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, 335 insertions, 0 deletions

diff --git a/lib/test_sysctl.c b/lib/test_sysctl.c
new file mode 100644
index 000000000..c02aa9c86
--- /dev/null
+++ b/lib/test_sysctl.c
@@ -0,0 +1,335 @@
+// SPDX-License-Identifier: GPL-2.0-or-later OR copyleft-next-0.3.1
+/*
+ * proc sysctl test driver
+ *
+ * Copyright (C) 2017 Luis R. Rodriguez <mcgrof@kernel.org>
+ */
+
+/*
+ * This module provides an interface to the proc sysctl interfaces.  This
+ * driver requires CONFIG_PROC_SYSCTL. It will not normally be loaded by the
+ * system unless explicitly requested by name. You can also build this driver
+ * into your kernel.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+
+#include <linux/init.h>
+#include <linux/list.h>
+#include <linux/module.h>
+#include <linux/printk.h>
+#include <linux/fs.h>
+#include <linux/miscdevice.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <linux/async.h>
+#include <linux/delay.h>
+#include <linux/vmalloc.h>
+
+static int i_zero;
+static int i_one_hundred = 100;
+static int match_int_ok = 1;
+
+enum {
+	TEST_H_SETUP_NODE,
+	TEST_H_MNT,
+	TEST_H_MNTERROR,
+	TEST_H_EMPTY_ADD,
+	TEST_H_EMPTY,
+	TEST_H_U8,
+	TEST_H_SIZE /* Always at the end */
+};
+
+static struct ctl_table_header *ctl_headers[TEST_H_SIZE] = {};
+struct test_sysctl_data {
+	int int_0001;
+	int int_0002;
+	int int_0003[4];
+
+	int boot_int;
+
+	unsigned int uint_0001;
+
+	char string_0001[65];
+
+#define SYSCTL_TEST_BITMAP_SIZE	65536
+	unsigned long *bitmap_0001;
+};
+
+static struct test_sysctl_data test_data = {
+	.int_0001 = 60,
+	.int_0002 = 1,
+
+	.int_0003[0] = 0,
+	.int_0003[1] = 1,
+	.int_0003[2] = 2,
+	.int_0003[3] = 3,
+
+	.boot_int = 0,
+
+	.uint_0001 = 314,
+
+	.string_0001 = "(none)",
+};
+
+/* These are all under /proc/sys/debug/test_sysctl/ */
+static const struct ctl_table test_table[] = {
+	{
+		.procname	= "int_0001",
+		.data		= &test_data.int_0001,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &i_zero,
+		.extra2         = &i_one_hundred,
+	},
+	{
+		.procname	= "int_0002",
+		.data		= &test_data.int_0002,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
+	{
+		.procname	= "int_0003",
+		.data		= &test_data.int_0003,
+		.maxlen		= sizeof(test_data.int_0003),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+	},
+	{
+		.procname	= "match_int",
+		.data		= &match_int_ok,
+		.maxlen		= sizeof(match_int_ok),
+		.mode		= 0444,
+		.proc_handler	= proc_dointvec,
+	},
+	{
+		.procname	= "boot_int",
+		.data		= &test_data.boot_int,
+		.maxlen		= sizeof(test_data.boot_int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec,
+		.extra1		= SYSCTL_ZERO,
+		.extra2         = SYSCTL_ONE,
+	},
+	{
+		.procname	= "uint_0001",
+		.data		= &test_data.uint_0001,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec,
+	},
+	{
+		.procname	= "string_0001",
+		.data		= &test_data.string_0001,
+		.maxlen		= sizeof(test_data.string_0001),
+		.mode		= 0644,
+		.proc_handler	= proc_dostring,
+	},
+	{
+		.procname	= "bitmap_0001",
+		.data		= &test_data.bitmap_0001,
+		.maxlen		= SYSCTL_TEST_BITMAP_SIZE,
+		.mode		= 0644,
+		.proc_handler	= proc_do_large_bitmap,
+	},
+};
+
+static void test_sysctl_calc_match_int_ok(void)
+{
+	int i;
+
+	struct {
+		int defined;
+		int wanted;
+	} match_int[] = {
+		{.defined = *(int *)SYSCTL_ZERO,	.wanted = 0},
+		{.defined = *(int *)SYSCTL_ONE,		.wanted = 1},
+		{.defined = *(int *)SYSCTL_TWO,		.wanted = 2},
+		{.defined = *(int *)SYSCTL_THREE,	.wanted = 3},
+		{.defined = *(int *)SYSCTL_FOUR,	.wanted = 4},
+		{.defined = *(int *)SYSCTL_ONE_HUNDRED, .wanted = 100},
+		{.defined = *(int *)SYSCTL_TWO_HUNDRED,	.wanted = 200},
+		{.defined = *(int *)SYSCTL_ONE_THOUSAND, .wanted = 1000},
+		{.defined = *(int *)SYSCTL_THREE_THOUSAND, .wanted = 3000},
+		{.defined = *(int *)SYSCTL_INT_MAX,	.wanted = INT_MAX},
+		{.defined = *(int *)SYSCTL_MAXOLDUID,	.wanted = 65535},
+		{.defined = *(int *)SYSCTL_NEG_ONE,	.wanted = -1},
+	};
+
+	for (i = 0; i < ARRAY_SIZE(match_int); i++)
+		if (match_int[i].defined != match_int[i].wanted)
+			match_int_ok = 0;
+}
+
+static int test_sysctl_setup_node_tests(void)
+{
+	test_sysctl_calc_match_int_ok();
+	test_data.bitmap_0001 = kzalloc(SYSCTL_TEST_BITMAP_SIZE/8, GFP_KERNEL);
+	if (!test_data.bitmap_0001)
+		return -ENOMEM;
+	ctl_headers[TEST_H_SETUP_NODE] = register_sysctl("debug/test_sysctl", test_table);
+	if (!ctl_headers[TEST_H_SETUP_NODE]) {
+		kfree(test_data.bitmap_0001);
+		return -ENOMEM;
+	}
+
+	return 0;
+}
+
+/* Used to test that unregister actually removes the directory */
+static const struct ctl_table test_table_unregister[] = {
+	{
+		.procname	= "unregister_error",
+		.data		= &test_data.int_0001,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+	},
+};
+
+static int test_sysctl_run_unregister_nested(void)
+{
+	struct ctl_table_header *unregister;
+
+	unregister = register_sysctl("debug/test_sysctl/unregister_error",
+				   test_table_unregister);
+	if (!unregister)
+		return -ENOMEM;
+
+	unregister_sysctl_table(unregister);
+	return 0;
+}
+
+static int test_sysctl_run_register_mount_point(void)
+{
+	ctl_headers[TEST_H_MNT]
+		= register_sysctl_mount_point("debug/test_sysctl/mnt");
+	if (!ctl_headers[TEST_H_MNT])
+		return -ENOMEM;
+
+	ctl_headers[TEST_H_MNTERROR]
+		= register_sysctl("debug/test_sysctl/mnt/mnt_error",
+				  test_table_unregister);
+	/*
+	 * Don't check the result.:
+	 * If it fails (expected behavior), return 0.
+	 * If successful (missbehavior of register mount point), we want to see
+	 * mnt_error when we run the sysctl test script
+	 */
+
+	return 0;
+}
+
+static const struct ctl_table test_table_empty[] = { };
+
+static int test_sysctl_run_register_empty(void)
+{
+	/* Tets that an empty dir can be created */
+	ctl_headers[TEST_H_EMPTY_ADD]
+		= register_sysctl("debug/test_sysctl/empty_add", test_table_empty);
+	if (!ctl_headers[TEST_H_EMPTY_ADD])
+		return -ENOMEM;
+
+	/* Test that register on top of an empty dir works */
+	ctl_headers[TEST_H_EMPTY]
+		= register_sysctl("debug/test_sysctl/empty_add/empty", test_table_empty);
+	if (!ctl_headers[TEST_H_EMPTY])
+		return -ENOMEM;
+
+	return 0;
+}
+
+static const struct ctl_table table_u8_over[] = {
+	{
+		.procname	= "u8_over",
+		.data		= &test_data.uint_0001,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_FOUR,
+		.extra2		= SYSCTL_ONE_THOUSAND,
+	},
+};
+
+static const struct ctl_table table_u8_under[] = {
+	{
+		.procname	= "u8_under",
+		.data		= &test_data.uint_0001,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_NEG_ONE,
+		.extra2		= SYSCTL_ONE_HUNDRED,
+	},
+};
+
+static const struct ctl_table table_u8_valid[] = {
+	{
+		.procname	= "u8_valid",
+		.data		= &test_data.uint_0001,
+		.maxlen		= sizeof(u8),
+		.mode		= 0644,
+		.proc_handler	= proc_dou8vec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_TWO_HUNDRED,
+	},
+};
+
+static int test_sysctl_register_u8_extra(void)
+{
+	/* should fail because it's over */
+	ctl_headers[TEST_H_U8]
+		= register_sysctl("debug/test_sysctl", table_u8_over);
+	if (ctl_headers[TEST_H_U8])
+		return -ENOMEM;
+
+	/* should fail because it's under */
+	ctl_headers[TEST_H_U8]
+		= register_sysctl("debug/test_sysctl", table_u8_under);
+	if (ctl_headers[TEST_H_U8])
+		return -ENOMEM;
+
+	/* should not fail because it's valid */
+	ctl_headers[TEST_H_U8]
+		= register_sysctl("debug/test_sysctl", table_u8_valid);
+	if (!ctl_headers[TEST_H_U8])
+		return -ENOMEM;
+
+	return 0;
+}
+
+static int __init test_sysctl_init(void)
+{
+	int err = 0;
+
+	int (*func_array[])(void) = {
+		test_sysctl_setup_node_tests,
+		test_sysctl_run_unregister_nested,
+		test_sysctl_run_register_mount_point,
+		test_sysctl_run_register_empty,
+		test_sysctl_register_u8_extra
+	};
+
+	for (int i = 0; !err && i < ARRAY_SIZE(func_array); i++)
+		err = func_array[i]();
+
+	return err;
+}
+module_init(test_sysctl_init);
+
+static void __exit test_sysctl_exit(void)
+{
+	kfree(test_data.bitmap_0001);
+	for (int i = 0; i < TEST_H_SIZE; i++) {
+		if (ctl_headers[i])
+			unregister_sysctl_table(ctl_headers[i]);
+	}
+}
+
+module_exit(test_sysctl_exit);
+
+MODULE_AUTHOR("Luis R. Rodriguez <mcgrof@kernel.org>");
+MODULE_DESCRIPTION("proc sysctl test driver");
+MODULE_LICENSE("GPL");