[PATCH bpf-next 4/7] bpf: btf: Remove unused bits from uapi/linux/btf.h

[Martin KaFai Lau <kafai@fb.com>]

This patch does the followings:
1. Limit BTF_MAX_TYPES and BTF_MAX_NAME_OFFSET to 64k.  We can
   raise it later.

2. Remove the BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID.  They are
   currently encoded at the highest bit of a u32.
   It is because the current use case does not require supporting
   parent type (i.e type_id referring to a type in another BTF file).
   It also does not support referring to a string in ELF.

   The BTF_TYPE_PARENT and BTF_STR_TBL_ELF_ID checks are replaced
   by BTF_TYPE_ID_CHECK and BTF_STR_OFFSET_CHECK which are
   defined in btf.c instead of uapi/linux/btf.h.

3. Limit the BTF_INFO_KIND from 5 bits to 4 bits which is enough.
   There is unused bits headroom if we ever needed it later.

4. The root bit in BTF_INFO is also removed because it is not
   used in the current use case.

The above can be added back later because the verifier
ensures the unused bits are zeros.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
 include/uapi/linux/btf.h | 20 +++++---------------
 kernel/bpf/btf.c         | 34 +++++++++++++++++++++-------------
 2 files changed, 26 insertions(+), 28 deletions(-)

diff --git a/include/uapi/linux/btf.h b/include/uapi/linux/btf.h
index 4fa479741a02..b89b56f2b099 100644
--- a/include/uapi/linux/btf.h
+++ b/include/uapi/linux/btf.h
@@ -22,28 +22,19 @@ struct btf_header {
 };
 
 /* Max # of type identifier */
-#define BTF_MAX_TYPE	0x7fffffff
+#define BTF_MAX_TYPE	0x0000ffff
 /* Max offset into the string section */
-#define BTF_MAX_NAME_OFFSET	0x7fffffff
+#define BTF_MAX_NAME_OFFSET	0x0000ffff
 /* Max # of struct/union/enum members or func args */
 #define BTF_MAX_VLEN	0xffff
 
-/* The type id is referring to a parent BTF */
-#define BTF_TYPE_PARENT(id)	(((id) >> 31) & 0x1)
-#define BTF_TYPE_ID(id)		((id) & BTF_MAX_TYPE)
-
-/* String is in the ELF string section */
-#define BTF_STR_TBL_ELF_ID(ref)	(((ref) >> 31) & 0x1)
-#define BTF_STR_OFFSET(ref)	((ref) & BTF_MAX_NAME_OFFSET)
-
 struct btf_type {
 	__u32 name_off;
 	/* "info" bits arrangement
 	 * bits  0-15: vlen (e.g. # of struct's members)
 	 * bits 16-23: unused
-	 * bits 24-28: kind (e.g. int, ptr, array...etc)
-	 * bits 29-30: unused
-	 * bits    31: root
+	 * bits 24-27: kind (e.g. int, ptr, array...etc)
+	 * bits 28-31: unused
 	 */
 	__u32 info;
 	/* "size" is used by INT, ENUM, STRUCT and UNION.
@@ -58,8 +49,7 @@ struct btf_type {
 	};
 };
 
-#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x1f)
-#define BTF_INFO_ISROOT(info)	(!!(((info) >> 24) & 0x80))
+#define BTF_INFO_KIND(info)	(((info) >> 24) & 0x0f)
 #define BTF_INFO_VLEN(info)	((info) & 0xffff)
 
 #define BTF_KIND_UNKN		0	/* Unknown	*/
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index b4e48dae2240..5d1967d4fb62 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -163,13 +163,15 @@
 #define BITS_ROUNDUP_BYTES(bits) \
 	(BITS_ROUNDDOWN_BYTES(bits) + !!BITS_PER_BYTE_MASKED(bits))
 
+#define BTF_INFO_MASK 0x0f00ffff
+#define BTF_TYPE_ID_CHECK(type_id) ((type_id) <= BTF_MAX_TYPE)
+#define BTF_STR_OFFSET_CHECK(name_off) ((name_off) <= BTF_MAX_NAME_OFFSET)
+
 /* 16MB for 64k structs and each has 16 members and
  * a few MB spaces for the string section.
  * The hard limit is S32_MAX.
  */
 #define BTF_MAX_SIZE (16 * 1024 * 1024)
-/* 64k. We can raise it later. The hard limit is S32_MAX. */
-#define BTF_MAX_NR_TYPES 65535
 
 #define for_each_member(i, struct_type, member)			\
 	for (i = 0, member = btf_type_member(struct_type);	\
@@ -422,16 +424,16 @@ static const struct btf_kind_operations *btf_type_ops(const struct btf_type *t)
 
 static bool btf_name_offset_valid(const struct btf *btf, u32 offset)
 {
-	return !BTF_STR_TBL_ELF_ID(offset) &&
-		BTF_STR_OFFSET(offset) < btf->hdr.str_len;
+	return BTF_STR_OFFSET_CHECK(offset) &&
+		offset < btf->hdr.str_len;
 }
 
 static const char *btf_name_by_offset(const struct btf *btf, u32 offset)
 {
-	if (!BTF_STR_OFFSET(offset))
+	if (!offset)
 		return "(anon)";
-	else if (BTF_STR_OFFSET(offset) < btf->hdr.str_len)
-		return &btf->strings[BTF_STR_OFFSET(offset)];
+	else if (offset < btf->hdr.str_len)
+		return &btf->strings[offset];
 	else
 		return "(invalid-name-offset)";
 }
@@ -599,13 +601,13 @@ static int btf_add_type(struct btf_verifier_env *env, struct btf_type *t)
 		struct btf_type **new_types;
 		u32 expand_by, new_size;
 
-		if (btf->types_size == BTF_MAX_NR_TYPES) {
+		if (btf->types_size == BTF_MAX_TYPE) {
 			btf_verifier_log(env, "Exceeded max num of types");
 			return -E2BIG;
 		}
 
 		expand_by = max_t(u32, btf->types_size >> 2, 16);
-		new_size = min_t(u32, BTF_MAX_NR_TYPES,
+		new_size = min_t(u32, BTF_MAX_TYPE,
 				 btf->types_size + expand_by);
 
 		new_types = kvzalloc(new_size * sizeof(*new_types),
@@ -1127,7 +1129,7 @@ static int btf_ref_type_check_meta(struct btf_verifier_env *env,
 		return -EINVAL;
 	}
 
-	if (BTF_TYPE_PARENT(t->type)) {
+	if (!BTF_TYPE_ID_CHECK(t->type)) {
 		btf_verifier_log_type(env, t, "Invalid type_id");
 		return -EINVAL;
 	}
@@ -1334,12 +1336,12 @@ static s32 btf_array_check_meta(struct btf_verifier_env *env,
 	/* Array elem type and index type cannot be in type void,
 	 * so !array->type and !array->index_type are not allowed.
 	 */
-	if (!array->type || BTF_TYPE_PARENT(array->type)) {
+	if (!array->type || !BTF_TYPE_ID_CHECK(array->type)) {
 		btf_verifier_log_type(env, t, "Invalid elem");
 		return -EINVAL;
 	}
 
-	if (!array->index_type || BTF_TYPE_PARENT(array->index_type)) {
+	if (!array->index_type || !BTF_TYPE_ID_CHECK(array->index_type)) {
 		btf_verifier_log_type(env, t, "Invalid index");
 		return -EINVAL;
 	}
@@ -1511,7 +1513,7 @@ static s32 btf_struct_check_meta(struct btf_verifier_env *env,
 		}
 
 		/* A member cannot be in type void */
-		if (!member->type || BTF_TYPE_PARENT(member->type)) {
+		if (!member->type || !BTF_TYPE_ID_CHECK(member->type)) {
 			btf_verifier_log_member(env, t, member,
 						"Invalid type_id");
 			return -EINVAL;
@@ -1764,6 +1766,12 @@ static s32 btf_check_meta(struct btf_verifier_env *env,
 	}
 	meta_left -= sizeof(*t);
 
+	if (t->info & ~BTF_INFO_MASK) {
+		btf_verifier_log(env, "[%u] Invalid btf_info:%x",
+				 env->log_type_id, t->info);
+		return -EINVAL;
+	}
+
 	if (BTF_INFO_KIND(t->info) > BTF_KIND_MAX ||
 	    BTF_INFO_KIND(t->info) == BTF_KIND_UNKN) {
 		btf_verifier_log(env, "[%u] Invalid kind:%u",
-- 
2.9.5