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, 179 insertions, 0 deletions

diff --git a/drivers/char/hangcheck-timer.c b/drivers/char/hangcheck-timer.c
new file mode 100644
index 000000000..231cbf7b3
--- /dev/null
+++ b/drivers/char/hangcheck-timer.c
@@ -0,0 +1,179 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * hangcheck-timer.c
+ *
+ * Driver for a little io fencing timer.
+ *
+ * Copyright (C) 2002, 2003 Oracle.  All rights reserved.
+ *
+ * Author: Joel Becker <joel.becker@oracle.com>
+ */
+
+/*
+ * The hangcheck-timer driver uses the TSC to catch delays that
+ * jiffies does not notice.  A timer is set.  When the timer fires, it
+ * checks whether it was delayed and if that delay exceeds a given
+ * margin of error.  The hangcheck_tick module parameter takes the timer
+ * duration in seconds.  The hangcheck_margin parameter defines the
+ * margin of error, in seconds.  The defaults are 60 seconds for the
+ * timer and 180 seconds for the margin of error.  IOW, a timer is set
+ * for 60 seconds.  When the timer fires, the callback checks the
+ * actual duration that the timer waited.  If the duration exceeds the
+ * allotted time and margin (here 60 + 180, or 240 seconds), the machine
+ * is restarted.  A healthy machine will have the duration match the
+ * expected timeout very closely.
+ */
+
+#include <linux/module.h>
+#include <linux/moduleparam.h>
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/reboot.h>
+#include <linux/init.h>
+#include <linux/delay.h>
+#include <linux/uaccess.h>
+#include <linux/sysrq.h>
+#include <linux/timer.h>
+#include <linux/hrtimer.h>
+
+#define VERSION_STR "0.9.1"
+
+#define DEFAULT_IOFENCE_MARGIN 60	/* Default fudge factor, in seconds */
+#define DEFAULT_IOFENCE_TICK 180	/* Default timer timeout, in seconds */
+
+static int hangcheck_tick = DEFAULT_IOFENCE_TICK;
+static int hangcheck_margin = DEFAULT_IOFENCE_MARGIN;
+static int hangcheck_reboot;  /* Defaults to not reboot */
+static int hangcheck_dump_tasks;  /* Defaults to not dumping SysRQ T */
+
+/* options - modular */
+module_param(hangcheck_tick, int, 0);
+MODULE_PARM_DESC(hangcheck_tick, "Timer delay.");
+module_param(hangcheck_margin, int, 0);
+MODULE_PARM_DESC(hangcheck_margin, "If the hangcheck timer has been delayed more than hangcheck_margin seconds, the driver will fire.");
+module_param(hangcheck_reboot, int, 0);
+MODULE_PARM_DESC(hangcheck_reboot, "If nonzero, the machine will reboot when the timer margin is exceeded.");
+module_param(hangcheck_dump_tasks, int, 0);
+MODULE_PARM_DESC(hangcheck_dump_tasks, "If nonzero, the machine will dump the system task state when the timer margin is exceeded.");
+
+MODULE_AUTHOR("Oracle");
+MODULE_DESCRIPTION("Hangcheck-timer detects when the system has gone out to lunch past a certain margin.");
+MODULE_LICENSE("GPL");
+MODULE_VERSION(VERSION_STR);
+
+/* options - nonmodular */
+#ifndef MODULE
+
+static int __init hangcheck_parse_tick(char *str)
+{
+	int par;
+
+	if (get_option(&str, &par))
+		hangcheck_tick = par;
+	return 1;
+}
+
+static int __init hangcheck_parse_margin(char *str)
+{
+	int par;
+
+	if (get_option(&str, &par))
+		hangcheck_margin = par;
+	return 1;
+}
+
+static int __init hangcheck_parse_reboot(char *str)
+{
+	int par;
+
+	if (get_option(&str, &par))
+		hangcheck_reboot = par;
+	return 1;
+}
+
+static int __init hangcheck_parse_dump_tasks(char *str)
+{
+	int par;
+
+	if (get_option(&str, &par))
+		hangcheck_dump_tasks = par;
+	return 1;
+}
+
+__setup("hcheck_tick", hangcheck_parse_tick);
+__setup("hcheck_margin", hangcheck_parse_margin);
+__setup("hcheck_reboot", hangcheck_parse_reboot);
+__setup("hcheck_dump_tasks", hangcheck_parse_dump_tasks);
+#endif /* not MODULE */
+
+#define TIMER_FREQ 1000000000ULL
+
+/* Last time scheduled */
+static unsigned long long hangcheck_tsc, hangcheck_tsc_margin;
+
+static void hangcheck_fire(struct timer_list *);
+
+static DEFINE_TIMER(hangcheck_ticktock, hangcheck_fire);
+
+static void hangcheck_fire(struct timer_list *unused)
+{
+	unsigned long long cur_tsc, tsc_diff;
+
+	cur_tsc = ktime_get_ns();
+
+	if (cur_tsc > hangcheck_tsc)
+		tsc_diff = cur_tsc - hangcheck_tsc;
+	else
+		tsc_diff = (cur_tsc + (~0ULL - hangcheck_tsc)); /* or something */
+
+	if (tsc_diff > hangcheck_tsc_margin) {
+		if (hangcheck_dump_tasks) {
+			pr_crit("Hangcheck: Task state:\n");
+#ifdef CONFIG_MAGIC_SYSRQ
+			handle_sysrq('t');
+#endif  /* CONFIG_MAGIC_SYSRQ */
+		}
+		if (hangcheck_reboot) {
+			pr_crit("Hangcheck: hangcheck is restarting the machine.\n");
+			emergency_restart();
+		} else {
+			pr_crit("Hangcheck: hangcheck value past margin!\n");
+		}
+	}
+#if 0
+	/*
+	 * Enable to investigate delays in detail
+	 */
+	pr_debug("Hangcheck: called %lld ns since last time (%lld ns overshoot)\n",
+			tsc_diff, tsc_diff - hangcheck_tick*TIMER_FREQ);
+#endif
+	mod_timer(&hangcheck_ticktock, jiffies + (hangcheck_tick*HZ));
+	hangcheck_tsc = ktime_get_ns();
+}
+
+
+static int __init hangcheck_init(void)
+{
+	pr_debug("Hangcheck: starting hangcheck timer %s (tick is %d seconds, margin is %d seconds).\n",
+	       VERSION_STR, hangcheck_tick, hangcheck_margin);
+	hangcheck_tsc_margin =
+		(unsigned long long)hangcheck_margin + hangcheck_tick;
+	hangcheck_tsc_margin *= TIMER_FREQ;
+
+	hangcheck_tsc = ktime_get_ns();
+	mod_timer(&hangcheck_ticktock, jiffies + (hangcheck_tick*HZ));
+
+	return 0;
+}
+
+
+static void __exit hangcheck_exit(void)
+{
+	timer_delete_sync(&hangcheck_ticktock);
+	pr_debug("Hangcheck: Stopped hangcheck timer.\n");
+}
+
+module_init(hangcheck_init);
+module_exit(hangcheck_exit);