[RFC 05/16] bus1: add module scaffolding

[David Rheinsberg <david@readahead.eu>]

Add ./ipc/bus1/ as the new home of the bus1 module. `uapi/linux/bus1.h`
will provide the user-space API, but is left blank for now.

The module setup and the user-space API are written in C with the intent
to expose a C API to the other parts of the kernel to use in the future,
if desired.

The internals of the bus1 module are written in Rust. The exposed C API
is defined in `lib.h` and only used inside of the bus1 module for now.
`bindgen` is used to generate Rust symbols for `lib.h` (and
`uapi/linux/bus1.h`) so we can avoid duplicating symbols of the C API in
the Rust code. This way, we can rely on the C header and the C API of
the Rust code to match.

Signed-off-by: David Rheinsberg <david@readahead.eu>
---
 include/uapi/linux/bus1.h |  5 +++++
 init/Kconfig              | 12 ++++++++++++
 ipc/Makefile              |  2 +-
 ipc/bus1/Makefile         | 28 ++++++++++++++++++++++++++++
 ipc/bus1/lib.h            | 18 ++++++++++++++++++
 ipc/bus1/lib.rs           | 18 ++++++++++++++++++
 ipc/bus1/main.c           | 19 +++++++++++++++++++
 7 files changed, 101 insertions(+), 1 deletion(-)
 create mode 100644 include/uapi/linux/bus1.h
 create mode 100644 ipc/bus1/Makefile
 create mode 100644 ipc/bus1/lib.h
 create mode 100644 ipc/bus1/lib.rs
 create mode 100644 ipc/bus1/main.c

diff --git a/include/uapi/linux/bus1.h b/include/uapi/linux/bus1.h
new file mode 100644
index 000000000000..4297e7a00ab9
--- /dev/null
+++ b/include/uapi/linux/bus1.h
@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _UAPI_LINUX_BUS1_H
+#define _UAPI_LINUX_BUS1_H
+
+#endif /* _UAPI_LINUX_BUS1_H */
diff --git a/init/Kconfig b/init/Kconfig
index 444ce811ea67..20e8a577d9a7 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -502,6 +502,18 @@ config POSIX_MQUEUE_SYSCTL
 	depends on SYSCTL
 	default y
 
+config BUS1
+	tristate "Bus1 Capability-based IPC"
+	depends on RUST
+	help
+	  The Bus1 linux subsystem provides capability-based inter-process
+	  communication. It allows services and operating system tasks to
+	  exchange messages, send notifications, and transfer data and
+	  resources, using an object-capability system for implicit access
+	  control. It is highly scalable, yet guarantees causal ordering across
+	  all its operations, including the builtin object lifetime
+	  notifications.
+
 config WATCH_QUEUE
 	bool "General notification queue"
 	default n
diff --git a/ipc/Makefile b/ipc/Makefile
index c2558c430f51..6fa8c3b1c47c 100644
--- a/ipc/Makefile
+++ b/ipc/Makefile
@@ -9,4 +9,4 @@ obj-$(CONFIG_SYSVIPC_SYSCTL) += ipc_sysctl.o
 obj-$(CONFIG_POSIX_MQUEUE) += mqueue.o msgutil.o
 obj-$(CONFIG_IPC_NS) += namespace.o
 obj-$(CONFIG_POSIX_MQUEUE_SYSCTL) += mq_sysctl.o
-
+obj-$(CONFIG_BUS1) += bus1/
diff --git a/ipc/bus1/Makefile b/ipc/bus1/Makefile
new file mode 100644
index 000000000000..1f2fbbe8603f
--- /dev/null
+++ b/ipc/bus1/Makefile
@@ -0,0 +1,28 @@
+# SPDX-License-Identifier: GPL-2.0
+
+quiet_cmd_bindgen = BINDGEN $@
+      cmd_bindgen = \
+	$(BINDGEN) $< -o $@ \
+		$(addprefix --allowlist-file=,$^) \
+		--ctypes-prefix kernel::ffi \
+		--no-debug '.*' \
+		--no-layout-tests \
+		--rust-target 1.68 \
+		--use-core \
+		-- \
+		$(c_flags) \
+		-D__BINDGEN__ \
+		-DMODULE \
+		-fno-builtin
+
+$(obj)/capi.rs: $(src)/lib.h $(srctree)/include/uapi/linux/bus1.h
+	$(call cmd,bindgen)
+
+$(obj)/lib.o: $(obj)/capi.rs
+$(obj)/lib.o: export BUS1_CAPI_PATH=$(abspath $(obj)/capi.rs)
+
+bus1-y :=		\
+	lib.o			\
+	main.o
+
+obj-$(CONFIG_BUS1) += bus1.o
diff --git a/ipc/bus1/lib.h b/ipc/bus1/lib.h
new file mode 100644
index 000000000000..e84c47f97031
--- /dev/null
+++ b/ipc/bus1/lib.h
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef __B1_LIB_H
+#define __B1_LIB_H
+
+/**
+ * DOC: C API of the Bus1 Rust Module
+ *
+ * This header exposes the C API of the Bus1 rust module. It provides the
+ * necessary hooks for the C module code to call into the rust code.
+ */
+
+#include <linux/cleanup.h>
+#include <linux/err.h>
+#include <linux/errno.h>
+#include <linux/types.h>
+#include <uapi/linux/bus1.h>
+
+#endif /* __B1_LIB_H */
diff --git a/ipc/bus1/lib.rs b/ipc/bus1/lib.rs
new file mode 100644
index 000000000000..7c3364651638
--- /dev/null
+++ b/ipc/bus1/lib.rs
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+//! # Kernel Bus1 Crate
+//!
+//! This is the in-kernel implementation of the Bus1 communication system in
+//! rust. Any user-space API is outside the scope of this module.
+
+#[allow(
+    dead_code,
+    missing_docs,
+    non_camel_case_types,
+    non_snake_case,
+    non_upper_case_globals,
+)]
+pub mod capi {
+    include!(env!("BUS1_CAPI_PATH"));
+}
+
+const __LOG_PREFIX: &[u8] = b"bus1\0";
diff --git a/ipc/bus1/main.c b/ipc/bus1/main.c
new file mode 100644
index 000000000000..bd6399b2ce3a
--- /dev/null
+++ b/ipc/bus1/main.c
@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/init.h>
+#include <linux/module.h>
+#include "lib.h"
+
+static int __init b1_main_init(void)
+{
+	return 0;
+}
+
+static void __exit b1_main_deinit(void)
+{
+}
+
+module_init(b1_main_init);
+module_exit(b1_main_deinit);
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Capability-based IPC");
-- 
2.53.0