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, 158 insertions, 0 deletions

diff --git a/samples/bpf/ibumad_user.c b/samples/bpf/ibumad_user.c
new file mode 100644
index 000000000..d074c978a
--- /dev/null
+++ b/samples/bpf/ibumad_user.c
@@ -0,0 +1,158 @@
+// SPDX-License-Identifier: GPL-2.0 OR Linux-OpenIB
+
+/*
+ * ibumad BPF sample user side
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ *
+ * Copyright(c) 2018 Ira Weiny, Intel Corporation
+ */
+
+#include <linux/bpf.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <limits.h>
+
+#include <getopt.h>
+#include <net/if.h>
+
+#include <bpf/bpf.h>
+#include "bpf_util.h"
+#include <bpf/libbpf.h>
+
+static struct bpf_link *tp_links[3];
+static struct bpf_object *obj;
+static int map_fd[2];
+static int tp_cnt;
+
+static void dump_counts(int fd)
+{
+	__u32 key;
+	__u64 value;
+
+	for (key = 0; key < 256; key++) {
+		if (bpf_map_lookup_elem(fd, &key, &value)) {
+			printf("failed to read key %u\n", key);
+			continue;
+		}
+		if (value)
+			printf("0x%02x : %llu\n", key, value);
+	}
+}
+
+static void dump_all_counts(void)
+{
+	printf("Read 'Class : count'\n");
+	dump_counts(map_fd[0]);
+	printf("Write 'Class : count'\n");
+	dump_counts(map_fd[1]);
+}
+
+static void dump_exit(int sig)
+{
+	dump_all_counts();
+	/* Detach tracepoints */
+	while (tp_cnt)
+		bpf_link__destroy(tp_links[--tp_cnt]);
+
+	bpf_object__close(obj);
+	exit(0);
+}
+
+static const struct option long_options[] = {
+	{"help",      no_argument,       NULL, 'h'},
+	{"delay",     required_argument, NULL, 'd'},
+};
+
+static void usage(char *cmd)
+{
+	printf("eBPF test program to count packets from various IP addresses\n"
+		"Usage: %s <options>\n"
+		"       --help,   -h  this menu\n"
+		"       --delay,  -d  <delay>  wait <delay> sec between prints [1 - 1000000]\n"
+		, cmd
+		);
+}
+
+int main(int argc, char **argv)
+{
+	struct bpf_program *prog;
+	unsigned long delay = 5;
+	char filename[256];
+	int longindex = 0;
+	int opt, err = -1;
+
+	while ((opt = getopt_long(argc, argv, "hd:rSw",
+				  long_options, &longindex)) != -1) {
+		switch (opt) {
+		case 'd':
+			delay = strtoul(optarg, NULL, 0);
+			if (delay == ULONG_MAX || delay < 0 ||
+			    delay > 1000000) {
+				fprintf(stderr, "ERROR: invalid delay : %s\n",
+					optarg);
+				usage(argv[0]);
+				return 1;
+			}
+			break;
+		default:
+		case 'h':
+			usage(argv[0]);
+			return 1;
+		}
+	}
+
+	/* Do one final dump when exiting */
+	signal(SIGINT, dump_exit);
+	signal(SIGTERM, dump_exit);
+
+	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+	obj = bpf_object__open_file(filename, NULL);
+	if (libbpf_get_error(obj)) {
+		fprintf(stderr, "ERROR: opening BPF object file failed\n");
+		return err;
+	}
+
+	/* load BPF program */
+	if (bpf_object__load(obj)) {
+		fprintf(stderr, "ERROR: loading BPF object file failed\n");
+		goto cleanup;
+	}
+
+	map_fd[0] = bpf_object__find_map_fd_by_name(obj, "read_count");
+	map_fd[1] = bpf_object__find_map_fd_by_name(obj, "write_count");
+	if (map_fd[0] < 0 || map_fd[1] < 0) {
+		fprintf(stderr, "ERROR: finding a map in obj file failed\n");
+		goto cleanup;
+	}
+
+	bpf_object__for_each_program(prog, obj) {
+		tp_links[tp_cnt] = bpf_program__attach(prog);
+		if (libbpf_get_error(tp_links[tp_cnt])) {
+			fprintf(stderr, "ERROR: bpf_program__attach failed\n");
+			tp_links[tp_cnt] = NULL;
+			goto cleanup;
+		}
+		tp_cnt++;
+	}
+
+	while (1) {
+		sleep(delay);
+		dump_all_counts();
+	}
+	err = 0;
+
+cleanup:
+	/* Detach tracepoints */
+	while (tp_cnt)
+		bpf_link__destroy(tp_links[--tp_cnt]);
+
+	bpf_object__close(obj);
+	return err;
+}