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, 138 insertions, 0 deletions

diff --git a/samples/hid/hid_mouse.c b/samples/hid/hid_mouse.c
new file mode 100644
index 000000000..4b80d4e4c
--- /dev/null
+++ b/samples/hid/hid_mouse.c
@@ -0,0 +1,138 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Copyright (c) 2022 Benjamin Tissoires
+ *
+ * This is a pure HID-BPF example, and should be considered as such:
+ * on the Etekcity Scroll 6E, the X and Y axes will be swapped and
+ * inverted. On any other device... Not sure what this will do.
+ *
+ * This C main file is generic though. To adapt the code and test, users
+ * must amend only the .bpf.c file, which this program will load any
+ * eBPF program it finds.
+ */
+
+#include <assert.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <libgen.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/resource.h>
+#include <unistd.h>
+
+#include <linux/bpf.h>
+#include <linux/errno.h>
+
+#include <bpf/bpf.h>
+#include <bpf/libbpf.h>
+
+#include "hid_mouse.skel.h"
+
+static bool running = true;
+
+static void int_exit(int sig)
+{
+	running = false;
+	exit(0);
+}
+
+static void usage(const char *prog)
+{
+	fprintf(stderr,
+		"%s: %s /sys/bus/hid/devices/0BUS:0VID:0PID:00ID\n\n",
+		__func__, prog);
+	fprintf(stderr,
+		"This program will upload and attach a HID-BPF program to the given device.\n"
+		"On the Etekcity Scroll 6E, the X and Y axis will be inverted, but on any other\n"
+		"device, chances are high that the device will not be working anymore\n\n"
+		"consider this as a demo and adapt the eBPF program to your needs\n"
+		"Hit Ctrl-C to unbind the program and reset the device\n");
+}
+
+static int get_hid_id(const char *path)
+{
+	const char *str_id, *dir;
+	char uevent[1024];
+	int fd;
+
+	memset(uevent, 0, sizeof(uevent));
+	snprintf(uevent, sizeof(uevent) - 1, "%s/uevent", path);
+
+	fd = open(uevent, O_RDONLY | O_NONBLOCK);
+	if (fd < 0)
+		return -ENOENT;
+
+	close(fd);
+
+	dir = basename((char *)path);
+
+	str_id = dir + sizeof("0003:0001:0A37.");
+	return (int)strtol(str_id, NULL, 16);
+}
+
+int main(int argc, char **argv)
+{
+	struct hid_mouse *skel;
+	struct bpf_link *link;
+	int err;
+	const char *optstr = "";
+	const char *sysfs_path;
+	int opt, hid_id;
+
+	while ((opt = getopt(argc, argv, optstr)) != -1) {
+		switch (opt) {
+		default:
+			usage(basename(argv[0]));
+			return 1;
+		}
+	}
+
+	if (optind == argc) {
+		usage(basename(argv[0]));
+		return 1;
+	}
+
+	sysfs_path = argv[optind];
+	if (!sysfs_path) {
+		perror("sysfs");
+		return 1;
+	}
+
+	skel = hid_mouse__open();
+	if (!skel) {
+		fprintf(stderr, "%s  %s:%d", __func__, __FILE__, __LINE__);
+		return -1;
+	}
+
+	hid_id = get_hid_id(sysfs_path);
+
+	if (hid_id < 0) {
+		fprintf(stderr, "can not open HID device: %m\n");
+		return 1;
+	}
+	skel->struct_ops.mouse_invert->hid_id = hid_id;
+
+	err = hid_mouse__load(skel);
+	if (err < 0) {
+		fprintf(stderr, "can not load HID-BPF program: %m\n");
+		return 1;
+	}
+
+	link = bpf_map__attach_struct_ops(skel->maps.mouse_invert);
+	if (!link) {
+		fprintf(stderr, "can not attach HID-BPF program: %m\n");
+		return 1;
+	}
+
+	signal(SIGINT, int_exit);
+	signal(SIGTERM, int_exit);
+
+	while (running)
+		sleep(1);
+
+	hid_mouse__destroy(skel);
+
+	return 0;
+}