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, 291 insertions, 0 deletions

diff --git a/drivers/md/dm-exception-store.c b/drivers/md/dm-exception-store.c
new file mode 100644
index 000000000..58cbc5e54
--- /dev/null
+++ b/drivers/md/dm-exception-store.c
@@ -0,0 +1,291 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2001-2002 Sistina Software (UK) Limited.
+ * Copyright (C) 2006-2008 Red Hat GmbH
+ *
+ * This file is released under the GPL.
+ */
+
+#include "dm-exception-store.h"
+
+#include <linux/ctype.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/vmalloc.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+
+#define DM_MSG_PREFIX "snapshot exception stores"
+
+static LIST_HEAD(_exception_store_types);
+static DEFINE_SPINLOCK(_lock);
+
+static struct dm_exception_store_type *__find_exception_store_type(const char *name)
+{
+	struct dm_exception_store_type *type;
+
+	list_for_each_entry(type, &_exception_store_types, list)
+		if (!strcmp(name, type->name))
+			return type;
+
+	return NULL;
+}
+
+static struct dm_exception_store_type *_get_exception_store_type(const char *name)
+{
+	struct dm_exception_store_type *type;
+
+	spin_lock(&_lock);
+
+	type = __find_exception_store_type(name);
+
+	if (type && !try_module_get(type->module))
+		type = NULL;
+
+	spin_unlock(&_lock);
+
+	return type;
+}
+
+/*
+ * get_type
+ * @type_name
+ *
+ * Attempt to retrieve the dm_exception_store_type by name.  If not already
+ * available, attempt to load the appropriate module.
+ *
+ * Exstore modules are named "dm-exstore-" followed by the 'type_name'.
+ * Modules may contain multiple types.
+ * This function will first try the module "dm-exstore-<type_name>",
+ * then truncate 'type_name' on the last '-' and try again.
+ *
+ * For example, if type_name was "clustered-shared", it would search
+ * 'dm-exstore-clustered-shared' then 'dm-exstore-clustered'.
+ *
+ * 'dm-exception-store-<type_name>' is too long of a name in my
+ * opinion, which is why I've chosen to have the files
+ * containing exception store implementations be 'dm-exstore-<type_name>'.
+ * If you want your module to be autoloaded, you will follow this
+ * naming convention.
+ *
+ * Returns: dm_exception_store_type* on success, NULL on failure
+ */
+static struct dm_exception_store_type *get_type(const char *type_name)
+{
+	char *p, *type_name_dup;
+	struct dm_exception_store_type *type;
+
+	type = _get_exception_store_type(type_name);
+	if (type)
+		return type;
+
+	type_name_dup = kstrdup(type_name, GFP_KERNEL);
+	if (!type_name_dup) {
+		DMERR("No memory left to attempt load for \"%s\"", type_name);
+		return NULL;
+	}
+
+	while (request_module("dm-exstore-%s", type_name_dup) ||
+	       !(type = _get_exception_store_type(type_name))) {
+		p = strrchr(type_name_dup, '-');
+		if (!p)
+			break;
+		p[0] = '\0';
+	}
+
+	if (!type)
+		DMWARN("Module for exstore type \"%s\" not found.", type_name);
+
+	kfree(type_name_dup);
+
+	return type;
+}
+
+static void put_type(struct dm_exception_store_type *type)
+{
+	spin_lock(&_lock);
+	module_put(type->module);
+	spin_unlock(&_lock);
+}
+
+int dm_exception_store_type_register(struct dm_exception_store_type *type)
+{
+	int r = 0;
+
+	spin_lock(&_lock);
+	if (!__find_exception_store_type(type->name))
+		list_add(&type->list, &_exception_store_types);
+	else
+		r = -EBUSY;
+	spin_unlock(&_lock);
+
+	return r;
+}
+EXPORT_SYMBOL(dm_exception_store_type_register);
+
+int dm_exception_store_type_unregister(struct dm_exception_store_type *type)
+{
+	spin_lock(&_lock);
+
+	if (!__find_exception_store_type(type->name)) {
+		spin_unlock(&_lock);
+		return -EINVAL;
+	}
+
+	list_del(&type->list);
+
+	spin_unlock(&_lock);
+
+	return 0;
+}
+EXPORT_SYMBOL(dm_exception_store_type_unregister);
+
+static int set_chunk_size(struct dm_exception_store *store,
+			  const char *chunk_size_arg, char **error)
+{
+	unsigned int chunk_size;
+
+	if (kstrtouint(chunk_size_arg, 10, &chunk_size)) {
+		*error = "Invalid chunk size";
+		return -EINVAL;
+	}
+
+	if (!chunk_size) {
+		store->chunk_size = store->chunk_mask = store->chunk_shift = 0;
+		return 0;
+	}
+
+	return dm_exception_store_set_chunk_size(store, chunk_size, error);
+}
+
+int dm_exception_store_set_chunk_size(struct dm_exception_store *store,
+				      unsigned int chunk_size,
+				      char **error)
+{
+	/* Check chunk_size is a power of 2 */
+	if (!is_power_of_2(chunk_size)) {
+		*error = "Chunk size is not a power of 2";
+		return -EINVAL;
+	}
+
+	/* Validate the chunk size against the device block size */
+	if (chunk_size %
+	    (bdev_logical_block_size(dm_snap_cow(store->snap)->bdev) >> 9) ||
+	    chunk_size %
+	    (bdev_logical_block_size(dm_snap_origin(store->snap)->bdev) >> 9)) {
+		*error = "Chunk size is not a multiple of device blocksize";
+		return -EINVAL;
+	}
+
+	if (chunk_size > INT_MAX >> SECTOR_SHIFT) {
+		*error = "Chunk size is too high";
+		return -EINVAL;
+	}
+
+	store->chunk_size = chunk_size;
+	store->chunk_mask = chunk_size - 1;
+	store->chunk_shift = __ffs(chunk_size);
+
+	return 0;
+}
+
+int dm_exception_store_create(struct dm_target *ti, int argc, char **argv,
+			      struct dm_snapshot *snap,
+			      unsigned int *args_used,
+			      struct dm_exception_store **store)
+{
+	int r = 0;
+	struct dm_exception_store_type *type = NULL;
+	struct dm_exception_store *tmp_store;
+	char persistent;
+
+	if (argc < 2) {
+		ti->error = "Insufficient exception store arguments";
+		return -EINVAL;
+	}
+
+	tmp_store = kzalloc_obj(*tmp_store);
+	if (!tmp_store) {
+		ti->error = "Exception store allocation failed";
+		return -ENOMEM;
+	}
+
+	persistent = toupper(*argv[0]);
+	if (persistent == 'P')
+		type = get_type("P");
+	else if (persistent == 'N')
+		type = get_type("N");
+	else {
+		ti->error = "Exception store type is not P or N";
+		r = -EINVAL;
+		goto bad_type;
+	}
+
+	if (!type) {
+		ti->error = "Exception store type not recognised";
+		r = -EINVAL;
+		goto bad_type;
+	}
+
+	tmp_store->type = type;
+	tmp_store->snap = snap;
+
+	r = set_chunk_size(tmp_store, argv[1], &ti->error);
+	if (r)
+		goto bad;
+
+	r = type->ctr(tmp_store, (strlen(argv[0]) > 1 ? &argv[0][1] : NULL));
+	if (r) {
+		ti->error = "Exception store type constructor failed";
+		goto bad;
+	}
+
+	*args_used = 2;
+	*store = tmp_store;
+	return 0;
+
+bad:
+	put_type(type);
+bad_type:
+	kfree(tmp_store);
+	return r;
+}
+EXPORT_SYMBOL(dm_exception_store_create);
+
+void dm_exception_store_destroy(struct dm_exception_store *store)
+{
+	store->type->dtr(store);
+	put_type(store->type);
+	kfree(store);
+}
+EXPORT_SYMBOL(dm_exception_store_destroy);
+
+int dm_exception_store_init(void)
+{
+	int r;
+
+	r = dm_transient_snapshot_init();
+	if (r) {
+		DMERR("Unable to register transient exception store type.");
+		goto transient_fail;
+	}
+
+	r = dm_persistent_snapshot_init();
+	if (r) {
+		DMERR("Unable to register persistent exception store type");
+		goto persistent_fail;
+	}
+
+	return 0;
+
+persistent_fail:
+	dm_transient_snapshot_exit();
+transient_fail:
+	return r;
+}
+
+void dm_exception_store_exit(void)
+{
+	dm_persistent_snapshot_exit();
+	dm_transient_snapshot_exit();
+}