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, 106 insertions, 0 deletions

diff --git a/scripts/livepatch/init.c b/scripts/livepatch/init.c
new file mode 100644
index 000000000..f14d8c8fb
--- /dev/null
+++ b/scripts/livepatch/init.c
@@ -0,0 +1,106 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Init code for a livepatch kernel module
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/livepatch.h>
+
+static struct klp_patch *patch;
+
+static int __init livepatch_mod_init(void)
+{
+	struct klp_object_ext *obj_exts;
+	size_t obj_exts_sec_size;
+	struct klp_object *objs;
+	unsigned int nr_objs;
+	int ret;
+
+	obj_exts = klp_find_section_by_name(THIS_MODULE, ".init.klp_objects",
+					    &obj_exts_sec_size);
+	nr_objs = obj_exts_sec_size / sizeof(*obj_exts);
+	if (!nr_objs) {
+		pr_err("nothing to patch!\n");
+		ret = -EINVAL;
+		goto err;
+	}
+
+	patch = kzalloc_obj(*patch);
+	if (!patch) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	objs = kzalloc(sizeof(struct klp_object) * (nr_objs + 1),  GFP_KERNEL);
+	if (!objs) {
+		ret = -ENOMEM;
+		goto err_free_patch;
+	}
+
+	for (int i = 0; i < nr_objs; i++) {
+		struct klp_object_ext *obj_ext = obj_exts + i;
+		struct klp_func_ext *funcs_ext = obj_ext->funcs;
+		unsigned int nr_funcs = obj_ext->nr_funcs;
+		struct klp_func *funcs = objs[i].funcs;
+		struct klp_object *obj = objs + i;
+
+		funcs = kzalloc(sizeof(struct klp_func) * (nr_funcs + 1), GFP_KERNEL);
+		if (!funcs) {
+			ret = -ENOMEM;
+			for (int j = 0; j < i; j++)
+				kfree(objs[i].funcs);
+			goto err_free_objs;
+		}
+
+		for (int j = 0; j < nr_funcs; j++) {
+			funcs[j].old_name   = funcs_ext[j].old_name;
+			funcs[j].new_func   = funcs_ext[j].new_func;
+			funcs[j].old_sympos = funcs_ext[j].sympos;
+		}
+
+		obj->name = obj_ext->name;
+		obj->funcs = funcs;
+
+		memcpy(&obj->callbacks, &obj_ext->callbacks, sizeof(struct klp_callbacks));
+	}
+
+	patch->mod = THIS_MODULE;
+	patch->objs = objs;
+
+	/* TODO patch->states */
+
+#ifdef KLP_NO_REPLACE
+	patch->replace = false;
+#else
+	patch->replace = true;
+#endif
+
+	return klp_enable_patch(patch);
+
+err_free_objs:
+	kfree(objs);
+err_free_patch:
+	kfree(patch);
+err:
+	return ret;
+}
+
+static void __exit livepatch_mod_exit(void)
+{
+	struct klp_object *obj;
+
+	klp_for_each_object_static(patch, obj)
+		kfree(obj->funcs);
+
+	kfree(patch->objs);
+	kfree(patch);
+}
+
+module_init(livepatch_mod_init);
+module_exit(livepatch_mod_exit);
+MODULE_LICENSE("GPL");
+MODULE_INFO(livepatch, "Y");
+MODULE_DESCRIPTION("Livepatch module");