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, 93 insertions, 0 deletions

diff --git a/samples/bpf/tracex5.bpf.c b/samples/bpf/tracex5.bpf.c
new file mode 100644
index 000000000..4d3d6c9b2
--- /dev/null
+++ b/samples/bpf/tracex5.bpf.c
@@ -0,0 +1,93 @@
+/* Copyright (c) 2015 PLUMgrid, http://plumgrid.com
+ *
+ * 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.
+ */
+#include "vmlinux.h"
+#include "syscall_nrs.h"
+#include <linux/version.h>
+#include <uapi/linux/unistd.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_core_read.h>
+
+#define __stringify(x) #x
+#define PROG(F) SEC("kprobe/"__stringify(F)) int bpf_func_##F
+
+struct {
+	__uint(type, BPF_MAP_TYPE_PROG_ARRAY);
+	__uint(key_size, sizeof(u32));
+	__uint(value_size, sizeof(u32));
+#ifdef __mips__
+	__uint(max_entries, 6000); /* MIPS n64 syscalls start at 5000 */
+#else
+	__uint(max_entries, 1024);
+#endif
+} progs SEC(".maps");
+
+SEC("kprobe/__seccomp_filter")
+int bpf_prog1(struct pt_regs *ctx)
+{
+	int sc_nr = (int)PT_REGS_PARM1(ctx);
+
+	/* dispatch into next BPF program depending on syscall number */
+	bpf_tail_call(ctx, &progs, sc_nr);
+
+	/* fall through -> unknown syscall */
+	if (sc_nr >= __NR_getuid && sc_nr <= __NR_getsid) {
+		char fmt[] = "syscall=%d (one of get/set uid/pid/gid)\n";
+		bpf_trace_printk(fmt, sizeof(fmt), sc_nr);
+	}
+	return 0;
+}
+
+/* we jump here when syscall number == __NR_write */
+PROG(SYS__NR_write)(struct pt_regs *ctx)
+{
+	struct seccomp_data sd;
+
+	bpf_core_read(&sd, sizeof(sd), (void *)PT_REGS_PARM2(ctx));
+	if (sd.args[2] == 512) {
+		char fmt[] = "write(fd=%d, buf=%p, size=%d)\n";
+		bpf_trace_printk(fmt, sizeof(fmt),
+				 sd.args[0], sd.args[1], sd.args[2]);
+	}
+	return 0;
+}
+
+PROG(SYS__NR_read)(struct pt_regs *ctx)
+{
+	struct seccomp_data sd;
+
+	bpf_core_read(&sd, sizeof(sd), (void *)PT_REGS_PARM2(ctx));
+	if (sd.args[2] > 128 && sd.args[2] <= 1024) {
+		char fmt[] = "read(fd=%d, buf=%p, size=%d)\n";
+		bpf_trace_printk(fmt, sizeof(fmt),
+				 sd.args[0], sd.args[1], sd.args[2]);
+	}
+	return 0;
+}
+
+#ifdef __NR_mmap2
+PROG(SYS__NR_mmap2)(struct pt_regs *ctx)
+{
+	char fmt[] = "mmap2\n";
+
+	bpf_trace_printk(fmt, sizeof(fmt));
+	return 0;
+}
+#endif
+
+#ifdef __NR_mmap
+PROG(SYS__NR_mmap)(struct pt_regs *ctx)
+{
+	char fmt[] = "mmap\n";
+
+	bpf_trace_printk(fmt, sizeof(fmt));
+	return 0;
+}
+#endif
+
+char _license[] SEC("license") = "GPL";
+u32 _version SEC("version") = LINUX_VERSION_CODE;