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, 236 insertions, 0 deletions

diff --git a/samples/connector/ucon.c b/samples/connector/ucon.c
new file mode 100644
index 000000000..fa17f8642
--- /dev/null
+++ b/samples/connector/ucon.c
@@ -0,0 +1,236 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * 	ucon.c
+ *
+ * Copyright (c) 2004+ Evgeniy Polyakov <zbr@ioremap.net>
+ */
+
+#include <asm/types.h>
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/poll.h>
+
+#include <linux/netlink.h>
+#include <linux/rtnetlink.h>
+
+#include <arpa/inet.h>
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+#include <errno.h>
+#include <time.h>
+#include <getopt.h>
+
+#include <linux/connector.h>
+
+#define DEBUG
+#define NETLINK_CONNECTOR 	11
+
+/* Hopefully your userspace connector.h matches this kernel */
+#define CN_TEST_IDX		CN_NETLINK_USERS + 3
+#define CN_TEST_VAL		0x456
+
+#ifdef DEBUG
+#define ulog(f, a...) fprintf(stdout, f, ##a)
+#else
+#define ulog(f, a...) do {} while (0)
+#endif
+
+static int need_exit;
+static __u32 seq;
+
+static int netlink_send(int s, struct cn_msg *msg)
+{
+	struct nlmsghdr *nlh;
+	unsigned int size;
+	int err;
+	char buf[128];
+	struct cn_msg *m;
+
+	size = NLMSG_SPACE(sizeof(struct cn_msg) + msg->len);
+
+	nlh = (struct nlmsghdr *)buf;
+	nlh->nlmsg_seq = seq++;
+	nlh->nlmsg_pid = getpid();
+	nlh->nlmsg_type = NLMSG_DONE;
+	nlh->nlmsg_len = size;
+	nlh->nlmsg_flags = 0;
+
+	m = NLMSG_DATA(nlh);
+#if 0
+	ulog("%s: [%08x.%08x] len=%u, seq=%u, ack=%u.\n",
+	       __func__, msg->id.idx, msg->id.val, msg->len, msg->seq, msg->ack);
+#endif
+	memcpy(m, msg, sizeof(*m) + msg->len);
+
+	err = send(s, nlh, size, 0);
+	if (err == -1)
+		ulog("Failed to send: %s [%d].\n",
+			strerror(errno), errno);
+
+	return err;
+}
+
+static void usage(void)
+{
+	printf(
+		"Usage: ucon [options] [output file]\n"
+		"\n"
+		"\t-h\tthis help screen\n"
+		"\t-s\tsend buffers to the test module\n"
+		"\n"
+		"The default behavior of ucon is to subscribe to the test module\n"
+		"and wait for state messages.  Any ones received are dumped to the\n"
+		"specified output file (or stdout).  The test module is assumed to\n"
+		"have an id of {%u.%u}\n"
+		"\n"
+		"If you get no output, then verify the cn_test module id matches\n"
+		"the expected id above.\n"
+		, CN_TEST_IDX, CN_TEST_VAL
+	);
+}
+
+int main(int argc, char *argv[])
+{
+	int s;
+	char buf[1024];
+	int len;
+	struct nlmsghdr *reply;
+	struct sockaddr_nl l_local;
+	struct cn_msg *data;
+	FILE *out;
+	time_t tm;
+	struct pollfd pfd;
+	bool send_msgs = false;
+
+	while ((s = getopt(argc, argv, "hs")) != -1) {
+		switch (s) {
+		case 's':
+			send_msgs = true;
+			break;
+
+		case 'h':
+			usage();
+			return 0;
+
+		default:
+			/* getopt() outputs an error for us */
+			usage();
+			return 1;
+		}
+	}
+
+	if (argc != optind) {
+		out = fopen(argv[optind], "a+");
+		if (!out) {
+			ulog("Unable to open %s for writing: %s\n",
+				argv[1], strerror(errno));
+			out = stdout;
+		}
+	} else
+		out = stdout;
+
+	memset(buf, 0, sizeof(buf));
+
+	s = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_CONNECTOR);
+	if (s == -1) {
+		perror("socket");
+		return -1;
+	}
+
+	l_local.nl_family = AF_NETLINK;
+	l_local.nl_groups = -1; /* bitmask of requested groups */
+	l_local.nl_pid = 0;
+
+	ulog("subscribing to %u.%u\n", CN_TEST_IDX, CN_TEST_VAL);
+
+	if (bind(s, (struct sockaddr *)&l_local, sizeof(struct sockaddr_nl)) == -1) {
+		perror("bind");
+		close(s);
+		return -1;
+	}
+
+#if 0
+	{
+		int on = 0x57; /* Additional group number */
+		setsockopt(s, SOL_NETLINK, NETLINK_ADD_MEMBERSHIP, &on, sizeof(on));
+	}
+#endif
+	if (send_msgs) {
+		int i, j;
+
+		memset(buf, 0, sizeof(buf));
+
+		data = (struct cn_msg *)buf;
+
+		data->id.idx = CN_TEST_IDX;
+		data->id.val = CN_TEST_VAL;
+		data->seq = seq++;
+		data->ack = 0;
+		data->len = 0;
+
+		for (j=0; j<10; ++j) {
+			for (i=0; i<1000; ++i) {
+				len = netlink_send(s, data);
+			}
+
+			ulog("%d messages have been sent to %08x.%08x.\n", i, data->id.idx, data->id.val);
+		}
+
+		return 0;
+	}
+
+
+	pfd.fd = s;
+
+	while (!need_exit) {
+		pfd.events = POLLIN;
+		pfd.revents = 0;
+		switch (poll(&pfd, 1, -1)) {
+			case 0:
+				need_exit = 1;
+				break;
+			case -1:
+				if (errno != EINTR) {
+					need_exit = 1;
+					break;
+				}
+				continue;
+		}
+		if (need_exit)
+			break;
+
+		memset(buf, 0, sizeof(buf));
+		len = recv(s, buf, sizeof(buf), 0);
+		if (len == -1) {
+			perror("recv buf");
+			close(s);
+			return -1;
+		}
+		reply = (struct nlmsghdr *)buf;
+
+		switch (reply->nlmsg_type) {
+		case NLMSG_ERROR:
+			fprintf(out, "Error message received.\n");
+			fflush(out);
+			break;
+		case NLMSG_DONE:
+			data = (struct cn_msg *)NLMSG_DATA(reply);
+
+			time(&tm);
+			fprintf(out, "%.24s : [%x.%x] [%08u.%08u].\n",
+				ctime(&tm), data->id.idx, data->id.val, data->seq, data->ack);
+			fflush(out);
+			break;
+		default:
+			break;
+		}
+	}
+
+	close(s);
+	return 0;
+}