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, 164 insertions, 0 deletions

diff --git a/tools/perf/tests/keep-tracking.c b/tools/perf/tests/keep-tracking.c
new file mode 100644
index 000000000..729cc9cc1
--- /dev/null
+++ b/tools/perf/tests/keep-tracking.c
@@ -0,0 +1,164 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/types.h>
+#include <limits.h>
+#include <unistd.h>
+#include <sys/prctl.h>
+#include <perf/cpumap.h>
+#include <perf/evlist.h>
+#include <perf/mmap.h>
+
+#include "debug.h"
+#include "parse-events.h"
+#include "evlist.h"
+#include "evsel.h"
+#include "record.h"
+#include "thread_map.h"
+#include "tests.h"
+#include "util/mmap.h"
+
+#define CHECK__(x) {				\
+	while ((x) < 0) {			\
+		pr_debug(#x " failed!\n");	\
+		goto out_err;			\
+	}					\
+}
+
+#define CHECK_NOT_NULL__(x) {			\
+	while ((x) == NULL) {			\
+		pr_debug(#x " failed!\n");	\
+		goto out_err;			\
+	}					\
+}
+
+static int find_comm(struct evlist *evlist, const char *comm)
+{
+	union perf_event *event;
+	struct mmap *md;
+	int i, found;
+
+	found = 0;
+	for (i = 0; i < evlist->core.nr_mmaps; i++) {
+		md = &evlist->mmap[i];
+		if (perf_mmap__read_init(&md->core) < 0)
+			continue;
+		while ((event = perf_mmap__read_event(&md->core)) != NULL) {
+			if (event->header.type == PERF_RECORD_COMM &&
+			    (pid_t)event->comm.pid == getpid() &&
+			    (pid_t)event->comm.tid == getpid() &&
+			    strcmp(event->comm.comm, comm) == 0)
+				found += 1;
+			perf_mmap__consume(&md->core);
+		}
+		perf_mmap__read_done(&md->core);
+	}
+	return found;
+}
+
+/**
+ * test__keep_tracking - test using a dummy software event to keep tracking.
+ *
+ * This function implements a test that checks that tracking events continue
+ * when an event is disabled but a dummy software event is not disabled.  If the
+ * test passes %0 is returned, otherwise %-1 is returned.
+ */
+static int test__keep_tracking(struct test_suite *test __maybe_unused, int subtest __maybe_unused)
+{
+	struct record_opts opts = {
+		.mmap_pages	     = UINT_MAX,
+		.user_freq	     = UINT_MAX,
+		.user_interval	     = ULLONG_MAX,
+		.target		     = {
+			.uses_mmap   = true,
+		},
+	};
+	struct perf_thread_map *threads = NULL;
+	struct perf_cpu_map *cpus = NULL;
+	struct evlist *evlist = NULL;
+	struct evsel *evsel = NULL;
+	int found, err = -1;
+	const char *comm;
+
+	threads = thread_map__new_by_tid(getpid());
+	CHECK_NOT_NULL__(threads);
+
+	cpus = perf_cpu_map__new_online_cpus();
+	CHECK_NOT_NULL__(cpus);
+
+	evlist = evlist__new();
+	CHECK_NOT_NULL__(evlist);
+
+	perf_evlist__set_maps(&evlist->core, cpus, threads);
+
+	CHECK__(parse_event(evlist, "dummy:u"));
+	CHECK__(parse_event(evlist, "cpu-cycles:u"));
+
+	evlist__config(evlist, &opts, NULL);
+
+	evsel = evlist__first(evlist);
+
+	evsel->core.attr.comm = 1;
+	evsel->core.attr.disabled = 1;
+	evsel->core.attr.enable_on_exec = 0;
+
+	if (evlist__open(evlist) < 0) {
+		pr_debug("Unable to open dummy and cycles event\n");
+		err = TEST_SKIP;
+		goto out_err;
+	}
+
+	CHECK__(evlist__mmap(evlist, UINT_MAX));
+
+	/*
+	 * First, test that a 'comm' event can be found when the event is
+	 * enabled.
+	 */
+
+	evlist__enable(evlist);
+
+	comm = "Test COMM 1";
+	CHECK__(prctl(PR_SET_NAME, (unsigned long)comm, 0, 0, 0));
+
+	evlist__disable(evlist);
+
+	found = find_comm(evlist, comm);
+	if (found != 1) {
+		pr_debug("First time, failed to find tracking event.\n");
+		goto out_err;
+	}
+
+	/*
+	 * Secondly, test that a 'comm' event can be found when the event is
+	 * disabled with the dummy event still enabled.
+	 */
+
+	evlist__enable(evlist);
+
+	evsel = evlist__last(evlist);
+
+	CHECK__(evsel__disable(evsel));
+
+	comm = "Test COMM 2";
+	CHECK__(prctl(PR_SET_NAME, (unsigned long)comm, 0, 0, 0));
+
+	evlist__disable(evlist);
+
+	found = find_comm(evlist, comm);
+	if (found != 1) {
+		pr_debug("Second time, failed to find tracking event.\n");
+		goto out_err;
+	}
+
+	err = 0;
+
+out_err:
+	if (evlist) {
+		evlist__disable(evlist);
+		evlist__delete(evlist);
+	}
+	perf_cpu_map__put(cpus);
+	perf_thread_map__put(threads);
+
+	return err;
+}
+
+DEFINE_SUITE("Use a dummy software event to keep tracking", keep_tracking);