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, 121 insertions, 0 deletions

diff --git a/drivers/dma/idxd/compat.c b/drivers/dma/idxd/compat.c
new file mode 100644
index 000000000..95b8ef958
--- /dev/null
+++ b/drivers/dma/idxd/compat.c
@@ -0,0 +1,121 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright(c) 2021 Intel Corporation. All rights rsvd. */
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/device.h>
+#include <linux/device/bus.h>
+#include "idxd.h"
+
+extern void device_driver_detach(struct device *dev);
+
+#define DRIVER_ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store)	\
+	struct driver_attribute driver_attr_##_name =		\
+	__ATTR_IGNORE_LOCKDEP(_name, _mode, _show, _store)
+
+static ssize_t unbind_store(struct device_driver *drv, const char *buf, size_t count)
+{
+	const struct bus_type *bus = drv->bus;
+	struct device *dev;
+	int rc = -ENODEV;
+
+	dev = bus_find_device_by_name(bus, NULL, buf);
+	if (!dev)
+		return -ENODEV;
+
+	if (dev->driver) {
+		device_driver_detach(dev);
+		rc = count;
+	}
+
+	put_device(dev);
+
+	return rc;
+}
+static DRIVER_ATTR_IGNORE_LOCKDEP(unbind, 0200, NULL, unbind_store);
+
+static ssize_t bind_store(struct device_driver *drv, const char *buf, size_t count)
+{
+	const struct bus_type *bus = drv->bus;
+	struct device *dev;
+	struct device_driver *alt_drv = NULL;
+	int rc = -ENODEV;
+	struct idxd_dev *idxd_dev;
+
+	dev = bus_find_device_by_name(bus, NULL, buf);
+	if (!dev)
+		return -ENODEV;
+
+	if (dev->driver || drv != &dsa_drv.drv)
+		goto err_put_dev;
+
+	idxd_dev = confdev_to_idxd_dev(dev);
+	if (is_idxd_dev(idxd_dev)) {
+		alt_drv = driver_find("idxd", bus);
+	} else if (is_idxd_wq_dev(idxd_dev)) {
+		struct idxd_wq *wq = confdev_to_wq(dev);
+
+		if (is_idxd_wq_kernel(wq))
+			alt_drv = driver_find("dmaengine", bus);
+		else if (is_idxd_wq_user(wq))
+			alt_drv = driver_find("user", bus);
+	}
+	if (!alt_drv)
+		goto err_put_dev;
+
+	rc = device_driver_attach(alt_drv, dev);
+	if (rc < 0)
+		goto err_put_dev;
+
+	put_device(dev);
+
+	return count;
+
+err_put_dev:
+	put_device(dev);
+
+	return rc;
+}
+static DRIVER_ATTR_IGNORE_LOCKDEP(bind, 0200, NULL, bind_store);
+
+static struct attribute *dsa_drv_compat_attrs[] = {
+	&driver_attr_bind.attr,
+	&driver_attr_unbind.attr,
+	NULL,
+};
+
+static const struct attribute_group dsa_drv_compat_attr_group = {
+	.attrs = dsa_drv_compat_attrs,
+};
+
+static const struct attribute_group *dsa_drv_compat_groups[] = {
+	&dsa_drv_compat_attr_group,
+	NULL,
+};
+
+static int idxd_dsa_drv_probe(struct idxd_dev *idxd_dev)
+{
+	return -ENODEV;
+}
+
+static void idxd_dsa_drv_remove(struct idxd_dev *idxd_dev)
+{
+}
+
+static enum idxd_dev_type dev_types[] = {
+	IDXD_DEV_NONE,
+};
+
+struct idxd_device_driver dsa_drv = {
+	.name = "dsa",
+	.probe = idxd_dsa_drv_probe,
+	.remove = idxd_dsa_drv_remove,
+	.type = dev_types,
+	.drv = {
+		.suppress_bind_attrs = true,
+		.groups = dsa_drv_compat_groups,
+	},
+};
+
+module_idxd_driver(dsa_drv);
+MODULE_IMPORT_NS("IDXD");