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, 212 insertions, 0 deletions

diff --git a/samples/livepatch/livepatch-shadow-mod.c b/samples/livepatch/livepatch-shadow-mod.c
new file mode 100644
index 000000000..5d83ad5a8
--- /dev/null
+++ b/samples/livepatch/livepatch-shadow-mod.c
@@ -0,0 +1,212 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2017 Joe Lawrence <joe.lawrence@redhat.com>
+ */
+
+/*
+ * livepatch-shadow-mod.c - Shadow variables, buggy module demo
+ *
+ * Purpose
+ * -------
+ *
+ * As a demonstration of livepatch shadow variable API, this module
+ * introduces memory leak behavior that livepatch modules
+ * livepatch-shadow-fix1.ko and livepatch-shadow-fix2.ko correct and
+ * enhance.
+ *
+ * WARNING - even though the livepatch-shadow-fix modules patch the
+ * memory leak, please load these modules at your own risk -- some
+ * amount of memory may leaked before the bug is patched.
+ *
+ *
+ * Usage
+ * -----
+ *
+ * Step 1 - Load the buggy demonstration module:
+ *
+ *   insmod samples/livepatch/livepatch-shadow-mod.ko
+ *
+ * Watch dmesg output for a few moments to see new dummy being allocated
+ * and a periodic cleanup check.  (Note: a small amount of memory is
+ * being leaked.)
+ *
+ *
+ * Step 2 - Load livepatch fix1:
+ *
+ *   insmod samples/livepatch/livepatch-shadow-fix1.ko
+ *
+ * Continue watching dmesg and note that now livepatch_fix1_dummy_free()
+ * and livepatch_fix1_dummy_alloc() are logging messages about leaked
+ * memory and eventually leaks prevented.
+ *
+ *
+ * Step 3 - Load livepatch fix2 (on top of fix1):
+ *
+ *   insmod samples/livepatch/livepatch-shadow-fix2.ko
+ *
+ * This module extends functionality through shadow variables, as a new
+ * "check" counter is added to the dummy structure.  Periodic dmesg
+ * messages will log these as dummies are cleaned up.
+ *
+ *
+ * Step 4 - Cleanup
+ *
+ * Unwind the demonstration by disabling the livepatch fix modules, then
+ * removing them and the demo module:
+ *
+ *   echo 0 > /sys/kernel/livepatch/livepatch_shadow_fix2/enabled
+ *   echo 0 > /sys/kernel/livepatch/livepatch_shadow_fix1/enabled
+ *   rmmod livepatch-shadow-fix2
+ *   rmmod livepatch-shadow-fix1
+ *   rmmod livepatch-shadow-mod
+ */
+
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/sched.h>
+#include <linux/slab.h>
+#include <linux/stat.h>
+#include <linux/workqueue.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Joe Lawrence <joe.lawrence@redhat.com>");
+MODULE_DESCRIPTION("Buggy module for shadow variable demo");
+
+/* Allocate new dummies every second */
+#define ALLOC_PERIOD	1
+/* Check for expired dummies after a few new ones have been allocated */
+#define CLEANUP_PERIOD	(3 * ALLOC_PERIOD)
+/* Dummies expire after a few cleanup instances */
+#define EXPIRE_PERIOD	(4 * CLEANUP_PERIOD)
+
+/*
+ * Keep a list of all the dummies so we can clean up any residual ones
+ * on module exit
+ */
+static LIST_HEAD(dummy_list);
+static DEFINE_MUTEX(dummy_list_mutex);
+
+struct dummy {
+	struct list_head list;
+	unsigned long jiffies_expire;
+};
+
+static __used noinline struct dummy *dummy_alloc(void)
+{
+	struct dummy *d;
+	int *leak;
+
+	d = kzalloc(sizeof(*d), GFP_KERNEL);
+	if (!d)
+		return NULL;
+
+	d->jiffies_expire = jiffies + secs_to_jiffies(EXPIRE_PERIOD);
+
+	/* Oops, forgot to save leak! */
+	leak = kzalloc(sizeof(*leak), GFP_KERNEL);
+	if (!leak) {
+		kfree(d);
+		return NULL;
+	}
+
+	pr_info("%s: dummy @ %p, expires @ %lx\n",
+		__func__, d, d->jiffies_expire);
+
+	return d;
+}
+
+static __used noinline void dummy_free(struct dummy *d)
+{
+	pr_info("%s: dummy @ %p, expired = %lx\n",
+		__func__, d, d->jiffies_expire);
+
+	kfree(d);
+}
+
+static __used noinline bool dummy_check(struct dummy *d,
+					   unsigned long jiffies)
+{
+	return time_after(jiffies, d->jiffies_expire);
+}
+
+/*
+ * alloc_work_func: allocates new dummy structures, allocates additional
+ *                  memory, aptly named "leak", but doesn't keep
+ *                  permanent record of it.
+ */
+
+static void alloc_work_func(struct work_struct *work);
+static DECLARE_DELAYED_WORK(alloc_dwork, alloc_work_func);
+
+static void alloc_work_func(struct work_struct *work)
+{
+	struct dummy *d;
+
+	d = dummy_alloc();
+	if (!d)
+		return;
+
+	mutex_lock(&dummy_list_mutex);
+	list_add(&d->list, &dummy_list);
+	mutex_unlock(&dummy_list_mutex);
+
+	schedule_delayed_work(&alloc_dwork, secs_to_jiffies(ALLOC_PERIOD));
+}
+
+/*
+ * cleanup_work_func: frees dummy structures.  Without knownledge of
+ *                    "leak", it leaks the additional memory that
+ *                    alloc_work_func created.
+ */
+
+static void cleanup_work_func(struct work_struct *work);
+static DECLARE_DELAYED_WORK(cleanup_dwork, cleanup_work_func);
+
+static void cleanup_work_func(struct work_struct *work)
+{
+	struct dummy *d, *tmp;
+	unsigned long j;
+
+	j = jiffies;
+	pr_info("%s: jiffies = %lx\n", __func__, j);
+
+	mutex_lock(&dummy_list_mutex);
+	list_for_each_entry_safe(d, tmp, &dummy_list, list) {
+
+		/* Kick out and free any expired dummies */
+		if (dummy_check(d, j)) {
+			list_del(&d->list);
+			dummy_free(d);
+		}
+	}
+	mutex_unlock(&dummy_list_mutex);
+
+	schedule_delayed_work(&cleanup_dwork, secs_to_jiffies(CLEANUP_PERIOD));
+}
+
+static int livepatch_shadow_mod_init(void)
+{
+	schedule_delayed_work(&alloc_dwork, secs_to_jiffies(ALLOC_PERIOD));
+	schedule_delayed_work(&cleanup_dwork, secs_to_jiffies(CLEANUP_PERIOD));
+
+	return 0;
+}
+
+static void livepatch_shadow_mod_exit(void)
+{
+	struct dummy *d, *tmp;
+
+	/* Wait for any dummies at work */
+	cancel_delayed_work_sync(&alloc_dwork);
+	cancel_delayed_work_sync(&cleanup_dwork);
+
+	/* Cleanup residual dummies */
+	list_for_each_entry_safe(d, tmp, &dummy_list, list) {
+		list_del(&d->list);
+		dummy_free(d);
+	}
+}
+
+module_init(livepatch_shadow_mod_init);
+module_exit(livepatch_shadow_mod_exit);