linux-kernel - [PATCH v10 04/11] module.h: MODULE_DEVICE

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Message-ID: <20230406190030.968972-5-allenwebb@google.com>
Date:   Thu,  6 Apr 2023 14:00:23 -0500
From:   Allen Webb <allenwebb@...gle.com>
To:     "linux-modules@...r.kernel.org" <linux-modules@...r.kernel.org>,
        "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     gregkh@...uxfoundation.org, mcgrof@...nel.org,
        christophe.leroy@...roup.eu, nick.alcock@...cle.com,
        Allen Webb <allenwebb@...gle.com>
Subject: [PATCH v10 04/11] module.h: MODULE_DEVICE_TABLE for built-in modules

Implement MODULE_DEVICE_TABLE for build-in modules to make it possible
to generate a builtin.alias file to complement modules.alias.

Signed-off-by: Allen Webb <allenwebb@...gle.com>
---
 include/linux/module.h | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 4435ad9439ab..b1cb12e06996 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -237,14 +237,36 @@ extern void cleanup_module(void);
 /* What your module does. */
 #define MODULE_DESCRIPTION(_description) MODULE_INFO(description, _description)
 
-#ifdef MODULE
-/* Creates an alias so file2alias.c can find device table. */
+/*
+ * Creates an alias so file2alias.c can find device table.
+ *
+ * Use this in cases where a device table is used to match devices because it
+ * surfaces match-id based module aliases to userspace for:
+ *   - Automatic module loading through modules.alias.
+ *   - Tools like USBGuard which block devices based on policy such as which
+ *     modules match a device.
+ *
+ * The only use-case for built-in drivers today is to enable userspace to
+ * prevent / allow probe for devices on certain subsystems even if the driver is
+ * already loaded. An example is the USB subsystem with its authorized_default
+ * sysfs attribute. For more details refer to the kernel's Documentation for USB
+ * about authorized_default.
+ *
+ * The module name is included in the alias for two reasons:
+ *   - It avoids creating two aliases with the same name for built-in modules.
+ *     Historically MODULE_DEVICE_TABLE was a no-op for built-in modules, so
+ *     there was nothing to stop different modules from having the same device
+ *     table name and consequently the same alias when building as a module.
+ *   - The module name is needed by files2alias.c to associate a particular
+ *     device table with its associated module for built-in modules since
+ *     files2alias would otherwise see the module name as `vmlinuz.o`.
+ */
 #define MODULE_DEVICE_TABLE(type, name)					\
-extern typeof(name) __mod_##type##__##name##_device_table		\
-  __attribute__ ((unused, alias(__stringify(name))))
-#else  /* !MODULE */
-#define MODULE_DEVICE_TABLE(type, name)
-#endif
+extern void *CONCATENATE(						\
+	CONCATENATE(__mod_##type##__##name##__,				\
+		__KBUILD_MODNAME),					\
+	_device_table)							\
+	__attribute__ ((unused, alias(__stringify(name))))
 
 /* Version of form [<epoch>:]<version>[-<extra-version>].
  * Or for CVS/RCS ID version, everything but the number is stripped.
-- 
2.39.2