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>

3 files changed, 217 insertions, 0 deletions

diff --git a/tools/testing/selftests/fchmodat2/.gitignore b/tools/testing/selftests/fchmodat2/.gitignore
new file mode 100644
index 000000000..82a4846cb
--- /dev/null
+++ b/tools/testing/selftests/fchmodat2/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/*_test
diff --git a/tools/testing/selftests/fchmodat2/Makefile b/tools/testing/selftests/fchmodat2/Makefile
new file mode 100644
index 000000000..4373cea79
--- /dev/null
+++ b/tools/testing/selftests/fchmodat2/Makefile
@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+CFLAGS += -Wall -O2 -g -fsanitize=address -fsanitize=undefined $(KHDR_INCLUDES)
+
+# gcc requires -static-libasan in order to ensure that Address Sanitizer's
+# library is the first one loaded. However, clang already statically links the
+# Address Sanitizer if -fsanitize is specified. Therefore, simply omit
+# -static-libasan for clang builds.
+ifeq ($(LLVM),)
+    CFLAGS += -static-libasan
+endif
+
+TEST_GEN_PROGS := fchmodat2_test
+
+include ../lib.mk
diff --git a/tools/testing/selftests/fchmodat2/fchmodat2_test.c b/tools/testing/selftests/fchmodat2/fchmodat2_test.c
new file mode 100644
index 000000000..8e1b6384b
--- /dev/null
+++ b/tools/testing/selftests/fchmodat2/fchmodat2_test.c
@@ -0,0 +1,200 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <syscall.h>
+#include <unistd.h>
+
+#include "kselftest.h"
+
+struct testdir {
+	char *dirname;
+	int dfd;
+};
+
+int sys_fchmodat2(int dfd, const char *filename, mode_t mode, int flags)
+{
+	int ret = syscall(__NR_fchmodat2, dfd, filename, mode, flags);
+
+	return ret >= 0 ? ret : -errno;
+}
+
+static void setup_testdir(struct testdir *testdir)
+{
+	int ret, dfd;
+	char dirname[] = "/tmp/ksft-fchmodat2.XXXXXX";
+
+	/* Make the top-level directory. */
+	if (!mkdtemp(dirname))
+		ksft_exit_fail_msg("%s: failed to create tmpdir\n", __func__);
+
+	dfd = open(dirname, O_PATH | O_DIRECTORY);
+	if (dfd < 0) {
+		ksft_perror("failed to open tmpdir");
+		goto err;
+	}
+
+	ret = openat(dfd, "regfile", O_CREAT | O_WRONLY | O_TRUNC, 0644);
+	if (ret < 0) {
+		ksft_perror("failed to create file in tmpdir");
+		goto err;
+	}
+	close(ret);
+
+	ret = symlinkat("regfile", dfd, "symlink");
+	if (ret < 0) {
+		ksft_perror("symlinkat() failed");
+		goto err_regfile;
+	}
+
+	testdir->dirname = strdup(dirname);
+	if (!testdir->dirname) {
+		ksft_perror("Out of memory");
+		goto err_symlink;
+	}
+	testdir->dfd = dfd;
+
+	return;
+
+err_symlink:
+	unlinkat(testdir->dfd, "symlink", 0);
+err_regfile:
+	unlinkat(testdir->dfd, "regfile", 0);
+err:
+	unlink(dirname);
+	ksft_exit_fail();
+}
+
+static void cleanup_testdir(struct testdir *testdir)
+{
+	unlinkat(testdir->dfd, "regfile", 0);
+	unlinkat(testdir->dfd, "symlink", 0);
+	rmdir(testdir->dirname);
+	free(testdir->dirname);
+}
+
+int expect_mode(int dfd, const char *filename, mode_t expect_mode)
+{
+	struct stat st;
+	int ret = fstatat(dfd, filename, &st, AT_SYMLINK_NOFOLLOW);
+
+	if (ret) {
+		ksft_perror("fstatat() failed\n");
+		return 0;
+	}
+
+	return (st.st_mode == expect_mode);
+}
+
+void test_regfile(void)
+{
+	struct testdir testdir;
+	int ret;
+
+	setup_testdir(&testdir);
+
+	ret = sys_fchmodat2(testdir.dfd, "regfile", 0640, 0);
+
+	if (ret < 0) {
+		ksft_perror("fchmodat2(noflag) failed");
+		goto out;
+	}
+
+	if (!expect_mode(testdir.dfd, "regfile", 0100640)) {
+		ksft_print_msg("%s: wrong file mode bits after fchmodat2\n",
+				__func__);
+		ret = 1;
+		goto out;
+	}
+
+	ret = sys_fchmodat2(testdir.dfd, "regfile", 0600, AT_SYMLINK_NOFOLLOW);
+
+	if (ret < 0) {
+		ksft_perror("fchmodat2(AT_SYMLINK_NOFOLLOW) failed");
+		goto out;
+	}
+
+	if (!expect_mode(testdir.dfd, "regfile", 0100600)) {
+		ksft_print_msg("%s: wrong file mode bits after fchmodat2 with nofollow\n",
+			       __func__);
+		ret = 1;
+	}
+
+out:
+	ksft_test_result(ret == 0, "fchmodat2(regfile)\n");
+	cleanup_testdir(&testdir);
+}
+
+void test_symlink(void)
+{
+	struct testdir testdir;
+	int ret;
+
+	setup_testdir(&testdir);
+
+	ret = sys_fchmodat2(testdir.dfd, "symlink", 0640, 0);
+
+	if (ret < 0) {
+		ksft_perror("fchmodat2(noflag) failed");
+		goto err;
+	}
+
+	if (!expect_mode(testdir.dfd, "regfile", 0100640)) {
+		ksft_print_msg("%s: wrong file mode bits after fchmodat2\n",
+			       __func__);
+		goto err;
+	}
+
+	if (!expect_mode(testdir.dfd, "symlink", 0120777)) {
+		ksft_print_msg("%s: wrong symlink mode bits after fchmodat2\n",
+			       __func__);
+		goto err;
+	}
+
+	ret = sys_fchmodat2(testdir.dfd, "symlink", 0600, AT_SYMLINK_NOFOLLOW);
+
+	/*
+	 * On certain filesystems (xfs or btrfs), chmod operation fails. So we
+	 * first check the symlink target but if the operation fails we mark the
+	 * test as skipped.
+	 *
+	 * https://sourceware.org/legacy-ml/libc-alpha/2020-02/msg00467.html
+	 */
+	if (ret == 0 && !expect_mode(testdir.dfd, "symlink", 0120600)) {
+		ksft_print_msg("%s: wrong symlink mode bits after fchmodat2 with nofollow\n",
+				__func__);
+		ret = 1;
+		goto err;
+	}
+
+	if (!expect_mode(testdir.dfd, "regfile", 0100640)) {
+		ksft_print_msg("%s: wrong file mode bits after fchmodat2 with nofollow\n",
+			       __func__);
+	}
+
+	if (ret != 0)
+		ksft_test_result_skip("fchmodat2(symlink)\n");
+	else
+		ksft_test_result_pass("fchmodat2(symlink)\n");
+	cleanup_testdir(&testdir);
+	return;
+
+err:
+	ksft_test_result_fail("fchmodat2(symlink)\n");
+	cleanup_testdir(&testdir);
+}
+
+#define NUM_TESTS 2
+
+int main(int argc, char **argv)
+{
+	ksft_print_header();
+	ksft_set_plan(NUM_TESTS);
+
+	test_regfile();
+	test_symlink();
+
+	ksft_finished();
+}