bpf: verifier (add verifier core)

This patch adds verifier core which simulates execution of every insn and
records the state of registers and program stack. Every branch instruction seen
during simulation is pushed into state stack. When verifier reaches BPF_EXIT,
it pops the state from the stack and continues until it reaches BPF_EXIT again.
For program:
1: bpf_mov r1, xxx
2: if (r1 == 0) goto 5
3: bpf_mov r0, 1
4: goto 6
5: bpf_mov r0, 2
6: bpf_exit
The verifier will walk insns: 1, 2, 3, 4, 6
then it will pop the state recorded at insn#2 and will continue: 5, 6

This way it walks all possible paths through the program and checks all
possible values of registers. While doing so, it checks for:
- invalid instructions
- uninitialized register access
- uninitialized stack access
- misaligned stack access
- out of range stack access
- invalid calling convention
- instruction encoding is not using reserved fields

Kernel subsystem configures the verifier with two callbacks:

- bool (*is_valid_access)(int off, int size, enum bpf_access_type type);
  that provides information to the verifer which fields of 'ctx'
  are accessible (remember 'ctx' is the first argument to eBPF program)

- const struct bpf_func_proto *(*get_func_proto)(enum bpf_func_id func_id);
  returns argument constraints of kernel helper functions that eBPF program
  may call, so that verifier can checks that R1-R5 types match the prototype

More details in Documentation/networking/filter.txt and in kernel/bpf/verifier.c
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpf.h

@@ -46,6 +46,31 @@ void bpf_register_map_type(struct bpf_map_type_list *tl);
 void bpf_map_put(struct bpf_map *map);
 struct bpf_map *bpf_map_get(struct fd f);
 
+/* function argument constraints */
+enum bpf_arg_type {
+	ARG_ANYTHING = 0,	/* any argument is ok */
+
+	/* the following constraints used to prototype
+	 * bpf_map_lookup/update/delete_elem() functions
+	 */
+	ARG_CONST_MAP_PTR,	/* const argument used as pointer to bpf_map */
+	ARG_PTR_TO_MAP_KEY,	/* pointer to stack used as map key */
+	ARG_PTR_TO_MAP_VALUE,	/* pointer to stack used as map value */
+
+	/* the following constraints used to prototype bpf_memcmp() and other
+	 * functions that access data on eBPF program stack
+	 */
+	ARG_PTR_TO_STACK,	/* any pointer to eBPF program stack */
+	ARG_CONST_STACK_SIZE,	/* number of bytes accessed from stack */
+};
+
+/* type of values returned from helper functions */
+enum bpf_return_type {
+	RET_INTEGER,			/* function returns integer */
+	RET_VOID,			/* function doesn't return anything */
+	RET_PTR_TO_MAP_VALUE_OR_NULL,	/* returns a pointer to map elem value or NULL */
+};
+
 /* 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
  * instructions after verifying
@@ -53,11 +78,33 @@ struct bpf_map *bpf_map_get(struct fd f);
 struct bpf_func_proto {
 	u64 (*func)(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
 	bool gpl_only;
+	enum bpf_return_type ret_type;
+	enum bpf_arg_type arg1_type;
+	enum bpf_arg_type arg2_type;
+	enum bpf_arg_type arg3_type;
+	enum bpf_arg_type arg4_type;
+	enum bpf_arg_type arg5_type;
+};
+
+/* bpf_context is intentionally undefined structure. Pointer to bpf_context is
+ * the first argument to eBPF programs.
+ * For socket filters: 'struct bpf_context *' == 'struct sk_buff *'
+ */
+struct bpf_context;
+
+enum bpf_access_type {
+	BPF_READ = 1,
+	BPF_WRITE = 2
 };
 
 struct bpf_verifier_ops {
 	/* return eBPF function prototype for verification */
 	const struct bpf_func_proto *(*get_func_proto)(enum bpf_func_id func_id);
+
+	/* return true if 'size' wide access at offset 'off' within bpf_context
+	 * with 'type' (read or write) is allowed
+	 */
+	bool (*is_valid_access)(int off, int size, enum bpf_access_type type);
 };
 
 struct bpf_prog_type_list {