diff --git a/cmd/bb_runner/main.go b/cmd/bb_runner/main.go
index 000ae003..6252e99b 100644
--- a/cmd/bb_runner/main.go
+++ b/cmd/bb_runner/main.go
@@ -68,6 +68,13 @@ func main() {
 			buildDirectoryPath,
 			commandCreator,
 			configuration.SetTmpdirEnvironmentVariable)
+		for _, mountinfo := range configuration.InputRootMounts {
+			r = runner.NewMountingRunner(
+				r,
+				buildDirectory,
+				mountinfo,
+			)
+		}
 
 		// Let bb_runner replace temporary directories with symbolic
 		// links pointing to the temporary directory set up by
diff --git a/pkg/proto/configuration/bb_runner/bb_runner.pb.go b/pkg/proto/configuration/bb_runner/bb_runner.pb.go
index 0b6159b7..aa7676c4 100644
--- a/pkg/proto/configuration/bb_runner/bb_runner.pb.go
+++ b/pkg/proto/configuration/bb_runner/bb_runner.pb.go
@@ -41,6 +41,7 @@ type ApplicationConfiguration struct {
 	SymlinkTemporaryDirectories    []string                                  `protobuf:"bytes,12,rep,name=symlink_temporary_directories,json=symlinkTemporaryDirectories,proto3" json:"symlink_temporary_directories,omitempty"`
 	RunCommandCleaner              []string                                  `protobuf:"bytes,13,rep,name=run_command_cleaner,json=runCommandCleaner,proto3" json:"run_command_cleaner,omitempty"`
 	AppleXcodeDeveloperDirectories map[string]string                         `protobuf:"bytes,14,rep,name=apple_xcode_developer_directories,json=appleXcodeDeveloperDirectories,proto3" json:"apple_xcode_developer_directories,omitempty" protobuf_key:"bytes,1,opt,name=key,proto3" protobuf_val:"bytes,2,opt,name=value,proto3"`
+	InputRootMounts                []*InputMountOptions                      `protobuf:"bytes,15,rep,name=input_root_mounts,json=inputRootMounts,proto3" json:"input_root_mounts,omitempty"`
 }
 
 func (x *ApplicationConfiguration) Reset() {
@@ -166,6 +167,76 @@ func (x *ApplicationConfiguration) GetAppleXcodeDeveloperDirectories() map[strin
 	return nil
 }
 
+func (x *ApplicationConfiguration) GetInputRootMounts() []*InputMountOptions {
+	if x != nil {
+		return x.InputRootMounts
+	}
+	return nil
+}
+
+type InputMountOptions struct {
+	state         protoimpl.MessageState
+	sizeCache     protoimpl.SizeCache
+	unknownFields protoimpl.UnknownFields
+
+	Mountpoint     string `protobuf:"bytes,1,opt,name=mountpoint,proto3" json:"mountpoint,omitempty"`
+	Source         string `protobuf:"bytes,2,opt,name=source,proto3" json:"source,omitempty"`
+	FilesystemType string `protobuf:"bytes,3,opt,name=filesystem_type,json=filesystemType,proto3" json:"filesystem_type,omitempty"`
+}
+
+func (x *InputMountOptions) Reset() {
+	*x = InputMountOptions{}
+	if protoimpl.UnsafeEnabled {
+		mi := &file_pkg_proto_configuration_bb_runner_bb_runner_proto_msgTypes[1]
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		ms.StoreMessageInfo(mi)
+	}
+}
+
+func (x *InputMountOptions) String() string {
+	return protoimpl.X.MessageStringOf(x)
+}
+
+func (*InputMountOptions) ProtoMessage() {}
+
+func (x *InputMountOptions) ProtoReflect() protoreflect.Message {
+	mi := &file_pkg_proto_configuration_bb_runner_bb_runner_proto_msgTypes[1]
+	if protoimpl.UnsafeEnabled && x != nil {
+		ms := protoimpl.X.MessageStateOf(protoimpl.Pointer(x))
+		if ms.LoadMessageInfo() == nil {
+			ms.StoreMessageInfo(mi)
+		}
+		return ms
+	}
+	return mi.MessageOf(x)
+}
+
+// Deprecated: Use InputMountOptions.ProtoReflect.Descriptor instead.
+func (*InputMountOptions) Descriptor() ([]byte, []int) {
+	return file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDescGZIP(), []int{1}
+}
+
+func (x *InputMountOptions) GetMountpoint() string {
+	if x != nil {
+		return x.Mountpoint
+	}
+	return ""
+}
+
+func (x *InputMountOptions) GetSource() string {
+	if x != nil {
+		return x.Source
+	}
+	return ""
+}
+
+func (x *InputMountOptions) GetFilesystemType() string {
+	if x != nil {
+		return x.FilesystemType
+	}
+	return ""
+}
+
 var File_pkg_proto_configuration_bb_runner_bb_runner_proto protoreflect.FileDescriptor
 
 var file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDesc = []byte{
@@ -183,7 +254,7 @@ var file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDesc = []byte{
 	0x6f, 0x62, 0x61, 0x6c, 0x2e, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x1a, 0x27, 0x70, 0x6b, 0x67, 0x2f,
 	0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2f, 0x63, 0x6f, 0x6e, 0x66, 0x69, 0x67, 0x75, 0x72, 0x61, 0x74,
 	0x69, 0x6f, 0x6e, 0x2f, 0x67, 0x72, 0x70, 0x63, 0x2f, 0x67, 0x72, 0x70, 0x63, 0x2e, 0x70, 0x72,
-	0x6f, 0x74, 0x6f, 0x22, 0xf3, 0x08, 0x0a, 0x18, 0x41, 0x70, 0x70, 0x6c, 0x69, 0x63, 0x61, 0x74,
+	0x6f, 0x74, 0x6f, 0x22, 0xd5, 0x09, 0x0a, 0x18, 0x41, 0x70, 0x70, 0x6c, 0x69, 0x63, 0x61, 0x74,
 	0x69, 0x6f, 0x6e, 0x43, 0x6f, 0x6e, 0x66, 0x69, 0x67, 0x75, 0x72, 0x61, 0x74, 0x69, 0x6f, 0x6e,
 	0x12, 0x30, 0x0a, 0x14, 0x62, 0x75, 0x69, 0x6c, 0x64, 0x5f, 0x64, 0x69, 0x72, 0x65, 0x63, 0x74,
 	0x6f, 0x72, 0x79, 0x5f, 0x70, 0x61, 0x74, 0x68, 0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x12,
@@ -249,17 +320,31 @@ var file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDesc = []byte{
 	0x44, 0x69, 0x72, 0x65, 0x63, 0x74, 0x6f, 0x72, 0x69, 0x65, 0x73, 0x45, 0x6e, 0x74, 0x72, 0x79,
 	0x52, 0x1e, 0x61, 0x70, 0x70, 0x6c, 0x65, 0x58, 0x63, 0x6f, 0x64, 0x65, 0x44, 0x65, 0x76, 0x65,
 	0x6c, 0x6f, 0x70, 0x65, 0x72, 0x44, 0x69, 0x72, 0x65, 0x63, 0x74, 0x6f, 0x72, 0x69, 0x65, 0x73,
-	0x1a, 0x51, 0x0a, 0x23, 0x41, 0x70, 0x70, 0x6c, 0x65, 0x58, 0x63, 0x6f, 0x64, 0x65, 0x44, 0x65,
-	0x76, 0x65, 0x6c, 0x6f, 0x70, 0x65, 0x72, 0x44, 0x69, 0x72, 0x65, 0x63, 0x74, 0x6f, 0x72, 0x69,
-	0x65, 0x73, 0x45, 0x6e, 0x74, 0x72, 0x79, 0x12, 0x10, 0x0a, 0x03, 0x6b, 0x65, 0x79, 0x18, 0x01,
-	0x20, 0x01, 0x28, 0x09, 0x52, 0x03, 0x6b, 0x65, 0x79, 0x12, 0x14, 0x0a, 0x05, 0x76, 0x61, 0x6c,
-	0x75, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x52, 0x05, 0x76, 0x61, 0x6c, 0x75, 0x65, 0x3a,
-	0x02, 0x38, 0x01, 0x4a, 0x04, 0x08, 0x09, 0x10, 0x0a, 0x42, 0x4c, 0x5a, 0x4a, 0x67, 0x69, 0x74,
-	0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f, 0x62, 0x75, 0x69, 0x6c, 0x64, 0x62, 0x61, 0x72,
-	0x6e, 0x2f, 0x62, 0x62, 0x2d, 0x72, 0x65, 0x6d, 0x6f, 0x74, 0x65, 0x2d, 0x65, 0x78, 0x65, 0x63,
-	0x75, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x70, 0x6b, 0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2f,
-	0x63, 0x6f, 0x6e, 0x66, 0x69, 0x67, 0x75, 0x72, 0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x62, 0x62,
-	0x5f, 0x72, 0x75, 0x6e, 0x6e, 0x65, 0x72, 0x62, 0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
+	0x12, 0x60, 0x0a, 0x11, 0x69, 0x6e, 0x70, 0x75, 0x74, 0x5f, 0x72, 0x6f, 0x6f, 0x74, 0x5f, 0x6d,
+	0x6f, 0x75, 0x6e, 0x74, 0x73, 0x18, 0x0f, 0x20, 0x03, 0x28, 0x0b, 0x32, 0x34, 0x2e, 0x62, 0x75,
+	0x69, 0x6c, 0x64, 0x62, 0x61, 0x72, 0x6e, 0x2e, 0x63, 0x6f, 0x6e, 0x66, 0x69, 0x67, 0x75, 0x72,
+	0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2e, 0x62, 0x62, 0x5f, 0x72, 0x75, 0x6e, 0x6e, 0x65, 0x72, 0x2e,
+	0x49, 0x6e, 0x70, 0x75, 0x74, 0x4d, 0x6f, 0x75, 0x6e, 0x74, 0x4f, 0x70, 0x74, 0x69, 0x6f, 0x6e,
+	0x73, 0x52, 0x0f, 0x69, 0x6e, 0x70, 0x75, 0x74, 0x52, 0x6f, 0x6f, 0x74, 0x4d, 0x6f, 0x75, 0x6e,
+	0x74, 0x73, 0x1a, 0x51, 0x0a, 0x23, 0x41, 0x70, 0x70, 0x6c, 0x65, 0x58, 0x63, 0x6f, 0x64, 0x65,
+	0x44, 0x65, 0x76, 0x65, 0x6c, 0x6f, 0x70, 0x65, 0x72, 0x44, 0x69, 0x72, 0x65, 0x63, 0x74, 0x6f,
+	0x72, 0x69, 0x65, 0x73, 0x45, 0x6e, 0x74, 0x72, 0x79, 0x12, 0x10, 0x0a, 0x03, 0x6b, 0x65, 0x79,
+	0x18, 0x01, 0x20, 0x01, 0x28, 0x09, 0x52, 0x03, 0x6b, 0x65, 0x79, 0x12, 0x14, 0x0a, 0x05, 0x76,
+	0x61, 0x6c, 0x75, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09, 0x52, 0x05, 0x76, 0x61, 0x6c, 0x75,
+	0x65, 0x3a, 0x02, 0x38, 0x01, 0x4a, 0x04, 0x08, 0x09, 0x10, 0x0a, 0x22, 0x74, 0x0a, 0x11, 0x49,
+	0x6e, 0x70, 0x75, 0x74, 0x4d, 0x6f, 0x75, 0x6e, 0x74, 0x4f, 0x70, 0x74, 0x69, 0x6f, 0x6e, 0x73,
+	0x12, 0x1e, 0x0a, 0x0a, 0x6d, 0x6f, 0x75, 0x6e, 0x74, 0x70, 0x6f, 0x69, 0x6e, 0x74, 0x18, 0x01,
+	0x20, 0x01, 0x28, 0x09, 0x52, 0x0a, 0x6d, 0x6f, 0x75, 0x6e, 0x74, 0x70, 0x6f, 0x69, 0x6e, 0x74,
+	0x12, 0x16, 0x0a, 0x06, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65, 0x18, 0x02, 0x20, 0x01, 0x28, 0x09,
+	0x52, 0x06, 0x73, 0x6f, 0x75, 0x72, 0x63, 0x65, 0x12, 0x27, 0x0a, 0x0f, 0x66, 0x69, 0x6c, 0x65,
+	0x73, 0x79, 0x73, 0x74, 0x65, 0x6d, 0x5f, 0x74, 0x79, 0x70, 0x65, 0x18, 0x03, 0x20, 0x01, 0x28,
+	0x09, 0x52, 0x0e, 0x66, 0x69, 0x6c, 0x65, 0x73, 0x79, 0x73, 0x74, 0x65, 0x6d, 0x54, 0x79, 0x70,
+	0x65, 0x42, 0x4c, 0x5a, 0x4a, 0x67, 0x69, 0x74, 0x68, 0x75, 0x62, 0x2e, 0x63, 0x6f, 0x6d, 0x2f,
+	0x62, 0x75, 0x69, 0x6c, 0x64, 0x62, 0x61, 0x72, 0x6e, 0x2f, 0x62, 0x62, 0x2d, 0x72, 0x65, 0x6d,
+	0x6f, 0x74, 0x65, 0x2d, 0x65, 0x78, 0x65, 0x63, 0x75, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x70, 0x6b,
+	0x67, 0x2f, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x2f, 0x63, 0x6f, 0x6e, 0x66, 0x69, 0x67, 0x75, 0x72,
+	0x61, 0x74, 0x69, 0x6f, 0x6e, 0x2f, 0x62, 0x62, 0x5f, 0x72, 0x75, 0x6e, 0x6e, 0x65, 0x72, 0x62,
+	0x06, 0x70, 0x72, 0x6f, 0x74, 0x6f, 0x33,
 }
 
 var (
@@ -274,26 +359,28 @@ func file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDescGZIP() []byte
 	return file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDescData
 }
 
-var file_pkg_proto_configuration_bb_runner_bb_runner_proto_msgTypes = make([]protoimpl.MessageInfo, 2)
+var file_pkg_proto_configuration_bb_runner_bb_runner_proto_msgTypes = make([]protoimpl.MessageInfo, 3)
 var file_pkg_proto_configuration_bb_runner_bb_runner_proto_goTypes = []interface{}{
 	(*ApplicationConfiguration)(nil),                 // 0: buildbarn.configuration.bb_runner.ApplicationConfiguration
-	nil,                                              // 1: buildbarn.configuration.bb_runner.ApplicationConfiguration.AppleXcodeDeveloperDirectoriesEntry
-	(*grpc.ServerConfiguration)(nil),                 // 2: buildbarn.configuration.grpc.ServerConfiguration
-	(*global.Configuration)(nil),                     // 3: buildbarn.configuration.global.Configuration
-	(*grpc.ClientConfiguration)(nil),                 // 4: buildbarn.configuration.grpc.ClientConfiguration
-	(*credentials.UNIXCredentialsConfiguration)(nil), // 5: buildbarn.configuration.credentials.UNIXCredentialsConfiguration
+	(*InputMountOptions)(nil),                        // 1: buildbarn.configuration.bb_runner.InputMountOptions
+	nil,                                              // 2: buildbarn.configuration.bb_runner.ApplicationConfiguration.AppleXcodeDeveloperDirectoriesEntry
+	(*grpc.ServerConfiguration)(nil),                 // 3: buildbarn.configuration.grpc.ServerConfiguration
+	(*global.Configuration)(nil),                     // 4: buildbarn.configuration.global.Configuration
+	(*grpc.ClientConfiguration)(nil),                 // 5: buildbarn.configuration.grpc.ClientConfiguration
+	(*credentials.UNIXCredentialsConfiguration)(nil), // 6: buildbarn.configuration.credentials.UNIXCredentialsConfiguration
 }
 var file_pkg_proto_configuration_bb_runner_bb_runner_proto_depIdxs = []int32{
-	2, // 0: buildbarn.configuration.bb_runner.ApplicationConfiguration.grpc_servers:type_name -> buildbarn.configuration.grpc.ServerConfiguration
-	3, // 1: buildbarn.configuration.bb_runner.ApplicationConfiguration.global:type_name -> buildbarn.configuration.global.Configuration
-	4, // 2: buildbarn.configuration.bb_runner.ApplicationConfiguration.temporary_directory_installer:type_name -> buildbarn.configuration.grpc.ClientConfiguration
-	5, // 3: buildbarn.configuration.bb_runner.ApplicationConfiguration.run_commands_as:type_name -> buildbarn.configuration.credentials.UNIXCredentialsConfiguration
-	1, // 4: buildbarn.configuration.bb_runner.ApplicationConfiguration.apple_xcode_developer_directories:type_name -> buildbarn.configuration.bb_runner.ApplicationConfiguration.AppleXcodeDeveloperDirectoriesEntry
-	5, // [5:5] is the sub-list for method output_type
-	5, // [5:5] is the sub-list for method input_type
-	5, // [5:5] is the sub-list for extension type_name
-	5, // [5:5] is the sub-list for extension extendee
-	0, // [0:5] is the sub-list for field type_name
+	3, // 0: buildbarn.configuration.bb_runner.ApplicationConfiguration.grpc_servers:type_name -> buildbarn.configuration.grpc.ServerConfiguration
+	4, // 1: buildbarn.configuration.bb_runner.ApplicationConfiguration.global:type_name -> buildbarn.configuration.global.Configuration
+	5, // 2: buildbarn.configuration.bb_runner.ApplicationConfiguration.temporary_directory_installer:type_name -> buildbarn.configuration.grpc.ClientConfiguration
+	6, // 3: buildbarn.configuration.bb_runner.ApplicationConfiguration.run_commands_as:type_name -> buildbarn.configuration.credentials.UNIXCredentialsConfiguration
+	2, // 4: buildbarn.configuration.bb_runner.ApplicationConfiguration.apple_xcode_developer_directories:type_name -> buildbarn.configuration.bb_runner.ApplicationConfiguration.AppleXcodeDeveloperDirectoriesEntry
+	1, // 5: buildbarn.configuration.bb_runner.ApplicationConfiguration.input_root_mounts:type_name -> buildbarn.configuration.bb_runner.InputMountOptions
+	6, // [6:6] is the sub-list for method output_type
+	6, // [6:6] is the sub-list for method input_type
+	6, // [6:6] is the sub-list for extension type_name
+	6, // [6:6] is the sub-list for extension extendee
+	0, // [0:6] is the sub-list for field type_name
 }
 
 func init() { file_pkg_proto_configuration_bb_runner_bb_runner_proto_init() }
@@ -314,6 +401,18 @@ func file_pkg_proto_configuration_bb_runner_bb_runner_proto_init() {
 				return nil
 			}
 		}
+		file_pkg_proto_configuration_bb_runner_bb_runner_proto_msgTypes[1].Exporter = func(v interface{}, i int) interface{} {
+			switch v := v.(*InputMountOptions); i {
+			case 0:
+				return &v.state
+			case 1:
+				return &v.sizeCache
+			case 2:
+				return &v.unknownFields
+			default:
+				return nil
+			}
+		}
 	}
 	type x struct{}
 	out := protoimpl.TypeBuilder{
@@ -321,7 +420,7 @@ func file_pkg_proto_configuration_bb_runner_bb_runner_proto_init() {
 			GoPackagePath: reflect.TypeOf(x{}).PkgPath(),
 			RawDescriptor: file_pkg_proto_configuration_bb_runner_bb_runner_proto_rawDesc,
 			NumEnums:      0,
-			NumMessages:   2,
+			NumMessages:   3,
 			NumExtensions: 0,
 			NumServices:   0,
 		},
diff --git a/pkg/proto/configuration/bb_runner/bb_runner.proto b/pkg/proto/configuration/bb_runner/bb_runner.proto
index 94c52514..240829b3 100644
--- a/pkg/proto/configuration/bb_runner/bb_runner.proto
+++ b/pkg/proto/configuration/bb_runner/bb_runner.proto
@@ -131,4 +131,37 @@ message ApplicationConfiguration {
   // https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/exec/local/XcodeLocalEnvProvider.java
   // https://www.smileykeith.com/2021/03/08/locking-xcode-in-bazel/
   map<string, string> apple_xcode_developer_directories = 14;
+
+  // Mount special filesystems in the input root. This is useful when
+  // running with `chroot_into_input_root`. some tools require access to
+  // special filesystems that are created when the operating system
+  // boots. An input root with a full userland implementation may need
+  // these.
+  //
+  // The mount point directories must exist in the input root.
+  //
+  // Typical choices are:
+  //
+  //   inputRootMounts: [
+  //     {
+  //       mountpoint: 'proc',
+  //       source: '/proc',
+  //       filesystemType: 'proc',
+  //     },
+  //     {
+  //       mountpoint: 'sys',
+  //       source: '/sys',
+  //       filesystemType: 'sysfs',
+  //     },
+  //   ],
+  repeated InputMountOptions input_root_mounts = 15;
+}
+
+message InputMountOptions {
+  // Mount a filesystem in the input root, a relative path.
+  string mountpoint = 1;
+  // Source filesystem from the runner's operating system, absolute path.
+  string source = 2;
+  // Type of filesystem, see `mount`'s man page.
+  string filesystem_type = 3;
 }
diff --git a/pkg/runner/BUILD.bazel b/pkg/runner/BUILD.bazel
index de0c7fe9..c5c37b9d 100644
--- a/pkg/runner/BUILD.bazel
+++ b/pkg/runner/BUILD.bazel
@@ -11,6 +11,7 @@ go_library(
         "local_runner_rss_kibibytes.go",
         "local_runner_unix.go",
         "local_runner_windows.go",
+        "mounting_runner.go",
         "path_existence_checking_runner.go",
         "temporary_directory_installing_runner.go",
         "temporary_directory_symlinking_runner.go",
@@ -19,6 +20,7 @@ go_library(
     visibility = ["//visibility:public"],
     deps = [
         "//pkg/cleaner",
+        "//pkg/proto/configuration/bb_runner",
         "//pkg/proto/runner",
         "//pkg/proto/tmp_installer",
         "@com_github_buildbarn_bb_storage//pkg/filesystem",
@@ -70,6 +72,7 @@ go_test(
         "apple_xcode_resolving_runner_test.go",
         "clean_runner_test.go",
         "local_runner_test.go",
+        "mounting_runner_test.go",
         "path_existence_checking_runner_test.go",
         "temporary_directory_symlinking_runner_test.go",
     ],
@@ -77,6 +80,7 @@ go_test(
         ":runner",
         "//internal/mock",
         "//pkg/cleaner",
+        "//pkg/proto/configuration/bb_runner",
         "//pkg/proto/resourceusage",
         "//pkg/proto/runner",
         "@com_github_buildbarn_bb_storage//pkg/filesystem",
diff --git a/pkg/runner/mounting_runner.go b/pkg/runner/mounting_runner.go
new file mode 100644
index 00000000..90b9d9f0
--- /dev/null
+++ b/pkg/runner/mounting_runner.go
@@ -0,0 +1,75 @@
+package runner
+
+import (
+	"context"
+	"path/filepath"
+
+	"github.com/buildbarn/bb-remote-execution/pkg/proto/configuration/bb_runner"
+	runner_pb "github.com/buildbarn/bb-remote-execution/pkg/proto/runner"
+	"github.com/buildbarn/bb-storage/pkg/filesystem"
+	"github.com/buildbarn/bb-storage/pkg/filesystem/path"
+	"github.com/buildbarn/bb-storage/pkg/util"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+	"google.golang.org/protobuf/types/known/emptypb"
+)
+
+type mountingRunner struct {
+	base           runner_pb.RunnerServer
+	buildDirectory filesystem.Directory
+	mount          *bb_runner.InputMountOptions
+}
+
+// NewMountingRunner is a decorator for Runner
+// that mounts `mount` before running a build action.
+//
+// This decorator can be used for chroot runners
+// that must mount special filesystems into the input root.
+func NewMountingRunner(base runner_pb.RunnerServer, buildDirectory filesystem.Directory, mount *bb_runner.InputMountOptions) runner_pb.RunnerServer {
+	return &mountingRunner{
+		buildDirectory: buildDirectory,
+		mount:          mount,
+		base:           base,
+	}
+}
+
+func (r *mountingRunner) Run(ctx context.Context, request *runner_pb.RunRequest) (response *runner_pb.RunResponse, err error) {
+	pathResolver := buildDirectoryPathResolver{
+		stack: util.NewNonEmptyStack(filesystem.NopDirectoryCloser(r.buildDirectory)),
+	}
+	scopeWalker := path.NewRelativeScopeWalker(&pathResolver)
+	defer pathResolver.closeAll()
+
+	fullpath := filepath.Join(request.InputRootDirectory, r.mount.Mountpoint)
+	if err := path.Resolve(fullpath, scopeWalker); err != nil {
+		return nil, util.StatusWrap(err, "Invalid mountpoint parent directory")
+	}
+
+	mountDir := pathResolver.stack.Peek()
+	defer mountDir.Close()
+	if pathResolver.TerminalName == nil {
+		return nil, status.Errorf(codes.InvalidArgument, "Could not resolve mountpoint basename: %#v", fullpath)
+	}
+
+	mountname := *pathResolver.TerminalName
+	if err := mountDir.Mount(mountname, r.mount.Source, r.mount.FilesystemType); err != nil {
+		return nil, util.StatusWrapf(err, "Failed to mount %#v in the input root", r.mount)
+	}
+
+	response, err = r.base.Run(ctx, request)
+	if err != nil {
+		return nil, err
+	}
+	if err2 := mountDir.Unmount(mountname); err2 != nil {
+		err = util.StatusFromMultiple([]error{
+			err,
+			util.StatusWrapf(err2, "Failed to unmount %#v in the input root", r.mount.Mountpoint),
+		})
+	}
+
+	return response, nil
+}
+
+func (r *mountingRunner) CheckReadiness(ctx context.Context, request *runner_pb.CheckReadinessRequest) (*emptypb.Empty, error) {
+	return r.base.CheckReadiness(ctx, request)
+}
diff --git a/pkg/runner/mounting_runner_test.go b/pkg/runner/mounting_runner_test.go
new file mode 100644
index 00000000..027eaab0
--- /dev/null
+++ b/pkg/runner/mounting_runner_test.go
@@ -0,0 +1,99 @@
+package runner_test
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+
+	"github.com/buildbarn/bb-remote-execution/internal/mock"
+	"github.com/buildbarn/bb-remote-execution/pkg/proto/configuration/bb_runner"
+	runner_pb "github.com/buildbarn/bb-remote-execution/pkg/proto/runner"
+	"github.com/buildbarn/bb-remote-execution/pkg/runner"
+	"github.com/buildbarn/bb-storage/pkg/filesystem/path"
+	"github.com/golang/mock/gomock"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMountingRunner(t *testing.T) {
+	ctrl, ctx := gomock.WithContext(context.Background(), t)
+
+	mountpoint := "proc_inside"
+	mountpointComponent := path.MustNewComponent(mountpoint)
+	source := "/proc"
+	fstype := "proc"
+
+	runRequest := &runner_pb.RunRequest{
+		Arguments: []string{"ls", "-l", mountpoint},
+	}
+
+	t.Run("Run", func(t *testing.T) {
+		mockRunner := mock.NewMockRunnerServer(ctrl)
+		mockDirectory := mock.NewMockDirectory(ctrl)
+
+		runnerServer := runner.NewMountingRunner(mockRunner, mockDirectory, &bb_runner.InputMountOptions{
+			Mountpoint:     mountpoint,
+			Source:         source,
+			FilesystemType: fstype,
+		})
+		mockDirectory.EXPECT().Mount(mountpointComponent, source, fstype)
+		mockDirectory.EXPECT().Unmount(mountpointComponent)
+
+		mockRunner.EXPECT().Run(ctx, runRequest).Return(&runner_pb.RunResponse{}, nil)
+		_, err := runnerServer.Run(ctx, runRequest)
+		require.NoError(t, err)
+	})
+
+	t.Run("RunChained", func(t *testing.T) {
+		mockRunner := mock.NewMockRunnerServer(ctrl)
+		mockDirectory := mock.NewMockDirectory(ctrl)
+
+		innerDecorator := runner.NewMountingRunner(mockRunner, mockDirectory, &bb_runner.InputMountOptions{
+			Mountpoint:     mountpoint,
+			Source:         source,
+			FilesystemType: fstype,
+		})
+		outer := []string{"sys_inside", "/sys", "sysfs"}
+		outerDecorator := runner.NewMountingRunner(innerDecorator, mockDirectory, &bb_runner.InputMountOptions{
+			Mountpoint:     outer[0],
+			Source:         outer[1],
+			FilesystemType: outer[2],
+		})
+
+		gomock.InOrder(
+			mockDirectory.EXPECT().Mount(path.MustNewComponent(outer[0]), outer[1], outer[2]),
+			mockDirectory.EXPECT().Mount(mountpointComponent, source, fstype),
+			mockDirectory.EXPECT().Unmount(mountpointComponent),
+			mockDirectory.EXPECT().Unmount(path.MustNewComponent(outer[0])),
+		)
+
+		// TODO: verify that the mounts are in the correct order.
+		mockRunner.EXPECT().Run(ctx, runRequest).Return(&runner_pb.RunResponse{}, nil)
+		_, err := outerDecorator.Run(ctx, runRequest)
+		require.NoError(t, err)
+	})
+
+	t.Run("RunNestedMount", func(t *testing.T) {
+		mockRunner := mock.NewMockRunnerServer(ctrl)
+		mockDirectory := mock.NewMockDirectory(ctrl)
+		mockDirectoryCloser := mock.NewMockDirectoryCloser(ctrl)
+
+		nestedMountpoint := path.MustNewComponent("nested")
+		parent := path.MustNewComponent("outer")
+		fullPath := filepath.Join(parent.String(), nestedMountpoint.String())
+		runnerServer := runner.NewMountingRunner(mockRunner, mockDirectory, &bb_runner.InputMountOptions{
+			Mountpoint:     fullPath,
+			Source:         source,
+			FilesystemType: fstype,
+		})
+
+		// The mount calls are called in a subdirectory
+		mockDirectory.EXPECT().EnterDirectory(parent).Return(mockDirectoryCloser, nil)
+		mockDirectoryCloser.EXPECT().Mount(nestedMountpoint, source, fstype)
+		mockDirectoryCloser.EXPECT().Unmount(nestedMountpoint)
+		mockDirectoryCloser.EXPECT().Close().Times(2) // Why twice ???
+
+		mockRunner.EXPECT().Run(ctx, runRequest).Return(&runner_pb.RunResponse{}, nil)
+		_, err := runnerServer.Run(ctx, runRequest)
+		require.NoError(t, err)
+	})
+}