kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 125 insertions, 0 deletions

diff --git a/include/drm/drm_module.h b/include/drm/drm_module.h
new file mode 100644
index 000000000..4db1ae03d
--- /dev/null
+++ b/include/drm/drm_module.h
@@ -0,0 +1,125 @@
+/* SPDX-License-Identifier: MIT */
+
+#ifndef DRM_MODULE_H
+#define DRM_MODULE_H
+
+#include <linux/pci.h>
+#include <linux/platform_device.h>
+
+#include <drm/drm_drv.h>
+
+/**
+ * DOC: overview
+ *
+ * This library provides helpers registering DRM drivers during module
+ * initialization and shutdown. The provided helpers act like bus-specific
+ * module helpers, such as module_pci_driver(), but respect additional
+ * parameters that control DRM driver registration.
+ *
+ * Below is an example of initializing a DRM driver for a device on the
+ * PCI bus.
+ *
+ * .. code-block:: c
+ *
+ *	struct pci_driver my_pci_drv = {
+ *	};
+ *
+ *	drm_module_pci_driver(my_pci_drv);
+ *
+ * The generated code will test if DRM drivers are enabled and register
+ * the PCI driver my_pci_drv. For more complex module initialization, you
+ * can still use module_init() and module_exit() in your driver.
+ */
+
+/*
+ * PCI drivers
+ */
+
+static inline int __init drm_pci_register_driver(struct pci_driver *pci_drv)
+{
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
+	return pci_register_driver(pci_drv);
+}
+
+/**
+ * drm_module_pci_driver - Register a DRM driver for PCI-based devices
+ * @__pci_drv: the PCI driver structure
+ *
+ * Registers a DRM driver for devices on the PCI bus. The helper
+ * macro behaves like module_pci_driver() but tests the state of
+ * drm_firmware_drivers_only(). For more complex module initialization,
+ * use module_init() and module_exit() directly.
+ *
+ * Each module may only use this macro once. Calling it replaces
+ * module_init() and module_exit().
+ */
+#define drm_module_pci_driver(__pci_drv) \
+	module_driver(__pci_drv, drm_pci_register_driver, pci_unregister_driver)
+
+static inline int __init
+drm_pci_register_driver_if_modeset(struct pci_driver *pci_drv, int modeset)
+{
+	if (drm_firmware_drivers_only() && modeset == -1)
+		return -ENODEV;
+	if (modeset == 0)
+		return -ENODEV;
+
+	return pci_register_driver(pci_drv);
+}
+
+static inline void __exit
+drm_pci_unregister_driver_if_modeset(struct pci_driver *pci_drv, int modeset)
+{
+	pci_unregister_driver(pci_drv);
+}
+
+/**
+ * drm_module_pci_driver_if_modeset - Register a DRM driver for PCI-based devices
+ * @__pci_drv: the PCI driver structure
+ * @__modeset: an additional parameter that disables the driver
+ *
+ * This macro is deprecated and only provided for existing drivers. For
+ * new drivers, use drm_module_pci_driver().
+ *
+ * Registers a DRM driver for devices on the PCI bus. The helper macro
+ * behaves like drm_module_pci_driver() with an additional driver-specific
+ * flag. If __modeset is 0, the driver has been disabled, if __modeset is
+ * -1 the driver state depends on the global DRM state. For all other
+ * values, the PCI driver has been enabled. The default should be -1.
+ */
+#define drm_module_pci_driver_if_modeset(__pci_drv, __modeset) \
+	module_driver(__pci_drv, drm_pci_register_driver_if_modeset, \
+		      drm_pci_unregister_driver_if_modeset, __modeset)
+
+/*
+ * Platform drivers
+ */
+
+static inline int __init
+drm_platform_driver_register(struct platform_driver *platform_drv)
+{
+	if (drm_firmware_drivers_only())
+		return -ENODEV;
+
+	return platform_driver_register(platform_drv);
+}
+
+/**
+ * drm_module_platform_driver - Register a DRM driver for platform devices
+ * @__platform_drv: the platform driver structure
+ *
+ * Registers a DRM driver for devices on the platform bus. The helper
+ * macro behaves like module_platform_driver() but tests the state of
+ * drm_firmware_drivers_only(). For more complex module initialization,
+ * use module_init() and module_exit() directly.
+ *
+ * Each module may only use this macro once. Calling it replaces
+ * module_init() and module_exit().
+ */
+#define drm_module_platform_driver(__platform_drv) \
+	module_driver(__platform_drv, drm_platform_driver_register, \
+		      platform_driver_unregister)
+
+#endif