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>

4 files changed, 253 insertions, 0 deletions

diff --git a/tools/testing/selftests/ir/.gitignore b/tools/testing/selftests/ir/.gitignore
new file mode 100644
index 000000000..0bbada8c1
--- /dev/null
+++ b/tools/testing/selftests/ir/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+ir_loopback
diff --git a/tools/testing/selftests/ir/Makefile b/tools/testing/selftests/ir/Makefile
new file mode 100644
index 000000000..ad06489c2
--- /dev/null
+++ b/tools/testing/selftests/ir/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_PROGS := ir_loopback.sh
+TEST_GEN_PROGS_EXTENDED := ir_loopback
+APIDIR := ../../../include/uapi
+CFLAGS += -Wall -O2 -I$(APIDIR)
+
+include ../lib.mk
diff --git a/tools/testing/selftests/ir/ir_loopback.c b/tools/testing/selftests/ir/ir_loopback.c
new file mode 100644
index 000000000..adfcf50b1
--- /dev/null
+++ b/tools/testing/selftests/ir/ir_loopback.c
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: GPL-2.0
+// test ir decoder
+//
+// Copyright (C) 2018 Sean Young <sean@mess.org>
+
+// When sending LIRC_MODE_SCANCODE, the IR will be encoded. rc-loopback
+// will send this IR to the receiver side, where we try to read the decoded
+// IR. Decoding happens in a separate kernel thread, so we will need to
+// wait until that is scheduled, hence we use poll to check for read
+// readiness.
+
+#include <linux/lirc.h>
+#include <errno.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdbool.h>
+#include <string.h>
+#include <unistd.h>
+#include <poll.h>
+#include <time.h>
+#include <sys/types.h>
+#include <sys/ioctl.h>
+#include <dirent.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include "kselftest.h"
+
+#define TEST_SCANCODES	10
+#define SYSFS_PATH_MAX 256
+#define DNAME_PATH_MAX 256
+
+/*
+ * Support ancient lirc.h which does not have these values. Can be removed
+ * once RHEL 8 is no longer a relevant testing platform.
+ */
+#if RC_PROTO_MAX < 26
+#define RC_PROTO_RCMM12 24
+#define RC_PROTO_RCMM24 25
+#define RC_PROTO_RCMM32 26
+#endif
+
+static const struct {
+	enum rc_proto proto;
+	const char *name;
+	unsigned int mask;
+	const char *decoder;
+} protocols[] = {
+	{ RC_PROTO_RC5, "rc-5", 0x1f7f, "rc-5" },
+	{ RC_PROTO_RC5X_20, "rc-5x-20", 0x1f7f3f, "rc-5" },
+	{ RC_PROTO_RC5_SZ, "rc-5-sz", 0x2fff, "rc-5-sz" },
+	{ RC_PROTO_JVC, "jvc", 0xffff, "jvc" },
+	{ RC_PROTO_SONY12, "sony-12", 0x1f007f, "sony" },
+	{ RC_PROTO_SONY15, "sony-15", 0xff007f, "sony" },
+	{ RC_PROTO_SONY20, "sony-20", 0x1fff7f, "sony" },
+	{ RC_PROTO_NEC, "nec", 0xffff, "nec" },
+	{ RC_PROTO_NECX, "nec-x", 0xffffff, "nec" },
+	{ RC_PROTO_NEC32, "nec-32", 0xffffffff, "nec" },
+	{ RC_PROTO_SANYO, "sanyo", 0x1fffff, "sanyo" },
+	{ RC_PROTO_RC6_0, "rc-6-0", 0xffff, "rc-6" },
+	{ RC_PROTO_RC6_6A_20, "rc-6-6a-20", 0xfffff, "rc-6" },
+	{ RC_PROTO_RC6_6A_24, "rc-6-6a-24", 0xffffff, "rc-6" },
+	{ RC_PROTO_RC6_6A_32, "rc-6-6a-32", 0xffffffff, "rc-6" },
+	{ RC_PROTO_RC6_MCE, "rc-6-mce", 0x00007fff, "rc-6" },
+	{ RC_PROTO_SHARP, "sharp", 0x1fff, "sharp" },
+	{ RC_PROTO_IMON, "imon", 0x7fffffff, "imon" },
+	{ RC_PROTO_RCMM12, "rcmm-12", 0x00000fff, "rc-mm" },
+	{ RC_PROTO_RCMM24, "rcmm-24", 0x00ffffff, "rc-mm" },
+	{ RC_PROTO_RCMM32, "rcmm-32", 0xffffffff, "rc-mm" },
+};
+
+int lirc_open(const char *rc)
+{
+	struct dirent *dent;
+	char buf[SYSFS_PATH_MAX + DNAME_PATH_MAX];
+	DIR *d;
+	int fd;
+
+	snprintf(buf, sizeof(buf), "/sys/class/rc/%s", rc);
+
+	d = opendir(buf);
+	if (!d)
+		ksft_exit_fail_msg("cannot open %s: %m\n", buf);
+
+	while ((dent = readdir(d)) != NULL) {
+		if (!strncmp(dent->d_name, "lirc", 4)) {
+			snprintf(buf, sizeof(buf), "/dev/%s", dent->d_name);
+			break;
+		}
+	}
+
+	if (!dent)
+		ksft_exit_skip("cannot find lirc device for %s\n", rc);
+
+	closedir(d);
+
+	fd = open(buf, O_RDWR | O_NONBLOCK);
+	if (fd == -1)
+		ksft_exit_fail_msg("cannot open: %s: %m\n", buf);
+
+	return fd;
+}
+
+int main(int argc, char **argv)
+{
+	unsigned int mode;
+	char buf[100];
+	int rlircfd, wlircfd, protocolfd, i, n;
+
+	srand(time(NULL));
+
+	if (argc != 3)
+		ksft_exit_fail_msg("Usage: %s <write rcN> <read rcN>\n",
+				   argv[0]);
+
+	rlircfd = lirc_open(argv[2]);
+	mode = LIRC_MODE_SCANCODE;
+	if (ioctl(rlircfd, LIRC_SET_REC_MODE, &mode))
+		ksft_exit_fail_msg("failed to set scancode rec mode %s: %m\n",
+				   argv[2]);
+
+	wlircfd = lirc_open(argv[1]);
+	if (ioctl(wlircfd, LIRC_SET_SEND_MODE, &mode))
+		ksft_exit_fail_msg("failed to set scancode send mode %s: %m\n",
+				   argv[1]);
+
+	snprintf(buf, sizeof(buf), "/sys/class/rc/%s/protocols", argv[2]);
+	protocolfd = open(buf, O_WRONLY);
+	if (protocolfd == -1)
+		ksft_exit_fail_msg("failed to open %s: %m\n", buf);
+
+	printf("Sending IR on %s and receiving IR on %s.\n", argv[1], argv[2]);
+
+	for (i = 0; i < ARRAY_SIZE(protocols); i++) {
+		if (write(protocolfd, protocols[i].decoder,
+			  strlen(protocols[i].decoder)) == -1)
+			ksft_exit_fail_msg("failed to set write decoder\n");
+
+		printf("Testing protocol %s for decoder %s (%d/%d)...\n",
+		       protocols[i].name, protocols[i].decoder,
+		       i + 1, (int)ARRAY_SIZE(protocols));
+
+		for (n = 0; n < TEST_SCANCODES; n++) {
+			unsigned int scancode = rand() & protocols[i].mask;
+			unsigned int rc_proto = protocols[i].proto;
+
+			if (rc_proto == RC_PROTO_RC6_MCE)
+				scancode |= 0x800f0000;
+
+			if (rc_proto == RC_PROTO_NECX &&
+			    (((scancode >> 16) ^ ~(scancode >> 8)) & 0xff) == 0)
+				continue;
+
+			if (rc_proto == RC_PROTO_NEC32 &&
+			    (((scancode >> 8) ^ ~scancode) & 0xff) == 0)
+				continue;
+
+			if (rc_proto == RC_PROTO_RCMM32 &&
+			    (scancode & 0x000c0000) != 0x000c0000 &&
+			    scancode & 0x00008000)
+				continue;
+
+			struct lirc_scancode lsc = {
+				.rc_proto = rc_proto,
+				.scancode = scancode
+			};
+
+			printf("Testing scancode:%x\n", scancode);
+
+			while (write(wlircfd, &lsc, sizeof(lsc)) < 0) {
+				if (errno == EINTR)
+					continue;
+
+				ksft_exit_fail_msg("failed to send ir: %m\n");
+			}
+
+			struct pollfd pfd = { .fd = rlircfd, .events = POLLIN };
+			struct lirc_scancode lsc2;
+
+			poll(&pfd, 1, 1000);
+
+			bool decoded = true;
+
+			while (read(rlircfd, &lsc2, sizeof(lsc2)) < 0) {
+				if (errno == EINTR)
+					continue;
+
+				ksft_test_result_error("no scancode decoded: %m\n");
+				decoded = false;
+				break;
+			}
+
+			if (!decoded)
+				continue;
+
+			if (lsc.rc_proto != lsc2.rc_proto)
+				ksft_test_result_error("decoded protocol is different: %d\n",
+						       lsc2.rc_proto);
+
+			else if (lsc.scancode != lsc2.scancode)
+				ksft_test_result_error("decoded scancode is different: %llx\n",
+						       lsc2.scancode);
+			else
+				ksft_inc_pass_cnt();
+		}
+
+		printf("OK\n");
+	}
+
+	close(rlircfd);
+	close(wlircfd);
+	close(protocolfd);
+
+	if (ksft_get_fail_cnt() > 0)
+		ksft_exit_fail();
+	else
+		ksft_exit_pass();
+
+	return 0;
+}
diff --git a/tools/testing/selftests/ir/ir_loopback.sh b/tools/testing/selftests/ir/ir_loopback.sh
new file mode 100755
index 000000000..aff9299c9
--- /dev/null
+++ b/tools/testing/selftests/ir/ir_loopback.sh
@@ -0,0 +1,25 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+
+# Kselftest framework requirement - SKIP code is 4.
+ksft_skip=4
+
+if [ $UID != 0 ]; then
+	echo "Please run ir_loopback test as root [SKIP]"
+	exit $ksft_skip
+fi
+
+if ! /sbin/modprobe -q -n rc-loopback; then
+        echo "ir_loopback: module rc-loopback is not found in /lib/modules/`uname -r` [SKIP]"
+        exit $ksft_skip
+fi
+
+/sbin/modprobe rc-loopback
+if [ $? -ne 0 ]; then
+	exit
+fi
+
+RCDEV=$(grep -l DRV_NAME=rc-loopback /sys/class/rc/rc*/uevent | grep -o 'rc[0-9]\+')
+
+./ir_loopback $RCDEV $RCDEV
+exit