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, 262 insertions, 0 deletions

diff --git a/tools/perf/bench/breakpoint.c b/tools/perf/bench/breakpoint.c
new file mode 100644
index 000000000..1b7cd4481
--- /dev/null
+++ b/tools/perf/bench/breakpoint.c
@@ -0,0 +1,262 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <subcmd/parse-options.h>
+#include <linux/hw_breakpoint.h>
+#include <linux/perf_event.h>
+#include <linux/time64.h>
+#include <sys/syscall.h>
+#include <sys/ioctl.h>
+#include <sys/time.h>
+#include <pthread.h>
+#include <stddef.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <errno.h>
+#include "bench.h"
+#include "futex.h"
+
+static struct {
+	unsigned int nbreakpoints;
+	unsigned int nparallel;
+	unsigned int nthreads;
+} thread_params = {
+	.nbreakpoints = 1,
+	.nparallel = 1,
+	.nthreads = 1,
+};
+
+static const struct option thread_options[] = {
+	OPT_UINTEGER('b', "breakpoints", &thread_params.nbreakpoints,
+		"Specify amount of breakpoints"),
+	OPT_UINTEGER('p', "parallelism", &thread_params.nparallel, "Specify amount of parallelism"),
+	OPT_UINTEGER('t', "threads", &thread_params.nthreads, "Specify amount of threads"),
+	OPT_END()
+};
+
+static const char * const thread_usage[] = {
+	"perf bench breakpoint thread <options>",
+	NULL
+};
+
+struct breakpoint {
+	int fd;
+	char watched;
+};
+
+static int breakpoint_setup(void *addr)
+{
+	struct perf_event_attr attr = { .size = 0, };
+	int fd;
+
+	attr.type = PERF_TYPE_BREAKPOINT;
+	attr.size = sizeof(attr);
+	attr.inherit = 1;
+	attr.exclude_kernel = 1;
+	attr.exclude_hv = 1;
+	attr.bp_addr = (unsigned long)addr;
+	attr.bp_type = HW_BREAKPOINT_RW;
+	attr.bp_len = HW_BREAKPOINT_LEN_1;
+	fd = syscall(SYS_perf_event_open, &attr, 0, -1, -1, 0);
+
+	if (fd < 0)
+		fd = -errno;
+
+	return fd;
+}
+
+static void *passive_thread(void *arg)
+{
+	unsigned int *done = (unsigned int *)arg;
+
+	while (!__atomic_load_n(done, __ATOMIC_RELAXED))
+		futex_wait(done, 0, NULL, 0);
+	return NULL;
+}
+
+static void *active_thread(void *arg)
+{
+	unsigned int *done = (unsigned int *)arg;
+
+	while (!__atomic_load_n(done, __ATOMIC_RELAXED));
+	return NULL;
+}
+
+static void *breakpoint_thread(void *arg)
+{
+	unsigned int i, done;
+	int *repeat = (int *)arg;
+	pthread_t *threads;
+
+	threads = calloc(thread_params.nthreads, sizeof(threads[0]));
+	if (!threads)
+		exit((perror("calloc"), EXIT_FAILURE));
+
+	while (__atomic_fetch_sub(repeat, 1, __ATOMIC_RELAXED) > 0) {
+		done = 0;
+		for (i = 0; i < thread_params.nthreads; i++) {
+			if (pthread_create(&threads[i], NULL, passive_thread, &done))
+				exit((perror("pthread_create"), EXIT_FAILURE));
+		}
+		__atomic_store_n(&done, 1, __ATOMIC_RELAXED);
+		futex_wake(&done, thread_params.nthreads, 0);
+		for (i = 0; i < thread_params.nthreads; i++)
+			pthread_join(threads[i], NULL);
+	}
+	free(threads);
+	return NULL;
+}
+
+// The benchmark creates nbreakpoints inheritable breakpoints,
+// then starts nparallel threads which create and join bench_repeat batches of nthreads threads.
+int bench_breakpoint_thread(int argc, const char **argv)
+{
+	unsigned int i, result_usec;
+	int repeat = bench_repeat;
+	struct breakpoint *breakpoints;
+	pthread_t *parallel;
+	struct timeval start, stop, diff;
+
+	if (parse_options(argc, argv, thread_options, thread_usage, 0)) {
+		usage_with_options(thread_usage, thread_options);
+		exit(EXIT_FAILURE);
+	}
+	breakpoints = calloc(thread_params.nbreakpoints, sizeof(breakpoints[0]));
+	parallel = calloc(thread_params.nparallel, sizeof(parallel[0]));
+	if (!breakpoints || !parallel)
+		exit((perror("calloc"), EXIT_FAILURE));
+
+	for (i = 0; i < thread_params.nbreakpoints; i++) {
+		breakpoints[i].fd = breakpoint_setup(&breakpoints[i].watched);
+
+		if (breakpoints[i].fd < 0) {
+			if (breakpoints[i].fd == -ENODEV) {
+				printf("Skipping perf bench breakpoint thread: No hardware support\n");
+				return 0;
+			}
+			exit((perror("perf_event_open"), EXIT_FAILURE));
+		}
+	}
+	gettimeofday(&start, NULL);
+	for (i = 0; i < thread_params.nparallel; i++) {
+		if (pthread_create(&parallel[i], NULL, breakpoint_thread, &repeat))
+			exit((perror("pthread_create"), EXIT_FAILURE));
+	}
+	for (i = 0; i < thread_params.nparallel; i++)
+		pthread_join(parallel[i], NULL);
+	gettimeofday(&stop, NULL);
+	timersub(&stop, &start, &diff);
+	for (i = 0; i < thread_params.nbreakpoints; i++)
+		close(breakpoints[i].fd);
+	free(parallel);
+	free(breakpoints);
+	switch (bench_format) {
+	case BENCH_FORMAT_DEFAULT:
+		printf("# Created/joined %d threads with %d breakpoints and %d parallelism\n",
+			bench_repeat, thread_params.nbreakpoints, thread_params.nparallel);
+		printf(" %14s: %lu.%03lu [sec]\n\n", "Total time",
+			(long)diff.tv_sec, (long)(diff.tv_usec / USEC_PER_MSEC));
+		result_usec = diff.tv_sec * USEC_PER_SEC + diff.tv_usec;
+		printf(" %14lf usecs/op\n",
+			(double)result_usec / bench_repeat / thread_params.nthreads);
+		printf(" %14lf usecs/op/cpu\n",
+			(double)result_usec / bench_repeat /
+			thread_params.nthreads * thread_params.nparallel);
+		break;
+	case BENCH_FORMAT_SIMPLE:
+		printf("%lu.%03lu\n", (long)diff.tv_sec, (long)(diff.tv_usec / USEC_PER_MSEC));
+		break;
+	default:
+		fprintf(stderr, "Unknown format: %d\n", bench_format);
+		exit(EXIT_FAILURE);
+	}
+	return 0;
+}
+
+static struct {
+	unsigned int npassive;
+	unsigned int nactive;
+} enable_params = {
+	.nactive = 0,
+	.npassive = 0,
+};
+
+static const struct option enable_options[] = {
+	OPT_UINTEGER('p', "passive", &enable_params.npassive, "Specify amount of passive threads"),
+	OPT_UINTEGER('a', "active", &enable_params.nactive, "Specify amount of active threads"),
+	OPT_END()
+};
+
+static const char * const enable_usage[] = {
+	"perf bench breakpoint enable <options>",
+	NULL
+};
+
+// The benchmark creates an inheritable breakpoint,
+// then starts npassive threads that block and nactive threads that actively spin
+// and then disables and enables the breakpoint bench_repeat times.
+int bench_breakpoint_enable(int argc, const char **argv)
+{
+	unsigned int i, nthreads, result_usec, done = 0;
+	char watched;
+	int fd;
+	pthread_t *threads;
+	struct timeval start, stop, diff;
+
+	if (parse_options(argc, argv, enable_options, enable_usage, 0)) {
+		usage_with_options(enable_usage, enable_options);
+		exit(EXIT_FAILURE);
+	}
+	fd = breakpoint_setup(&watched);
+
+	if (fd < 0) {
+		if (fd == -ENODEV) {
+			printf("Skipping perf bench breakpoint enable: No hardware support\n");
+			return 0;
+		}
+		exit((perror("perf_event_open"), EXIT_FAILURE));
+	}
+	nthreads = enable_params.npassive + enable_params.nactive;
+	threads = calloc(nthreads, sizeof(threads[0]));
+	if (!threads)
+		exit((perror("calloc"), EXIT_FAILURE));
+
+	for (i = 0; i < nthreads; i++) {
+		if (pthread_create(&threads[i], NULL,
+			i < enable_params.npassive ? passive_thread : active_thread, &done))
+			exit((perror("pthread_create"), EXIT_FAILURE));
+	}
+	usleep(10000);  // let the threads block
+	gettimeofday(&start, NULL);
+	for (i = 0; i < bench_repeat; i++) {
+		if (ioctl(fd, PERF_EVENT_IOC_DISABLE, 0))
+			exit((perror("ioctl(PERF_EVENT_IOC_DISABLE)"), EXIT_FAILURE));
+		if (ioctl(fd, PERF_EVENT_IOC_ENABLE, 0))
+			exit((perror("ioctl(PERF_EVENT_IOC_ENABLE)"), EXIT_FAILURE));
+	}
+	gettimeofday(&stop, NULL);
+	timersub(&stop, &start, &diff);
+	__atomic_store_n(&done, 1, __ATOMIC_RELAXED);
+	futex_wake(&done, enable_params.npassive, 0);
+	for (i = 0; i < nthreads; i++)
+		pthread_join(threads[i], NULL);
+	free(threads);
+	close(fd);
+	switch (bench_format) {
+	case BENCH_FORMAT_DEFAULT:
+		printf("# Enabled/disabled breakpoint %d time with %d passive and %d active threads\n",
+			bench_repeat, enable_params.npassive, enable_params.nactive);
+		printf(" %14s: %lu.%03lu [sec]\n\n", "Total time",
+			(long)diff.tv_sec, (long)(diff.tv_usec / USEC_PER_MSEC));
+		result_usec = diff.tv_sec * USEC_PER_SEC + diff.tv_usec;
+		printf(" %14lf usecs/op\n", (double)result_usec / bench_repeat);
+		break;
+	case BENCH_FORMAT_SIMPLE:
+		printf("%lu.%03lu\n", (long)diff.tv_sec, (long)(diff.tv_usec / USEC_PER_MSEC));
+		break;
+	default:
+		fprintf(stderr, "Unknown format: %d\n", bench_format);
+		exit(EXIT_FAILURE);
+	}
+	return 0;
+}