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 ARG_PTR_TO_MEM arguments without
the corresponding ARG_CONST_SIZE, given that they define the memory
region size in struct bpf_func_proto.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>

include/linux/bpf.h

@@ -457,6 +457,16 @@ struct bpf_func_proto {
 		};
 		u32 *arg_btf_id[5];
 	};
+	union {
+		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

@@ -5529,6 +5529,11 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		 * next is_mem_size argument below.
 		 */
 		meta->raw_mode = (arg_type == ARG_PTR_TO_UNINIT_MEM);
+		if (fn->arg_size[arg]) {
+			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);
 
@@ -5868,13 +5873,12 @@ 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 (arg_type_is_mem_ptr(arg_curr) &&
-	        !arg_type_is_mem_size(arg_next)) ||
-	       (!arg_type_is_mem_ptr(arg_curr) &&
-		arg_type_is_mem_size(arg_next));
+	if (arg_type_is_mem_ptr(fn->arg_type[arg]))
+		return arg_type_is_mem_size(fn->arg_type[arg + 1]) ==
+			!!fn->arg_size[arg];
+	return arg_type_is_mem_size(fn->arg_type[arg + 1]) || fn->arg_size[arg];
 }
 
 static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
@@ -5885,11 +5889,11 @@ static bool check_arg_pair_ok(const struct bpf_func_proto *fn)
 	 * helper function specification.
 	 */
 	if (arg_type_is_mem_size(fn->arg1_type) ||
-	    arg_type_is_mem_ptr(fn->arg5_type)  ||
-	    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))
+	    (arg_type_is_mem_ptr(fn->arg5_type) && !fn->arg5_size) ||
+	    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;