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, 109 insertions, 0 deletions

diff --git a/drivers/tc/tc-driver.c b/drivers/tc/tc-driver.c
new file mode 100644
index 000000000..2f6d14759
--- /dev/null
+++ b/drivers/tc/tc-driver.c
@@ -0,0 +1,109 @@
+/*
+ *	TURBOchannel driver services.
+ *
+ *	Copyright (c) 2005  James Simmons
+ *	Copyright (c) 2006  Maciej W. Rozycki
+ *
+ *	Loosely based on drivers/dio/dio-driver.c and
+ *	drivers/pci/pci-driver.c.
+ *
+ *	This file is subject to the terms and conditions of the GNU
+ *	General Public License.  See the file "COPYING" in the main
+ *	directory of this archive for more details.
+ */
+
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/tc.h>
+
+/**
+ * tc_register_driver - register a new TC driver
+ * @drv: the driver structure to register
+ *
+ * Adds the driver structure to the list of registered drivers
+ * Returns a negative value on error, otherwise 0.
+ * If no error occurred, the driver remains registered even if
+ * no device was claimed during registration.
+ */
+int tc_register_driver(struct tc_driver *tdrv)
+{
+	return driver_register(&tdrv->driver);
+}
+EXPORT_SYMBOL(tc_register_driver);
+
+/**
+ * tc_unregister_driver - unregister a TC driver
+ * @drv: the driver structure to unregister
+ *
+ * Deletes the driver structure from the list of registered TC drivers,
+ * gives it a chance to clean up by calling its remove() function for
+ * each device it was responsible for, and marks those devices as
+ * driverless.
+ */
+void tc_unregister_driver(struct tc_driver *tdrv)
+{
+	driver_unregister(&tdrv->driver);
+}
+EXPORT_SYMBOL(tc_unregister_driver);
+
+/**
+ * tc_match_device - tell if a TC device structure has a matching
+ *                   TC device ID structure
+ * @tdrv: the TC driver to earch for matching TC device ID strings
+ * @tdev: the TC device structure to match against
+ *
+ * Used by a driver to check whether a TC device present in the
+ * system is in its list of supported devices.  Returns the matching
+ * tc_device_id structure or %NULL if there is no match.
+ */
+static const struct tc_device_id *tc_match_device(const struct tc_driver *tdrv,
+						  struct tc_dev *tdev)
+{
+	const struct tc_device_id *id = tdrv->id_table;
+
+	if (id) {
+		while (id->name[0] || id->vendor[0]) {
+			if (strcmp(tdev->name, id->name) == 0 &&
+			    strcmp(tdev->vendor, id->vendor) == 0)
+				return id;
+			id++;
+		}
+	}
+	return NULL;
+}
+
+/**
+ * tc_bus_match - Tell if a device structure has a matching
+ *                TC device ID structure
+ * @dev: the device structure to match against
+ * @drv: the device driver to search for matching TC device ID strings
+ *
+ * Used by a driver to check whether a TC device present in the
+ * system is in its list of supported devices.  Returns 1 if there
+ * is a match or 0 otherwise.
+ */
+static int tc_bus_match(struct device *dev, const struct device_driver *drv)
+{
+	struct tc_dev *tdev = to_tc_dev(dev);
+	const struct tc_driver *tdrv = to_tc_driver(drv);
+	const struct tc_device_id *id;
+
+	id = tc_match_device(tdrv, tdev);
+	if (id)
+		return 1;
+
+	return 0;
+}
+
+const struct bus_type tc_bus_type = {
+	.name	= "tc",
+	.match	= tc_bus_match,
+};
+EXPORT_SYMBOL(tc_bus_type);
+
+static int __init tc_driver_init(void)
+{
+	return bus_register(&tc_bus_type);
+}
+
+postcore_initcall(tc_driver_init);