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, 287 insertions, 0 deletions

diff --git a/drivers/md/dm-target.c b/drivers/md/dm-target.c
new file mode 100644
index 000000000..3d294365e
--- /dev/null
+++ b/drivers/md/dm-target.c
@@ -0,0 +1,287 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2001 Sistina Software (UK) Limited
+ *
+ * This file is released under the GPL.
+ */
+
+#include "dm-core.h"
+
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/kmod.h>
+#include <linux/bio.h>
+#include <linux/dax.h>
+
+#define DM_MSG_PREFIX "target"
+
+static LIST_HEAD(_targets);
+static DECLARE_RWSEM(_lock);
+
+static inline struct target_type *__find_target_type(const char *name)
+{
+	struct target_type *tt;
+
+	list_for_each_entry(tt, &_targets, list)
+		if (!strcmp(name, tt->name))
+			return tt;
+
+	return NULL;
+}
+
+static struct target_type *get_target_type(const char *name)
+{
+	struct target_type *tt;
+
+	down_read(&_lock);
+
+	tt = __find_target_type(name);
+	if (tt && !try_module_get(tt->module))
+		tt = NULL;
+
+	up_read(&_lock);
+	return tt;
+}
+
+static void load_module(const char *name)
+{
+	request_module("dm-%s", name);
+}
+
+struct target_type *dm_get_target_type(const char *name)
+{
+	struct target_type *tt = get_target_type(name);
+
+	if (!tt) {
+		load_module(name);
+		tt = get_target_type(name);
+	}
+
+	return tt;
+}
+
+void dm_put_target_type(struct target_type *tt)
+{
+	down_read(&_lock);
+	module_put(tt->module);
+	up_read(&_lock);
+}
+
+int dm_target_iterate(void (*iter_func)(struct target_type *tt,
+					void *param), void *param)
+{
+	struct target_type *tt;
+
+	down_read(&_lock);
+	list_for_each_entry(tt, &_targets, list)
+		iter_func(tt, param);
+	up_read(&_lock);
+
+	return 0;
+}
+
+int dm_register_target(struct target_type *tt)
+{
+	int rv = 0;
+
+	down_write(&_lock);
+	if (__find_target_type(tt->name)) {
+		DMERR("%s: '%s' target already registered",
+		      __func__, tt->name);
+		rv = -EBUSY;
+	} else {
+		list_add(&tt->list, &_targets);
+	}
+	up_write(&_lock);
+
+	return rv;
+}
+EXPORT_SYMBOL(dm_register_target);
+
+void dm_unregister_target(struct target_type *tt)
+{
+	down_write(&_lock);
+	if (!__find_target_type(tt->name)) {
+		DMCRIT("Unregistering unrecognised target: %s", tt->name);
+		BUG();
+	}
+
+	list_del(&tt->list);
+
+	up_write(&_lock);
+}
+EXPORT_SYMBOL(dm_unregister_target);
+
+/*
+ * io-err: always fails an io, useful for bringing
+ * up LVs that have holes in them.
+ */
+struct io_err_c {
+	struct dm_dev *dev;
+	sector_t start;
+};
+
+static int io_err_get_args(struct dm_target *tt, unsigned int argc, char **args)
+{
+	unsigned long long start;
+	struct io_err_c *ioec;
+	char dummy;
+	int ret;
+
+	ioec = kmalloc_obj(*ioec);
+	if (!ioec) {
+		tt->error = "Cannot allocate io_err context";
+		return -ENOMEM;
+	}
+
+	ret = -EINVAL;
+	if (sscanf(args[1], "%llu%c", &start, &dummy) != 1 ||
+	    start != (sector_t)start) {
+		tt->error = "Invalid device sector";
+		goto bad;
+	}
+	ioec->start = start;
+
+	ret = dm_get_device(tt, args[0], dm_table_get_mode(tt->table), &ioec->dev);
+	if (ret) {
+		tt->error = "Device lookup failed";
+		goto bad;
+	}
+
+	tt->private = ioec;
+
+	return 0;
+
+bad:
+	kfree(ioec);
+
+	return ret;
+}
+
+static int io_err_ctr(struct dm_target *tt, unsigned int argc, char **args)
+{
+	/*
+	 * If we have arguments, assume it is the path to the backing
+	 * block device and its mapping start sector (same as dm-linear).
+	 * In this case, get the device so that we can get its limits.
+	 */
+	if (argc == 2) {
+		int ret = io_err_get_args(tt, argc, args);
+
+		if (ret)
+			return ret;
+	}
+
+	/*
+	 * Return error for discards instead of -EOPNOTSUPP
+	 */
+	tt->num_discard_bios = 1;
+	tt->discards_supported = true;
+
+	return 0;
+}
+
+static void io_err_dtr(struct dm_target *tt)
+{
+	struct io_err_c *ioec = tt->private;
+
+	if (ioec) {
+		dm_put_device(tt, ioec->dev);
+		kfree(ioec);
+	}
+}
+
+static int io_err_map(struct dm_target *tt, struct bio *bio)
+{
+	return DM_MAPIO_KILL;
+}
+
+static int io_err_clone_and_map_rq(struct dm_target *ti, struct request *rq,
+				   union map_info *map_context,
+				   struct request **clone)
+{
+	return DM_MAPIO_KILL;
+}
+
+static void io_err_release_clone_rq(struct request *clone,
+				    union map_info *map_context)
+{
+}
+
+#ifdef CONFIG_BLK_DEV_ZONED
+static sector_t io_err_map_sector(struct dm_target *ti, sector_t bi_sector)
+{
+	struct io_err_c *ioec = ti->private;
+
+	return ioec->start + dm_target_offset(ti, bi_sector);
+}
+
+static int io_err_report_zones(struct dm_target *ti,
+		struct dm_report_zones_args *args, unsigned int nr_zones)
+{
+	struct io_err_c *ioec = ti->private;
+
+	/*
+	 * This should never be called when we do not have a backing device
+	 * as that mean the target is not a zoned one.
+	 */
+	if (WARN_ON_ONCE(!ioec))
+		return -EIO;
+
+	return dm_report_zones(ioec->dev->bdev, ioec->start,
+			       io_err_map_sector(ti, args->next_sector),
+			       args, nr_zones);
+}
+#else
+#define io_err_report_zones NULL
+#endif
+
+static int io_err_iterate_devices(struct dm_target *ti,
+				  iterate_devices_callout_fn fn, void *data)
+{
+	struct io_err_c *ioec = ti->private;
+
+	if (!ioec)
+		return 0;
+
+	return fn(ti, ioec->dev, ioec->start, ti->len, data);
+}
+
+static void io_err_io_hints(struct dm_target *ti, struct queue_limits *limits)
+{
+	limits->max_hw_discard_sectors = UINT_MAX;
+	limits->discard_granularity = 512;
+}
+
+static long io_err_dax_direct_access(struct dm_target *ti, pgoff_t pgoff,
+		long nr_pages, enum dax_access_mode mode, void **kaddr,
+		unsigned long *pfn)
+{
+	return -EIO;
+}
+
+static struct target_type error_target = {
+	.name = "error",
+	.version = {1, 7, 0},
+	.features = DM_TARGET_WILDCARD | DM_TARGET_ZONED_HM |
+		DM_TARGET_PASSES_INTEGRITY,
+	.ctr  = io_err_ctr,
+	.dtr  = io_err_dtr,
+	.map  = io_err_map,
+	.clone_and_map_rq = io_err_clone_and_map_rq,
+	.release_clone_rq = io_err_release_clone_rq,
+	.iterate_devices = io_err_iterate_devices,
+	.io_hints = io_err_io_hints,
+	.direct_access = io_err_dax_direct_access,
+	.report_zones = io_err_report_zones,
+};
+
+int __init dm_target_init(void)
+{
+	return dm_register_target(&error_target);
+}
+
+void dm_target_exit(void)
+{
+	dm_unregister_target(&error_target);
+}