bpf: Introduce composable reg, ret and arg types.

There are some common properties shared between bpf reg, ret and arg
values. For instance, a value may be a NULL pointer, or a pointer to
a read-only memory. Previously, to express these properties, enumeration
was used. For example, in order to test whether a reg value can be NULL,
reg_type_may_be_null() simply enumerates all types that are possibly
NULL. The problem of this approach is that it's not scalable and causes
a lot of duplication. These properties can be combined, for example, a
type could be either MAYBE_NULL or RDONLY, or both.

This patch series rewrites the layout of reg_type, arg_type and
ret_type, so that common properties can be extracted and represented as
composable flag. For example, one can write

 ARG_PTR_TO_MEM | PTR_MAYBE_NULL

which is equivalent to the previous

 ARG_PTR_TO_MEM_OR_NULL

The type ARG_PTR_TO_MEM are called "base type" in this patch. Base
types can be extended with flags. A flag occupies the higher bits while
base types sits in the lower bits.

This patch in particular sets up a set of macro for this purpose. The
following patches will rewrite arg_types, ret_types and reg_types
respectively.
Signed-off-by: Hao Luo <haoluo@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211217003152.48334-2-haoluo@google.com

include/linux/bpf.h

@@ -297,6 +297,29 @@ bool bpf_map_meta_equal(const struct bpf_map *meta0,
 
 extern const struct bpf_map_ops bpf_map_offload_ops;
 
+/* bpf_type_flag contains a set of flags that are applicable to the values of
+ * arg_type, ret_type and reg_type. For example, a pointer value may be null,
+ * or a memory is read-only. We classify types into two categories: base types
+ * and extended types. Extended types are base types combined with a type flag.
+ *
+ * Currently there are no more than 32 base types in arg_type, ret_type and
+ * reg_types.
+ */
+#define BPF_BASE_TYPE_BITS	8
+
+enum bpf_type_flag {
+	/* PTR may be NULL. */
+	PTR_MAYBE_NULL		= BIT(0 + BPF_BASE_TYPE_BITS),
+
+	__BPF_TYPE_LAST_FLAG	= PTR_MAYBE_NULL,
+};
+
+/* Max number of base types. */
+#define BPF_BASE_TYPE_LIMIT	(1UL << BPF_BASE_TYPE_BITS)
+
+/* Max number of all types. */
+#define BPF_TYPE_LIMIT		(__BPF_TYPE_LAST_FLAG | (__BPF_TYPE_LAST_FLAG - 1))
+
 /* function argument constraints */
 enum bpf_arg_type {
 	ARG_DONTCARE = 0,	/* unused argument in helper function */
@@ -343,7 +366,13 @@ enum bpf_arg_type {
 	ARG_PTR_TO_CONST_STR,	/* pointer to a null terminated read-only string */
 	ARG_PTR_TO_TIMER,	/* pointer to bpf_timer */
 	__BPF_ARG_TYPE_MAX,
+
+	/* This must be the last entry. Its purpose is to ensure the enum is
+	 * wide enough to hold the higher bits reserved for bpf_type_flag.
+	 */
+	__BPF_ARG_TYPE_LIMIT	= BPF_TYPE_LIMIT,
 };
+static_assert(__BPF_ARG_TYPE_MAX <= BPF_BASE_TYPE_LIMIT);
 
 /* type of values returned from helper functions */
 enum bpf_return_type {
@@ -359,7 +388,14 @@ enum bpf_return_type {
 	RET_PTR_TO_MEM_OR_BTF_ID_OR_NULL, /* returns a pointer to a valid memory or a btf_id or NULL */
 	RET_PTR_TO_MEM_OR_BTF_ID,	/* returns a pointer to a valid memory or a btf_id */
 	RET_PTR_TO_BTF_ID,		/* returns a pointer to a btf_id */
+	__BPF_RET_TYPE_MAX,
+
+	/* This must be the last entry. Its purpose is to ensure the enum is
+	 * wide enough to hold the higher bits reserved for bpf_type_flag.
+	 */
+	__BPF_RET_TYPE_LIMIT	= BPF_TYPE_LIMIT,
 };
+static_assert(__BPF_RET_TYPE_MAX <= BPF_BASE_TYPE_LIMIT);
 
 /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
  * to in-kernel helper functions and for adjusting imm32 field in BPF_CALL
@@ -461,7 +497,13 @@ enum bpf_reg_type {
 	PTR_TO_FUNC,		 /* reg points to a bpf program function */
 	PTR_TO_MAP_KEY,		 /* reg points to a map element key */
 	__BPF_REG_TYPE_MAX,
+
+	/* This must be the last entry. Its purpose is to ensure the enum is
+	 * wide enough to hold the higher bits reserved for bpf_type_flag.
+	 */
+	__BPF_REG_TYPE_LIMIT	= BPF_TYPE_LIMIT,
 };
+static_assert(__BPF_REG_TYPE_MAX <= BPF_BASE_TYPE_LIMIT);
 
 /* The information passed from prog-specific *_is_valid_access
  * back to the verifier.

include/linux/bpf_verifier.h

@@ -546,5 +546,18 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    struct bpf_attach_target_info *tgt_info);
 void bpf_free_kfunc_btf_tab(struct bpf_kfunc_btf_tab *tab);
 
+#define BPF_BASE_TYPE_MASK	GENMASK(BPF_BASE_TYPE_BITS - 1, 0)
+
+/* extract base type from bpf_{arg, return, reg}_type. */
+static inline u32 base_type(u32 type)
+{
+	return type & BPF_BASE_TYPE_MASK;
+}
+
+/* extract flags from an extended type. See bpf_type_flag in bpf.h. */
+static inline u32 type_flag(u32 type)
+{
+	return type & ~BPF_BASE_TYPE_MASK;
+}
 
 #endif /* _LINUX_BPF_VERIFIER_H */