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, 163 insertions, 0 deletions

diff --git a/tools/perf/tests/openat-syscall-tp-fields.c b/tools/perf/tests/openat-syscall-tp-fields.c
new file mode 100644
index 000000000..9ff8caff9
--- /dev/null
+++ b/tools/perf/tests/openat-syscall-tp-fields.c
@@ -0,0 +1,163 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdbool.h>
+#include <linux/err.h>
+#include <linux/string.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include "evlist.h"
+#include "evsel.h"
+#include "thread_map.h"
+#include "record.h"
+#include "tests.h"
+#include "debug.h"
+#include "util/mmap.h"
+#include <errno.h>
+#include <perf/mmap.h>
+#include "util/sample.h"
+
+#ifndef O_DIRECTORY
+#define O_DIRECTORY    00200000
+#endif
+#ifndef AT_FDCWD
+#define AT_FDCWD       -100
+#endif
+
+static int test__syscall_openat_tp_fields(struct test_suite *test __maybe_unused,
+					  int subtest __maybe_unused)
+{
+	struct record_opts opts = {
+		.target = {
+			.uses_mmap = true,
+		},
+		.no_buffering = true,
+		.freq	      = 1,
+		.mmap_pages   = 256,
+		.raw_samples  = true,
+	};
+	const char *filename = "/etc/passwd";
+	int flags = O_RDONLY | O_DIRECTORY;
+	struct evlist *evlist = evlist__new();
+	struct evsel *evsel;
+	int ret = TEST_FAIL, err, i, nr_events = 0, nr_polls = 0;
+	char sbuf[STRERR_BUFSIZE];
+
+	if (evlist == NULL) {
+		pr_debug("%s: evlist__new\n", __func__);
+		goto out;
+	}
+
+	evsel = evsel__newtp("syscalls", "sys_enter_openat");
+	if (IS_ERR(evsel)) {
+		pr_debug("%s: evsel__newtp\n", __func__);
+		ret = PTR_ERR(evsel) == -EACCES ? TEST_SKIP : TEST_FAIL;
+		goto out_delete_evlist;
+	}
+
+	evlist__add(evlist, evsel);
+
+	err = evlist__create_maps(evlist, &opts.target);
+	if (err < 0) {
+		pr_debug("%s: evlist__create_maps\n", __func__);
+		goto out_delete_evlist;
+	}
+
+	evsel__config(evsel, &opts, NULL);
+
+	perf_thread_map__set_pid(evlist->core.threads, 0, getpid());
+
+	err = evlist__open(evlist);
+	if (err < 0) {
+		pr_debug("perf_evlist__open: %s\n",
+			 str_error_r(errno, sbuf, sizeof(sbuf)));
+		goto out_delete_evlist;
+	}
+
+	err = evlist__mmap(evlist, UINT_MAX);
+	if (err < 0) {
+		pr_debug("evlist__mmap: %s\n",
+			 str_error_r(errno, sbuf, sizeof(sbuf)));
+		goto out_delete_evlist;
+	}
+
+	evlist__enable(evlist);
+
+	/*
+	 * Generate the event:
+	 */
+	openat(AT_FDCWD, filename, flags);
+
+	while (1) {
+		int before = nr_events;
+
+		for (i = 0; i < evlist->core.nr_mmaps; i++) {
+			union perf_event *event;
+			struct mmap *md;
+
+			md = &evlist->mmap[i];
+			if (perf_mmap__read_init(&md->core) < 0)
+				continue;
+
+			while ((event = perf_mmap__read_event(&md->core)) != NULL) {
+				const u32 type = event->header.type;
+				int tp_flags;
+				struct perf_sample sample;
+
+				++nr_events;
+
+				if (type != PERF_RECORD_SAMPLE) {
+					perf_mmap__consume(&md->core);
+					continue;
+				}
+
+				perf_sample__init(&sample, /*all=*/false);
+				err = evsel__parse_sample(evsel, event, &sample);
+				if (err) {
+					pr_debug("Can't parse sample, err = %d\n", err);
+					perf_sample__exit(&sample);
+					goto out_delete_evlist;
+				}
+
+				tp_flags = perf_sample__intval(&sample, "flags");
+				perf_sample__exit(&sample);
+				/* C library wrapper may set additional flags,
+				   access mode must be unchanged */
+				if ((tp_flags & O_ACCMODE) != (flags & O_ACCMODE) ||
+				    (tp_flags & flags) != flags) {
+					pr_debug("%s: Expected flags=%#x, got %#x\n",
+						 __func__, flags, tp_flags);
+					goto out_delete_evlist;
+				}
+
+				goto out_ok;
+			}
+			perf_mmap__read_done(&md->core);
+		}
+
+		if (nr_events == before)
+			evlist__poll(evlist, 10);
+
+		if (++nr_polls > 5) {
+			pr_debug("%s: no events!\n", __func__);
+			goto out_delete_evlist;
+		}
+	}
+out_ok:
+	ret = TEST_OK;
+out_delete_evlist:
+	evlist__delete(evlist);
+out:
+	return ret;
+}
+
+static struct test_case tests__syscall_openat_tp_fields[] = {
+	TEST_CASE_REASON("syscalls:sys_enter_openat event fields",
+			 syscall_openat_tp_fields,
+			 "permissions"),
+	{	.name = NULL, }
+};
+
+struct test_suite suite__syscall_openat_tp_fields = {
+	.desc = "syscalls:sys_enter_openat event fields",
+	.test_cases = tests__syscall_openat_tp_fields,
+};