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, 116 insertions, 0 deletions

diff --git a/lib/test_memcat_p.c b/lib/test_memcat_p.c
new file mode 100644
index 000000000..62f1633b3
--- /dev/null
+++ b/lib/test_memcat_p.c
@@ -0,0 +1,116 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test cases for memcat_p() in lib/memcat_p.c
+ */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/string.h>
+#include <linux/slab.h>
+#include <linux/module.h>
+
+struct test_struct {
+	int		num;
+	unsigned int	magic;
+};
+
+#define MAGIC		0xf00ff00f
+/* Size of each of the NULL-terminated input arrays */
+#define INPUT_MAX	128
+/* Expected number of non-NULL elements in the output array */
+#define EXPECT		(INPUT_MAX * 2 - 2)
+
+static int __init test_memcat_p_init(void)
+{
+	struct test_struct **in0, **in1, **out, **p;
+	int err = -ENOMEM, i, r, total = 0;
+
+	in0 = kzalloc_objs(*in0, INPUT_MAX);
+	if (!in0)
+		return err;
+
+	in1 = kzalloc_objs(*in1, INPUT_MAX);
+	if (!in1)
+		goto err_free_in0;
+
+	for (i = 0, r = 1; i < INPUT_MAX - 1; i++) {
+		in0[i] = kmalloc_obj(**in0);
+		if (!in0[i])
+			goto err_free_elements;
+
+		in1[i] = kmalloc_obj(**in1);
+		if (!in1[i]) {
+			kfree(in0[i]);
+			goto err_free_elements;
+		}
+
+		/* lifted from test_sort.c */
+		r = (r * 725861) % 6599;
+		in0[i]->num = r;
+		in1[i]->num = -r;
+		in0[i]->magic = MAGIC;
+		in1[i]->magic = MAGIC;
+	}
+
+	in0[i] = in1[i] = NULL;
+
+	out = memcat_p(in0, in1);
+	if (!out)
+		goto err_free_all_elements;
+
+	err = -EINVAL;
+	for (i = 0, p = out; *p && (i < INPUT_MAX * 2 - 1); p++, i++) {
+		total += (*p)->num;
+
+		if ((*p)->magic != MAGIC) {
+			pr_err("test failed: wrong magic at %d: %u\n", i,
+			       (*p)->magic);
+			goto err_free_out;
+		}
+	}
+
+	if (total) {
+		pr_err("test failed: expected zero total, got %d\n", total);
+		goto err_free_out;
+	}
+
+	if (i != EXPECT) {
+		pr_err("test failed: expected output size %d, got %d\n",
+		       EXPECT, i);
+		goto err_free_out;
+	}
+
+	for (i = 0; i < INPUT_MAX - 1; i++)
+		if (out[i] != in0[i] || out[i + INPUT_MAX - 1] != in1[i]) {
+			pr_err("test failed: wrong element order at %d\n", i);
+			goto err_free_out;
+		}
+
+	err = 0;
+	pr_info("test passed\n");
+
+err_free_out:
+	kfree(out);
+err_free_all_elements:
+	i = INPUT_MAX;
+err_free_elements:
+	for (i--; i >= 0; i--) {
+		kfree(in1[i]);
+		kfree(in0[i]);
+	}
+
+	kfree(in1);
+err_free_in0:
+	kfree(in0);
+
+	return err;
+}
+
+static void __exit test_memcat_p_exit(void)
+{
+}
+
+module_init(test_memcat_p_init);
+module_exit(test_memcat_p_exit);
+
+MODULE_DESCRIPTION("Test cases for memcat_p() in lib/memcat_p.c");
+MODULE_LICENSE("GPL");