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>

2 files changed, 105 insertions, 0 deletions

diff --git a/samples/kmemleak/Makefile b/samples/kmemleak/Makefile
new file mode 100644
index 000000000..8a999ab43
--- /dev/null
+++ b/samples/kmemleak/Makefile
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_SAMPLE_KMEMLEAK) += kmemleak-test.o
diff --git a/samples/kmemleak/kmemleak-test.c b/samples/kmemleak/kmemleak-test.c
new file mode 100644
index 000000000..8609812a3
--- /dev/null
+++ b/samples/kmemleak/kmemleak-test.c
@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * samples/kmemleak/kmemleak-test.c
+ *
+ * Copyright (C) 2008 ARM Limited
+ * Written by Catalin Marinas <catalin.marinas@arm.com>
+ */
+
+#define pr_fmt(fmt) "kmemleak: " fmt
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/vmalloc.h>
+#include <linux/list.h>
+#include <linux/percpu.h>
+#include <linux/fdtable.h>
+
+#include <linux/kmemleak.h>
+
+struct test_node {
+	long header[25];
+	struct list_head list;
+	long footer[25];
+};
+
+static LIST_HEAD(test_list);
+static DEFINE_PER_CPU(void *, kmemleak_test_pointer);
+
+/*
+ * Some very simple testing. This function needs to be extended for
+ * proper testing.
+ */
+static int kmemleak_test_init(void)
+{
+	struct test_node *elem;
+	int i;
+
+	pr_info("Kmemleak testing\n");
+
+	/* make some orphan objects */
+	pr_info("kmalloc(32) = 0x%px\n", kmalloc(32, GFP_KERNEL));
+	pr_info("kmalloc(32) = 0x%px\n", kmalloc(32, GFP_KERNEL));
+	pr_info("kmalloc(1024) = 0x%px\n", kmalloc(1024, GFP_KERNEL));
+	pr_info("kmalloc(1024) = 0x%px\n", kmalloc(1024, GFP_KERNEL));
+	pr_info("kmalloc(2048) = 0x%px\n", kmalloc(2048, GFP_KERNEL));
+	pr_info("kmalloc(2048) = 0x%px\n", kmalloc(2048, GFP_KERNEL));
+	pr_info("kmalloc(4096) = 0x%px\n", kmalloc(4096, GFP_KERNEL));
+	pr_info("kmalloc(4096) = 0x%px\n", kmalloc(4096, GFP_KERNEL));
+#ifndef CONFIG_MODULES
+	pr_info("kmem_cache_alloc(files_cachep) = 0x%px\n",
+		kmem_cache_alloc(files_cachep, GFP_KERNEL));
+	pr_info("kmem_cache_alloc(files_cachep) = 0x%px\n",
+		kmem_cache_alloc(files_cachep, GFP_KERNEL));
+#endif
+	pr_info("vmalloc(64) = 0x%px\n", vmalloc(64));
+	pr_info("vmalloc(64) = 0x%px\n", vmalloc(64));
+	pr_info("vmalloc(64) = 0x%px\n", vmalloc(64));
+	pr_info("vmalloc(64) = 0x%px\n", vmalloc(64));
+	pr_info("vmalloc(64) = 0x%px\n", vmalloc(64));
+
+	/*
+	 * Add elements to a list. They should only appear as orphan
+	 * after the module is removed.
+	 */
+	for (i = 0; i < 10; i++) {
+		elem = kzalloc(sizeof(*elem), GFP_KERNEL);
+		pr_info("kzalloc(sizeof(*elem)) = 0x%px\n", elem);
+		if (!elem)
+			return -ENOMEM;
+		INIT_LIST_HEAD(&elem->list);
+		list_add_tail(&elem->list, &test_list);
+	}
+
+	for_each_possible_cpu(i) {
+		per_cpu(kmemleak_test_pointer, i) = kmalloc(129, GFP_KERNEL);
+		pr_info("kmalloc(129) = 0x%px\n",
+			per_cpu(kmemleak_test_pointer, i));
+	}
+
+	pr_info("__alloc_percpu(64, 4) = 0x%px\n", __alloc_percpu(64, 4));
+
+	return 0;
+}
+module_init(kmemleak_test_init);
+
+static void __exit kmemleak_test_exit(void)
+{
+	struct test_node *elem, *tmp;
+
+	/*
+	 * Remove the list elements without actually freeing the
+	 * memory.
+	 */
+	list_for_each_entry_safe(elem, tmp, &test_list, list)
+		list_del(&elem->list);
+}
+module_exit(kmemleak_test_exit);
+
+MODULE_DESCRIPTION("Sample module to leak memory for kmemleak testing");
+MODULE_LICENSE("GPL");