libbpf: Allow modification of BTF and add btf__add_str API

Allow internal BTF representation to switch from default read-only mode, in
which raw BTF data is a single non-modifiable block of memory with BTF header,
types, and strings layed out sequentially and contiguously in memory, into
a writable representation with types and strings data split out into separate
memory regions, that can be dynamically expanded.

Such writable internal representation is transparent to users of libbpf APIs,
but allows to append new types and strings at the end of BTF, which is
a typical use case when generating BTF programmatically. All the basic
guarantees of BTF types and strings layout is preserved, i.e., user can get
`struct btf_type *` pointer and read it directly. Such btf_type pointers might
be invalidated if BTF is modified, so some care is required in such mixed
read/write scenarios.

Switch from read-only to writable configuration happens automatically the
first time when user attempts to modify BTF by either adding a new type or new
string. It is still possible to get raw BTF data, which is a single piece of
memory that can be persisted in ELF section or into a file as raw BTF. Such
raw data memory is also still owned by BTF and will be freed either when BTF
object is freed or if another modification to BTF happens, as any modification
invalidates BTF raw representation.

This patch adds the first two BTF manipulation APIs: btf__add_str(), which
allows to add arbitrary strings to BTF string section, and btf__find_str()
which allows to find existing string offset, but not add it if it's missing.
All the added strings are automatically deduplicated. This is achieved by
maintaining an additional string lookup index for all unique strings. Such
index is built when BTF is switched to modifiable mode. If at that time BTF
strings section contained duplicate strings, they are not de-duplicated. This
is done specifically to not modify the existing content of BTF (types, their
string offsets, etc), which can cause confusion and is especially important
property if there is struct btf_ext associated with struct btf. By following
this "imperfect deduplication" process, btf_ext is kept consitent and correct.
If deduplication of strings is necessary, it can be forced by doing BTF
deduplication, at which point all the strings will be eagerly deduplicated and
all string offsets both in struct btf and struct btf_ext will be updated.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200926011357.2366158-6-andriin@fb.com

tools/lib/bpf/btf.c

@@ -44,16 +44,46 @@ struct btf {
 	 * hdr        |         |
 	 * types_data-+         |
 	 * strs_data------------+
+	 *
+	 * 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.
+	 * raw_data is nulled out at that point, but can be later allocated
+	 * and cached again if user calls btf__get_raw_data(), at which point
+	 * raw_data will contain a contiguous copy of header, types, and
+	 * strings:
+	 *
+	 * +----------+  +---------+  +-----------+
+	 * |  Header  |  |  Types  |  |  Strings  |
+	 * +----------+  +---------+  +-----------+
+	 * ^             ^            ^
+	 * |             |            |
+	 * hdr           |            |
+	 * types_data----+            |
+	 * strs_data------------------+
+	 *
+	 *               +----------+---------+-----------+
+	 *               |  Header  |  Types  |  Strings  |
+	 * raw_data----->+----------+---------+-----------+
 	 */
 	struct btf_header *hdr;
+
 	void *types_data;
-	void *strs_data;
+	size_t types_data_cap; /* used size stored in hdr->type_len */
 
 	/* type ID to `struct btf_type *` lookup index */
 	__u32 *type_offs;
 	size_t type_offs_cap;
 	__u32 nr_types;
 
+	void *strs_data;
+	size_t strs_data_cap; /* used size stored in hdr->str_len */
+
+	/* lookup index for each unique string in strings section */
+	struct hashmap *strs_hash;
+	/* whether strings are already deduplicated */
+	bool strs_deduped;
 	/* BTF object FD, if loaded into kernel */
 	int fd;
 
@@ -506,6 +536,11 @@ __s32 btf__find_by_name_kind(const struct btf *btf, const char *type_name,
 	return -ENOENT;
 }
 
+static bool btf_is_modifiable(const struct btf *btf)
+{
+	return (void *)btf->hdr != btf->raw_data;
+}
+
 void btf__free(struct btf *btf)
 {
 	if (IS_ERR_OR_NULL(btf))
@@ -514,6 +549,17 @@ void btf__free(struct btf *btf)
 	if (btf->fd >= 0)
 		close(btf->fd);
 
+	if (btf_is_modifiable(btf)) {
+		/* if BTF was modified after loading, it will have a split
+		 * in-memory representation for header, types, and strings
+		 * sections, so we need to free all of them individually. It
+		 * might still have a cached contiguous raw data present,
+		 * which will be unconditionally freed below.
+		 */
+		free(btf->hdr);
+		free(btf->types_data);
+		free(btf->strs_data);
+	}
 	free(btf->raw_data);
 	free(btf->type_offs);
 	free(btf);
@@ -922,8 +968,29 @@ void btf__set_fd(struct btf *btf, int fd)
 	btf->fd = fd;
 }
 
-const void *btf__get_raw_data(const struct btf *btf, __u32 *size)
+const void *btf__get_raw_data(const struct btf *btf_ro, __u32 *size)
 {
+	struct btf *btf = (struct btf *)btf_ro;
+
+	if (!btf->raw_data) {
+		struct btf_header *hdr = btf->hdr;
+		void *data;
+
+		btf->raw_size = hdr->hdr_len + hdr->type_len + hdr->str_len;
+		btf->raw_data = calloc(1, btf->raw_size);
+		if (!btf->raw_data)
+			return NULL;
+		data = btf->raw_data;
+
+		memcpy(data, hdr, hdr->hdr_len);
+		data += hdr->hdr_len;
+
+		memcpy(data, btf->types_data, hdr->type_len);
+		data += hdr->type_len;
+
+		memcpy(data, btf->strs_data, hdr->str_len);
+		data += hdr->str_len;
+	}
 	*size = btf->raw_size;
 	return btf->raw_data;
 }
@@ -1071,6 +1138,181 @@ int btf__get_map_kv_tids(const struct btf *btf, const char *map_name,
 	return 0;
 }
 
+static size_t strs_hash_fn(const void *key, void *ctx)
+{
+	struct btf *btf = ctx;
+	const char *str = btf->strs_data + (long)key;
+
+	return str_hash(str);
+}
+
+static bool strs_hash_equal_fn(const void *key1, const void *key2, void *ctx)
+{
+	struct btf *btf = ctx;
+	const char *str1 = btf->strs_data + (long)key1;
+	const char *str2 = btf->strs_data + (long)key2;
+
+	return strcmp(str1, str2) == 0;
+}
+
+/* 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.
+ */
+static int btf_ensure_modifiable(struct btf *btf)
+{
+	void *hdr, *types, *strs, *strs_end, *s;
+	struct hashmap *hash = NULL;
+	long off;
+	int err;
+
+	if (btf_is_modifiable(btf)) {
+		/* any BTF modification invalidates raw_data */
+		if (btf->raw_data) {
+			free(btf->raw_data);
+			btf->raw_data = NULL;
+		}
+		return 0;
+	}
+
+	/* split raw data into three memory regions */
+	hdr = malloc(btf->hdr->hdr_len);
+	types = malloc(btf->hdr->type_len);
+	strs = malloc(btf->hdr->str_len);
+	if (!hdr || !types || !strs)
+		goto err_out;
+
+	memcpy(hdr, btf->hdr, btf->hdr->hdr_len);
+	memcpy(types, btf->types_data, btf->hdr->type_len);
+	memcpy(strs, btf->strs_data, btf->hdr->str_len);
+
+	/* build lookup index for all strings */
+	hash = hashmap__new(strs_hash_fn, strs_hash_equal_fn, btf);
+	if (IS_ERR(hash)) {
+		err = PTR_ERR(hash);
+		hash = NULL;
+		goto err_out;
+	}
+
+	strs_end = strs + btf->hdr->str_len;
+	for (off = 0, s = strs; s < strs_end; off += strlen(s) + 1, s = strs + off) {
+		/* hashmap__add() returns EEXIST if string with the same
+		 * content already is in the hash map
+		 */
+		err = hashmap__add(hash, (void *)off, (void *)off);
+		if (err == -EEXIST)
+			continue; /* duplicate */
+		if (err)
+			goto err_out;
+	}
+
+	/* 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 = strs;
+	btf->strs_data_cap = btf->hdr->str_len;
+	btf->strs_hash = hash;
+	/* if BTF was created from scratch, all strings are guaranteed to be
+	 * unique and deduplicated
+	 */
+	btf->strs_deduped = btf->hdr->str_len <= 1;
+
+	/* invalidate raw_data representation */
+	free(btf->raw_data);
+	btf->raw_data = NULL;
+
+	return 0;
+
+err_out:
+	hashmap__free(hash);
+	free(hdr);
+	free(types);
+	free(strs);
+	return -ENOMEM;
+}
+
+static void *btf_add_str_mem(struct btf *btf, size_t add_sz)
+{
+	return btf_add_mem(&btf->strs_data, &btf->strs_data_cap, 1,
+			   btf->hdr->str_len, BTF_MAX_STR_OFFSET, add_sz);
+}
+
+/* Find an offset in BTF string section that corresponds to a given string *s*.
+ * Returns:
+ *   - >0 offset into string section, if string is found;
+ *   - -ENOENT, if string is not in the string section;
+ *   - <0, on any other error.
+ */
+int btf__find_str(struct btf *btf, const char *s)
+{
+	long old_off, new_off, len;
+	void *p;
+
+	/* BTF needs to be in a modifiable state to build string lookup index */
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	/* see btf__add_str() for why we do this */
+	len = strlen(s) + 1;
+	p = btf_add_str_mem(btf, len);
+	if (!p)
+		return -ENOMEM;
+
+	new_off = btf->hdr->str_len;
+	memcpy(p, s, len);
+
+	if (hashmap__find(btf->strs_hash, (void *)new_off, (void **)&old_off))
+		return old_off;
+
+	return -ENOENT;
+}
+
+/* Add a string s to the BTF string section.
+ * Returns:
+ *   - > 0 offset into string section, on success;
+ *   - < 0, on error.
+ */
+int btf__add_str(struct btf *btf, const char *s)
+{
+	long old_off, new_off, len;
+	void *p;
+	int err;
+
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
+	/* Hashmap keys are always offsets within btf->strs_data, so to even
+	 * look up some string from the "outside", we need to first append it
+	 * at the end, so that it can be addressed with an offset. Luckily,
+	 * until btf->hdr->str_len is incremented, that string is just a piece
+	 * of garbage for the rest of BTF code, so no harm, no foul. On the
+	 * other hand, if the string is unique, it's already appended and
+	 * ready to be used, only a simple btf->hdr->str_len increment away.
+	 */
+	len = strlen(s) + 1;
+	p = btf_add_str_mem(btf, len);
+	if (!p)
+		return -ENOMEM;
+
+	new_off = btf->hdr->str_len;
+	memcpy(p, s, len);
+
+	/* Now attempt to add the string, but only if the string with the same
+	 * contents doesn't exist already (HASHMAP_ADD strategy). If such
+	 * string exists, we'll get its offset in old_off (that's old_key).
+	 */
+	err = hashmap__insert(btf->strs_hash, (void *)new_off, (void *)new_off,
+			      HASHMAP_ADD, (const void **)&old_off, NULL);
+	if (err == -EEXIST)
+		return old_off; /* duplicated string, return existing offset */
+	if (err)
+		return err;
+
+	btf->hdr->str_len += len; /* new unique string, adjust data length */
+	return new_off;
+}
+
 struct btf_ext_sec_setup_param {
 	__u32 off;
 	__u32 len;
@@ -1537,6 +1779,9 @@ int btf__dedup(struct btf *btf, struct btf_ext *btf_ext,
 		return -EINVAL;
 	}
 
+	if (btf_ensure_modifiable(btf))
+		return -ENOMEM;
+
 	err = btf_dedup_strings(d);
 	if (err < 0) {
 		pr_debug("btf_dedup_strings failed:%d\n", err);
@@ -1926,6 +2171,9 @@ static int btf_dedup_strings(struct btf_dedup *d)
 	int i, j, err = 0, grp_idx;
 	bool grp_used;
 
+	if (d->btf->strs_deduped)
+		return 0;
+
 	/* build index of all strings */
 	while (p < end) {
 		if (strs.cnt + 1 > strs.cap) {
@@ -2018,6 +2266,7 @@ static int btf_dedup_strings(struct btf_dedup *d)
 		goto done;
 
 	d->btf->hdr->str_len = end - start;
+	d->btf->strs_deduped = true;
 
 done:
 	free(tmp_strs);
@@ -3021,12 +3270,7 @@ static int btf_dedup_compact_types(struct btf_dedup *d)
 	if (!new_offs)
 		return -ENOMEM;
 	d->btf->type_offs = new_offs;
-
-	/* make sure string section follows type information without gaps */
-	d->btf->hdr->str_off = p - d->btf->types_data;
-	memmove(p, d->btf->strs_data, d->btf->hdr->str_len);
-	d->btf->strs_data = p;
-
+	d->btf->hdr->str_off = d->btf->hdr->type_len;
 	d->btf->raw_size = d->btf->hdr->hdr_len + d->btf->hdr->type_len + d->btf->hdr->str_len;
 	return 0;
 }

tools/lib/bpf/btf.h

@@ -5,6 +5,7 @@
 #define __LIBBPF_BTF_H
 
 #include <stdarg.h>
+#include <stdbool.h>
 #include <linux/btf.h>
 #include <linux/types.h>
 
@@ -72,6 +73,9 @@ LIBBPF_API __u32 btf_ext__line_info_rec_size(const struct btf_ext *btf_ext);
 
 LIBBPF_API struct btf *libbpf_find_kernel_btf(void);
 
+LIBBPF_API int btf__find_str(struct btf *btf, const char *s);
+LIBBPF_API int btf__add_str(struct btf *btf, const char *s);
+
 struct btf_dedup_opts {
 	unsigned int dedup_table_size;
 	bool dont_resolve_fwds;

tools/lib/bpf/libbpf.map

@@ -305,6 +305,8 @@ LIBBPF_0.2.0 {
 		bpf_prog_bind_map;
 		bpf_prog_test_run_opts;
 		bpf_program__section_name;
+		btf__add_str;
+		btf__find_str;
 		perf_buffer__buffer_cnt;
 		perf_buffer__buffer_fd;
 		perf_buffer__epoll_fd;