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, 102 insertions, 0 deletions

diff --git a/drivers/mtd/chips/chipreg.c b/drivers/mtd/chips/chipreg.c
new file mode 100644
index 000000000..a05e10368
--- /dev/null
+++ b/drivers/mtd/chips/chipreg.c
@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Registration for chip drivers
+ *
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/kmod.h>
+#include <linux/spinlock.h>
+#include <linux/slab.h>
+#include <linux/mtd/map.h>
+#include <linux/mtd/mtd.h>
+
+static DEFINE_SPINLOCK(chip_drvs_lock);
+static LIST_HEAD(chip_drvs_list);
+
+void register_mtd_chip_driver(struct mtd_chip_driver *drv)
+{
+	spin_lock(&chip_drvs_lock);
+	list_add(&drv->list, &chip_drvs_list);
+	spin_unlock(&chip_drvs_lock);
+}
+
+void unregister_mtd_chip_driver(struct mtd_chip_driver *drv)
+{
+	spin_lock(&chip_drvs_lock);
+	list_del(&drv->list);
+	spin_unlock(&chip_drvs_lock);
+}
+
+static struct mtd_chip_driver *get_mtd_chip_driver (const char *name)
+{
+	struct mtd_chip_driver *ret = NULL, *this;
+
+	spin_lock(&chip_drvs_lock);
+
+	list_for_each_entry(this, &chip_drvs_list, list) {
+		if (!strcmp(this->name, name)) {
+			ret = this;
+			break;
+		}
+	}
+	if (ret && !try_module_get(ret->module))
+		ret = NULL;
+
+	spin_unlock(&chip_drvs_lock);
+
+	return ret;
+}
+
+	/* Hide all the horrid details, like some silly person taking
+	   get_module_symbol() away from us, from the caller. */
+
+struct mtd_info *do_map_probe(const char *name, struct map_info *map)
+{
+	struct mtd_chip_driver *drv;
+	struct mtd_info *ret;
+
+	drv = get_mtd_chip_driver(name);
+
+	if (!drv && !request_module("%s", name))
+		drv = get_mtd_chip_driver(name);
+
+	if (!drv)
+		return NULL;
+
+	ret = drv->probe(map);
+
+	/* We decrease the use count here. It may have been a
+	   probe-only module, which is no longer required from this
+	   point, having given us a handle on (and increased the use
+	   count of) the actual driver code.
+	*/
+	module_put(drv->module);
+
+	return ret;
+}
+/*
+ * Destroy an MTD device which was created for a map device.
+ * Make sure the MTD device is already unregistered before calling this
+ */
+void map_destroy(struct mtd_info *mtd)
+{
+	struct map_info *map = mtd->priv;
+
+	if (map->fldrv->destroy)
+		map->fldrv->destroy(mtd);
+
+	module_put(map->fldrv->module);
+
+	kfree(mtd);
+}
+
+EXPORT_SYMBOL(register_mtd_chip_driver);
+EXPORT_SYMBOL(unregister_mtd_chip_driver);
+EXPORT_SYMBOL(do_map_probe);
+EXPORT_SYMBOL(map_destroy);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("David Woodhouse <dwmw2@infradead.org>");
+MODULE_DESCRIPTION("Core routines for registering and invoking MTD chip drivers");