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, 262 insertions, 0 deletions

diff --git a/drivers/block/zram/zcomp.c b/drivers/block/zram/zcomp.c
new file mode 100644
index 000000000..974c46918
--- /dev/null
+++ b/drivers/block/zram/zcomp.c
@@ -0,0 +1,262 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/kernel.h>
+#include <linux/string.h>
+#include <linux/err.h>
+#include <linux/slab.h>
+#include <linux/wait.h>
+#include <linux/sched.h>
+#include <linux/cpuhotplug.h>
+#include <linux/vmalloc.h>
+#include <linux/sysfs.h>
+
+#include "zcomp.h"
+
+#include "backend_lzo.h"
+#include "backend_lzorle.h"
+#include "backend_lz4.h"
+#include "backend_lz4hc.h"
+#include "backend_zstd.h"
+#include "backend_deflate.h"
+#include "backend_842.h"
+
+static const struct zcomp_ops *backends[] = {
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_LZO)
+	&backend_lzorle,
+	&backend_lzo,
+#endif
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_LZ4)
+	&backend_lz4,
+#endif
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_LZ4HC)
+	&backend_lz4hc,
+#endif
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_ZSTD)
+	&backend_zstd,
+#endif
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_DEFLATE)
+	&backend_deflate,
+#endif
+#if IS_ENABLED(CONFIG_ZRAM_BACKEND_842)
+	&backend_842,
+#endif
+	NULL
+};
+
+static void zcomp_strm_free(struct zcomp *comp, struct zcomp_strm *zstrm)
+{
+	comp->ops->destroy_ctx(&zstrm->ctx);
+	vfree(zstrm->local_copy);
+	vfree(zstrm->buffer);
+	zstrm->buffer = NULL;
+}
+
+static int zcomp_strm_init(struct zcomp *comp, struct zcomp_strm *zstrm)
+{
+	int ret;
+
+	ret = comp->ops->create_ctx(comp->params, &zstrm->ctx);
+	if (ret)
+		return ret;
+
+	zstrm->local_copy = vzalloc(PAGE_SIZE);
+	/*
+	 * allocate 2 pages. 1 for compressed data, plus 1 extra for the
+	 * case when compressed size is larger than the original one
+	 */
+	zstrm->buffer = vzalloc(2 * PAGE_SIZE);
+	if (!zstrm->buffer || !zstrm->local_copy) {
+		zcomp_strm_free(comp, zstrm);
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+static const struct zcomp_ops *lookup_backend_ops(const char *comp)
+{
+	int i = 0;
+
+	while (backends[i]) {
+		if (sysfs_streq(comp, backends[i]->name))
+			break;
+		i++;
+	}
+	return backends[i];
+}
+
+const char *zcomp_lookup_backend_name(const char *comp)
+{
+	const struct zcomp_ops *backend = lookup_backend_ops(comp);
+
+	if (backend)
+		return backend->name;
+
+	return NULL;
+}
+
+/* show available compressors */
+ssize_t zcomp_available_show(const char *comp, char *buf, ssize_t at)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(backends) - 1; i++) {
+		if (!strcmp(comp, backends[i]->name)) {
+			at += sysfs_emit_at(buf, at, "[%s] ",
+					    backends[i]->name);
+		} else {
+			at += sysfs_emit_at(buf, at, "%s ", backends[i]->name);
+		}
+	}
+
+	at += sysfs_emit_at(buf, at, "\n");
+	return at;
+}
+
+struct zcomp_strm *zcomp_stream_get(struct zcomp *comp)
+{
+	for (;;) {
+		struct zcomp_strm *zstrm = raw_cpu_ptr(comp->stream);
+
+		/*
+		 * Inspired by zswap
+		 *
+		 * stream is returned with ->mutex locked which prevents
+		 * cpu_dead() from releasing this stream under us, however
+		 * there is still a race window between raw_cpu_ptr() and
+		 * mutex_lock(), during which we could have been migrated
+		 * from a CPU that has already destroyed its stream.  If
+		 * so then unlock and re-try on the current CPU.
+		 */
+		mutex_lock(&zstrm->lock);
+		if (likely(zstrm->buffer))
+			return zstrm;
+		mutex_unlock(&zstrm->lock);
+	}
+}
+
+void zcomp_stream_put(struct zcomp_strm *zstrm)
+{
+	mutex_unlock(&zstrm->lock);
+}
+
+int zcomp_compress(struct zcomp *comp, struct zcomp_strm *zstrm,
+		   const void *src, unsigned int *dst_len)
+{
+	struct zcomp_req req = {
+		.src = src,
+		.dst = zstrm->buffer,
+		.src_len = PAGE_SIZE,
+		.dst_len = 2 * PAGE_SIZE,
+	};
+	int ret;
+
+	might_sleep();
+	ret = comp->ops->compress(comp->params, &zstrm->ctx, &req);
+	if (!ret)
+		*dst_len = req.dst_len;
+	return ret;
+}
+
+int zcomp_decompress(struct zcomp *comp, struct zcomp_strm *zstrm,
+		     const void *src, unsigned int src_len, void *dst)
+{
+	struct zcomp_req req = {
+		.src = src,
+		.dst = dst,
+		.src_len = src_len,
+		.dst_len = PAGE_SIZE,
+	};
+
+	might_sleep();
+	return comp->ops->decompress(comp->params, &zstrm->ctx, &req);
+}
+
+int zcomp_cpu_up_prepare(unsigned int cpu, struct hlist_node *node)
+{
+	struct zcomp *comp = hlist_entry(node, struct zcomp, node);
+	struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
+	int ret;
+
+	ret = zcomp_strm_init(comp, zstrm);
+	if (ret)
+		pr_err("Can't allocate a compression stream\n");
+	return ret;
+}
+
+int zcomp_cpu_dead(unsigned int cpu, struct hlist_node *node)
+{
+	struct zcomp *comp = hlist_entry(node, struct zcomp, node);
+	struct zcomp_strm *zstrm = per_cpu_ptr(comp->stream, cpu);
+
+	mutex_lock(&zstrm->lock);
+	zcomp_strm_free(comp, zstrm);
+	mutex_unlock(&zstrm->lock);
+	return 0;
+}
+
+static int zcomp_init(struct zcomp *comp, struct zcomp_params *params)
+{
+	int ret, cpu;
+
+	comp->stream = alloc_percpu(struct zcomp_strm);
+	if (!comp->stream)
+		return -ENOMEM;
+
+	comp->params = params;
+	ret = comp->ops->setup_params(comp->params);
+	if (ret)
+		goto cleanup;
+
+	for_each_possible_cpu(cpu)
+		mutex_init(&per_cpu_ptr(comp->stream, cpu)->lock);
+
+	ret = cpuhp_state_add_instance(CPUHP_ZCOMP_PREPARE, &comp->node);
+	if (ret < 0)
+		goto cleanup;
+
+	return 0;
+
+cleanup:
+	comp->ops->release_params(comp->params);
+	free_percpu(comp->stream);
+	return ret;
+}
+
+void zcomp_destroy(struct zcomp *comp)
+{
+	cpuhp_state_remove_instance(CPUHP_ZCOMP_PREPARE, &comp->node);
+	comp->ops->release_params(comp->params);
+	free_percpu(comp->stream);
+	kfree(comp);
+}
+
+struct zcomp *zcomp_create(const char *alg, struct zcomp_params *params)
+{
+	struct zcomp *comp;
+	int error;
+
+	/*
+	 * The backends array has a sentinel NULL value, so the minimum
+	 * size is 1. In order to be valid the array, apart from the
+	 * sentinel NULL element, should have at least one compression
+	 * backend selected.
+	 */
+	BUILD_BUG_ON(ARRAY_SIZE(backends) <= 1);
+
+	comp = kzalloc_obj(struct zcomp);
+	if (!comp)
+		return ERR_PTR(-ENOMEM);
+
+	comp->ops = lookup_backend_ops(alg);
+	if (!comp->ops) {
+		kfree(comp);
+		return ERR_PTR(-EINVAL);
+	}
+
+	error = zcomp_init(comp, params);
+	if (error) {
+		kfree(comp);
+		return ERR_PTR(error);
+	}
+	return comp;
+}