bpf: Allow helpers to accept pointers with a fixed size

Before this commit, the BPF verifier required ARG_PTR_TO_MEM arguments
to be followed by ARG_CONST_SIZE holding the size of the memory region.
The helpers had to check that size in runtime.

There are cases where the size expected by a helper is a compile-time
constant. Checking it in runtime is an unnecessary overhead and waste of
BPF registers.

This commit allows helpers to accept pointers to memory without the
corresponding ARG_CONST_SIZE, given that they define the memory region
size in struct bpf_func_proto and use ARG_PTR_TO_FIXED_SIZE_MEM type.

arg_size is unionized with arg_btf_id to reduce the kernel image size,
and it's valid because they are used by different argument types.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20220615134847.3753567-3-maximmi@nvidia.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -401,6 +401,9 @@ enum bpf_type_flag {
 	/* DYNPTR points to a ringbuf record. */
 	DYNPTR_TYPE_RINGBUF	= BIT(9 + BPF_BASE_TYPE_BITS),
 
+	/* Size is known at compile time. */
+	MEM_FIXED_SIZE		= BIT(10 + BPF_BASE_TYPE_BITS),
+
 	__BPF_TYPE_FLAG_MAX,
 	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,
 };
@@ -464,6 +467,8 @@ enum bpf_arg_type {
 	 * all bytes or clear them in error case.
 	 */
 	ARG_PTR_TO_UNINIT_MEM		= MEM_UNINIT | ARG_PTR_TO_MEM,
+	/* Pointer to valid memory of size known at compile time. */
+	ARG_PTR_TO_FIXED_SIZE_MEM	= MEM_FIXED_SIZE | ARG_PTR_TO_MEM,
 
 	/* 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.
@@ -529,6 +534,14 @@ struct bpf_func_proto {
 			u32 *arg5_btf_id;
 		};
 		u32 *arg_btf_id[5];
+		struct {
+			size_t arg1_size;
+			size_t arg2_size;
+			size_t arg3_size;
+			size_t arg4_size;
+			size_t arg5_size;
+		};
+		size_t arg_size[5];
 	};
 	int *ret_btf_id; /* return value btf_id */
 	bool (*allowed)(const struct bpf_prog *prog);

kernel/bpf/verifier.c

@@ -5848,6 +5848,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 	struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
 	enum bpf_arg_type arg_type = fn->arg_type[arg];
 	enum bpf_reg_type type = reg->type;
+	u32 *arg_btf_id = NULL;
 	int err = 0;
 
 	if (arg_type == ARG_DONTCARE)
@@ -5884,7 +5885,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		 */
 		goto skip_type_check;
 
-	err = check_reg_type(env, regno, arg_type, fn->arg_btf_id[arg], meta);
+	/* arg_btf_id and arg_size are in a union. */
+	if (base_type(arg_type) == ARG_PTR_TO_BTF_ID)
+		arg_btf_id = fn->arg_btf_id[arg];
+
+	err = check_reg_type(env, regno, arg_type, arg_btf_id, meta);
 	if (err)
 		return err;
 
@@ -6011,6 +6016,11 @@ skip_type_check:
 		 * next is_mem_size argument below.
 		 */
 		meta->raw_mode = arg_type & MEM_UNINIT;
+		if (arg_type & MEM_FIXED_SIZE) {
+			err = check_helper_mem_access(env, regno,
+						      fn->arg_size[arg], false,
+						      meta);
+		}
 	} else if (arg_type_is_mem_size(arg_type)) {
 		bool zero_size_allowed = (arg_type == ARG_CONST_SIZE_OR_ZERO);
 
@@ -6400,11 +6410,19 @@ static bool check_raw_mode_ok(const struct bpf_func_proto *fn)
 	return count <= 1;
 }
 
-static bool check_args_pair_invalid(enum bpf_arg_type arg_curr,
-				    enum bpf_arg_type arg_next)
+static bool check_args_pair_invalid(const struct bpf_func_proto *fn, int arg)
 {
-	return (base_type(arg_curr) == ARG_PTR_TO_MEM) !=
-		arg_type_is_mem_size(arg_next);
+	bool is_fixed = fn->arg_type[arg] & MEM_FIXED_SIZE;
+	bool has_size = fn->arg_size[arg] != 0;
+	bool is_next_size = false;
+
+	if (arg + 1 < ARRAY_SIZE(fn->arg_type))
+		is_next_size = arg_type_is_mem_size(fn->arg_type[arg + 1]);
+
+	if (base_type(fn->arg_type[arg]) != ARG_PTR_TO_MEM)
+		return is_next_size;
+
+	return has_size == is_next_size || is_next_size == is_fixed;
 }
 
 static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
@@ -6415,11 +6433,11 @@ static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
 	 * helper function specification.
 	 */
 	if (arg_type_is_mem_size(fn->arg1_type) ||
-	    base_type(fn->arg5_type) == ARG_PTR_TO_MEM ||
-	    check_args_pair_invalid(fn->arg1_type, fn->arg2_type) ||
-	    check_args_pair_invalid(fn->arg2_type, fn->arg3_type) ||
-	    check_args_pair_invalid(fn->arg3_type, fn->arg4_type) ||
-	    check_args_pair_invalid(fn->arg4_type, fn->arg5_type))
+	    check_args_pair_invalid(fn, 0) ||
+	    check_args_pair_invalid(fn, 1) ||
+	    check_args_pair_invalid(fn, 2) ||
+	    check_args_pair_invalid(fn, 3) ||
+	    check_args_pair_invalid(fn, 4))
 		return false;
 
 	return true;
@@ -6460,7 +6478,10 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
 		if (base_type(fn->arg_type[i]) == ARG_PTR_TO_BTF_ID && !fn->arg_btf_id[i])
 			return false;
 
-		if (base_type(fn->arg_type[i]) != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i])
+		if (base_type(fn->arg_type[i]) != ARG_PTR_TO_BTF_ID && fn->arg_btf_id[i] &&
+		    /* arg_btf_id and arg_size are in a union. */
+		    (base_type(fn->arg_type[i]) != ARG_PTR_TO_MEM ||
+		     !(fn->arg_type[i] & MEM_FIXED_SIZE)))
 			return false;
 	}