struct btf_type *t)
>> +/* for unknown kinds, consult kind layout. */
>> +static int btf_type_size_unknown(const struct btf *btf, const struct btf_type *t)
>> +{
>> +       int size = sizeof(struct btf_type);
>> +       struct btf_kind_layout *k = NULL;
>> +       __u16 vlen = btf_vlen(t);
>> +       __u8 kind = btf_kind(t);
>> +
>> +       if (btf->kind_layout)
>> +               k = &((struct btf_kind_layout *)btf->kind_layout)[kind];
>> +
>> +       if (!k || (void *)k > ((void *)btf->kind_layout + btf->hdr->kind_layout_len)) {
>> +               pr_debug("Unsupported BTF_KIND: %u\n", btf_kind(t));
>> +               return -EINVAL;
> 
> I'm missing the point around kind_layout->flags.
> I was expecting that this helper and others at least
> would check that flags == 0, but none of it is happening.
> The patches say that flags is unused and do nothing.
> Why add flags field at all?
>

The intent of the flags field is to provide space to add additional
information about BTF kind encoding that may prove useful. E.g. at time
of encoding for this kind, was the kind flag supported? Perhaps if the
size/type field specifies a type or a size might be another useful flag
setting. But basically the idea is to provide space for additional
information around kind encoding for future use.

So in that context, should we check that flags are 0 now? I'm not sure,
because in some cases we'd like to have older libbpf be able to handle
newer kind layouts which might make use of flags.

>> +       }
>> +
>> +       size += k->info_sz;
>> +       size += vlen * k->elem_sz;
>> +
>> +       return size;
>> +}

Re: [PATCH v5 bpf-next 3/9] libbpf: use kind layout to compute an unknown kind size

[Alexei Starovoitov]

On Thu, May 29, 2025 at 5:53 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> On 29/05/2025 06:35, Alexei Starovoitov wrote:
> > On Wed, May 28, 2025 at 2:58 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> >>
> >> This allows BTF parsing to proceed even if we do not know the
> >> kind.
> >>
> >> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> >> ---
> >>  tools/lib/bpf/btf.c | 35 ++++++++++++++++++++++++++++-------
> >>  1 file changed, 28 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> >> index 43d1fce8977c..7a197dbfc689 100644
> >> --- a/tools/lib/bpf/btf.c
> >> +++ b/tools/lib/bpf/btf.c
> >> @@ -355,7 +355,29 @@ static int btf_parse_kind_layout_sec(struct btf *btf)
> >>         return 0;
> >>  }
> >>
> >> -static int btf_type_size(const struct btf_type *t)
> >> +/* for unknown kinds, consult kind layout. */
> >> +static int btf_type_size_unknown(const struct btf *btf, const struct btf_type *t)
> >> +{
> >> +       int size = sizeof(struct btf_type);
> >> +       struct btf_kind_layout *k = NULL;
> >> +       __u16 vlen = btf_vlen(t);
> >> +       __u8 kind = btf_kind(t);
> >> +
> >> +       if (btf->kind_layout)
> >> +               k = &((struct btf_kind_layout *)btf->kind_layout)[kind];
> >> +
> >> +       if (!k || (void *)k > ((void *)btf->kind_layout + btf->hdr->kind_layout_len)) {
> >> +               pr_debug("Unsupported BTF_KIND: %u\n", btf_kind(t));
> >> +               return -EINVAL;
> >
> > I'm missing the point around kind_layout->flags.
> > I was expecting that this helper and others at least
> > would check that flags == 0, but none of it is happening.
> > The patches say that flags is unused and do nothing.
> > Why add flags field at all?
> >
>
> The intent of the flags field is to provide space to add additional
> information about BTF kind encoding that may prove useful. E.g. at time
> of encoding for this kind, was the kind flag supported? Perhaps if the
> size/type field specifies a type or a size might be another useful flag
> setting. But basically the idea is to provide space for additional
> information around kind encoding for future use.

I feel there is a desire to add "flags" as an escape hatch,
but even for this example of "is kflag supported by this kind"
I suspect it would be incompatible to add it later.
Currently kflag is used by some, but not all kinds.
Say, we decide to make kflag meaningful for kind_int.
Currently kernel BTF validator will reject such BTF.
If we add a new flag to kind_laoyout->flags to indicate
that "at the time of encoding this kflag is supported"
the older kernel will ignore kind_layout->flags
and will error on future BTF with int's kflag.
So kind_layout->flags is really only meaningful for user tooling
that may or may not act on it.
Long ago there was an idea to use flags for "is it ok to skip
this unknown kind". imo it's the same issue. If we don't define
this flag now we won't be able to add it later and give it that meaning.
The "ok to ignore" flag will appear, but different versions
of libbpf/bpftool will either ignore it or will act on it.
That breaks the semantics of "ok to ignore".

> So in that context, should we check that flags are 0 now? I'm not sure,
> because in some cases we'd like to have older libbpf be able to handle
> newer kind layouts which might make use of flags.

yeah, if we make libbpf act as not-an-error on flags != 0 and
future version will make use of it, we're introducing an unusual
concept of flags that was never used in the kernel and libbpf.
Maybe it's ok, but I cannot wrap my head around it, since
prior knowledge and concepts don't apply to this scheme.
imo if we want extensibility for kind_layout we better use
known techniques like adding a size_of_kind_layout field to
struct btf_header, so we can add new fields to kind_layout and
do a similar check to bpf_check_uarg_tail_zero().
Then we don't have to add 'flags' there today.
We can add them later and bump the size.
I would also make
__u8 info_sz;
__u8 elem_sz;
to be __u32, since size is already __u32 in btf_type.
There is no need to save these few bytes.
With these two fields libbpf/bpftool can skip unknown kinds.
I would also add name to kind_layout, since extra hundred
bytes of strings is cheap, but warning about unknown kind
will be readable.

Re: [PATCH v5 bpf-next 3/9] libbpf: use kind layout to compute an unknown kind size

[Alan Maguire]

On 29/05/2025 17:30, Alexei Starovoitov wrote:
> On Thu, May 29, 2025 at 5:53 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>
>> On 29/05/2025 06:35, Alexei Starovoitov wrote:
>>> On Wed, May 28, 2025 at 2:58 AM Alan Maguire <alan.maguire@oracle.com> wrote:
>>>>
>>>> This allows BTF parsing to proceed even if we do not know the
>>>> kind.
>>>>
>>>> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
>>>> ---
>>>>  tools/lib/bpf/btf.c | 35 ++++++++++++++++++++++++++++-------
>>>>  1 file changed, 28 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>>>> index 43d1fce8977c..7a197dbfc689 100644
>>>> --- a/tools/lib/bpf/btf.c
>>>> +++ b/tools/lib/bpf/btf.c
>>>> @@ -355,7 +355,29 @@ static int btf_parse_kind_layout_sec(struct btf *btf)
>>>>         return 0;
>>>>  }
>>>>
>>>> -static int btf_type_size(const struct btf_type *t)
>>>> +/* for unknown kinds, consult kind layout. */
>>>> +static int btf_type_size_unknown(const struct btf *btf, const struct btf_type *t)
>>>> +{
>>>> +       int size = sizeof(struct btf_type);
>>>> +       struct btf_kind_layout *k = NULL;
>>>> +       __u16 vlen = btf_vlen(t);
>>>> +       __u8 kind = btf_kind(t);
>>>> +
>>>> +       if (btf->kind_layout)
>>>> +               k = &((struct btf_kind_layout *)btf->kind_layout)[kind];
>>>> +
>>>> +       if (!k || (void *)k > ((void *)btf->kind_layout + btf->hdr->kind_layout_len)) {
>>>> +               pr_debug("Unsupported BTF_KIND: %u\n", btf_kind(t));
>>>> +               return -EINVAL;
>>>
>>> I'm missing the point around kind_layout->flags.
>>> I was expecting that this helper and others at least
>>> would check that flags == 0, but none of it is happening.
>>> The patches say that flags is unused and do nothing.
>>> Why add flags field at all?
>>>
>>
>> The intent of the flags field is to provide space to add additional
>> information about BTF kind encoding that may prove useful. E.g. at time
>> of encoding for this kind, was the kind flag supported? Perhaps if the
>> size/type field specifies a type or a size might be another useful flag
>> setting. But basically the idea is to provide space for additional
>> information around kind encoding for future use.
> 
> I feel there is a desire to add "flags" as an escape hatch,
> but even for this example of "is kflag supported by this kind"
> I suspect it would be incompatible to add it later.
> Currently kflag is used by some, but not all kinds.
> Say, we decide to make kflag meaningful for kind_int.
> Currently kernel BTF validator will reject such BTF.
> If we add a new flag to kind_laoyout->flags to indicate
> that "at the time of encoding this kflag is supported"
> the older kernel will ignore kind_layout->flags
> and will error on future BTF with int's kflag.
> So kind_layout->flags is really only meaningful for user tooling
> that may or may not act on it.
> Long ago there was an idea to use flags for "is it ok to skip
> this unknown kind". imo it's the same issue. If we don't define
> this flag now we won't be able to add it later and give it that meaning.
> The "ok to ignore" flag will appear, but different versions
> of libbpf/bpftool will either ignore it or will act on it.
> That breaks the semantics of "ok to ignore".

Right. The challenge here is that there are two problems we might want
to solve here. One is the easier one; separating parsing BTF from being
able to use it. The basic kind_layout info solves this because it
associates the singular and vlen-specified lengths with each kind.

The second, harder problem - which I had hoped we might be able to crack
- is how to make BTF _useful_ to older tools when it contains kinds they
do not support.  We've often seen the case where distro kernels are
built with a newer pahole so have newer kind info and older tools run on
those kernels then fall over trying to read their vmlinux BTF. My
original hope was that we could provide enough context in the kind
layout to sanitize BTF with new kinds, but the challenge is to ensure
that they do not participate in the type graph (and then whether they
can be safely skipped) we need to know where type references live in
each kind representation.

Another more dynamic sort of approach to this problem would be a kind of
auto-negotiation with the kernel. Since the kernel is the source of the
newer BTF info, we could imagine asking for a mostly-equivalent BTF
representation that supports the more limited subset of features.
Because it knows what the kinds are, it is in a position to generate
such a subset. This would be in-kernel BTF sanitization similar to what
is done in libbpf for BPF program BTF. We also sometimes have the case
that BTF generated during compilation is too new for the kernel, but
libbpf's BTF sanitization helps there.

I guess what I'm getting at here is we can potentially separate these
problems and focus on making BTF parseable for now.

> 
>> So in that context, should we check that flags are 0 now? I'm not sure,
>> because in some cases we'd like to have older libbpf be able to handle
>> newer kind layouts which might make use of flags.
> 
> yeah, if we make libbpf act as not-an-error on flags != 0 and
> future version will make use of it, we're introducing an unusual
> concept of flags that was never used in the kernel and libbpf.
> Maybe it's ok, but I cannot wrap my head around it, since
> prior knowledge and concepts don't apply to this scheme.
> imo if we want extensibility for kind_layout we better use
> known techniques like adding a size_of_kind_layout field to
> struct btf_header, so we can add new fields to kind_layout and
> do a similar check to bpf_check_uarg_tail_zero().
> Then we don't have to add 'flags' there today.
> We can add them later and bump the size.

Makes sense.

> I would also make
> __u8 info_sz;
> __u8 elem_sz;
> to be __u32, since size is already __u32 in btf_type.
> There is no need to save these few bytes.
> With these two fields libbpf/bpftool can skip unknown kinds.
> I would also add name to kind_layout, since extra hundred
> bytes of strings is cheap, but warning about unknown kind
> will be readable.

Sure, will do. Thanks!

Alan

Re: [PATCH v5 bpf-next 8/9] bpftool: add BTF dump "format meta" to dump header/metadata

[Quentin Monnet]

2025-05-28 10:57 UTC+0100 ~ Alan Maguire <alan.maguire@oracle.com>
> Provide a way to dump BTF metadata info via bpftool; this
> consists of BTF size, header fields and kind layout info
> (if available); for example
> 
> $ bpftool btf dump file vmlinux format meta
> size 5169836
> magic 0xeb9f
> version 1
> flags 0x1
> hdr_len 40
> type_len 3041436
> type_off 0
> str_len 2128279
> str_off 3041436
> kind_layout_len 80
> kind_layout_off 5169716
> kind 0    UNKNOWN    flags 0x0    info_sz 0    elem_sz 0
> kind 1    INT        flags 0x0    info_sz 4    elem_sz 0
> kind 2    PTR        flags 0x0    info_sz 0    elem_sz 0
> kind 3    ARRAY      flags 0x0    info_sz 12   elem_sz 0
> kind 4    STRUCT     flags 0x0    info_sz 0    elem_sz 12
> ...
> 
> JSON output is also supported:
> 
> $ bpftool -j btf dump file vmlinux format meta | jq


Just so that you know, we have "bpftool -p" (which is the short version
for "bpftool --json --pretty") that will prettify the output without
requiring you to pipe to jq.


> {
>   "size": 5169836,
>   "header": {
>     "magic": 60319,
>     "version": 1,
>     "flags": 1,
>     "hdr_len": 40,
>     "type_len": 3041436,
>     "type_off": 0,
>     "str_len": 2128279,
>     "str_off": 3041436,
>     "kind_layout_len": 80,
>     "kind_layout_offset": 5169716,
>   },
>   "kind_layouts": [
>     {
>       "kind": 0,
>       "name": "UNKNOWN",
>       "flags": 0,
>       "info_sz": 0,
>       "elem_sz": 0
>     },
>     {
>       "kind": 1,
>       "name": "INT",
>       "flags": 0,
>       "info_sz": 4,
>       "elem_sz": 0
>     },
> ...
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  tools/bpf/bpftool/bash-completion/bpftool |  2 +-
>  tools/bpf/bpftool/btf.c                   | 90 ++++++++++++++++++++++-
>  2 files changed, 88 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/bpf/bpftool/bash-completion/bpftool b/tools/bpf/bpftool/bash-completion/bpftool
> index 1ce409a6cbd9..8accc9e153a7 100644
> --- a/tools/bpf/bpftool/bash-completion/bpftool
> +++ b/tools/bpf/bpftool/bash-completion/bpftool
> @@ -928,7 +928,7 @@ _bpftool()
>                              return 0
>                              ;;
>                          format)
> -                            COMPREPLY=( $( compgen -W "c raw" -- "$cur" ) )
> +                            COMPREPLY=( $( compgen -W "c raw meta" -- "$cur" ) )
>                              ;;
>                          root_id)
>                              return 0;


Thanks for the bash completion; could you also please update bpftool's
documentation? The bpftool-btf.rst page (two locations: the "BTF
COMMANDS" summary at the top, and the command description - you could
even add the example from your commit description as an example at the
bottom of the page, if you'd like).


> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 6b14cbfa58aa..686608fb7b6c 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -835,6 +835,86 @@ static int dump_btf_c(const struct btf *btf,
>  	return err;
>  }
>  
> +static int dump_btf_meta(const struct btf *btf)
> +{
> +	const struct btf_header *hdr;
> +	const struct btf_kind_layout *k;
> +	__u8 i, nr_kinds = 0;
> +	const void *data;
> +	__u32 data_sz;
> +
> +	data = btf__raw_data(btf, &data_sz);
> +	if (!data)
> +		return -ENOMEM;
> +	hdr = data;
> +	if (json_output) {
> +		jsonw_start_object(json_wtr);		/* metadata object */
> +		jsonw_uint_field(json_wtr, "size", data_sz);
> +		jsonw_name(json_wtr, "header");
> +		jsonw_start_object(json_wtr);		/* header object */
> +		jsonw_uint_field(json_wtr, "magic", hdr->magic);
> +		jsonw_uint_field(json_wtr, "version", hdr->version);
> +		jsonw_uint_field(json_wtr, "flags", hdr->flags);
> +		jsonw_uint_field(json_wtr, "hdr_len", hdr->hdr_len);
> +		jsonw_uint_field(json_wtr, "type_len", hdr->type_len);
> +		jsonw_uint_field(json_wtr, "type_off", hdr->type_off);
> +		jsonw_uint_field(json_wtr, "str_len", hdr->str_len);
> +		jsonw_uint_field(json_wtr, "str_off", hdr->str_off);
> +	} else {
> +		printf("size %-10u\n", data_sz);
> +		printf("magic 0x%-10x\nversion %-10d\nflags 0x%-10x\nhdr_len %-10u\n",
> +		       hdr->magic, hdr->version, hdr->flags, hdr->hdr_len);
> +		printf("type_len %-10u\ntype_off %-10u\n", hdr->type_len, hdr->type_off);
> +		printf("str_len %-10u\nstr_off %-10u\n", hdr->str_len, hdr->str_off);
> +	}
> +
> +	if (hdr->hdr_len < sizeof(struct btf_header)) {
> +		if (json_output) {
> +			jsonw_end_object(json_wtr);	/* end header object */
> +			jsonw_end_object(json_wtr);	/* end metadata object */
> +		}
> +		return 0;
> +	}
> +	if (hdr->kind_layout_len > 0 && hdr->kind_layout_off > 0) {
> +		k = (void *)hdr + hdr->hdr_len + hdr->kind_layout_off;
> +		nr_kinds = hdr->kind_layout_len / sizeof(*k);
> +	}
> +	if (json_output) {
> +		jsonw_uint_field(json_wtr, "kind_layout_len", hdr->kind_layout_len);
> +		jsonw_uint_field(json_wtr, "kind_layout_offset", hdr->kind_layout_off);
> +		jsonw_end_object(json_wtr);		/* end header object */
> +
> +		if (nr_kinds > 0) {
> +			jsonw_name(json_wtr, "kind_layouts");
> +			jsonw_start_array(json_wtr);
> +			for (i = 0; i < nr_kinds; i++) {
> +				jsonw_start_object(json_wtr);
> +				jsonw_uint_field(json_wtr, "kind", i);
> +				if (i < NR_BTF_KINDS)
> +					jsonw_string_field(json_wtr, "name", btf_kind_str[i]);
> +				else
> +					jsonw_null_field(json_wtr, "name");


Thanks!


> +				jsonw_uint_field(json_wtr, "flags", k[i].flags);
> +				jsonw_uint_field(json_wtr, "info_sz", k[i].info_sz);
> +				jsonw_uint_field(json_wtr, "elem_sz", k[i].elem_sz);
> +				jsonw_end_object(json_wtr);
> +			}
> +			jsonw_end_array(json_wtr);
> +		}
> +		jsonw_end_object(json_wtr);		/* end metadata object */


Why not have the fields unconditionally in JSON?

	...
	  "kind_layout_len": 0,
	  "kind_layout_offset": 0,
	},
	"kind_layouts": [
	]

This would allow tools to know explicitely that there's no kind layout
info, or to loop on kind_layouts without having to check for existence
first.

If the fields are not set, I find it always difficult to know if there's
no kind info or if bpftool didn't support dumping the kind info (in that
case this concern doesn't really apply because we add support for kind
info dump and the "meta" format at the same time, but users don't
necessarily know that).

Thanks,
Quentin