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, 136 insertions, 0 deletions

diff --git a/include/trace/events/ipi.h b/include/trace/events/ipi.h
new file mode 100644
index 000000000..fae4f8eac
--- /dev/null
+++ b/include/trace/events/ipi.h
@@ -0,0 +1,136 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM ipi
+
+#if !defined(_TRACE_IPI_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_IPI_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(ipi_send_cpu,
+
+	TP_PROTO(const unsigned int cpu, unsigned long callsite, void *callback),
+
+	TP_ARGS(cpu, callsite, callback),
+
+	TP_STRUCT__entry(
+		__field(unsigned int, cpu)
+		__field(void *, callsite)
+		__field(void *, callback)
+	),
+
+	TP_fast_assign(
+		__entry->cpu = cpu;
+		__entry->callsite = (void *)callsite;
+		__entry->callback = callback;
+	),
+
+	TP_printk("cpu=%u callsite=%pS callback=%pS",
+		  __entry->cpu, __entry->callsite, __entry->callback)
+);
+
+TRACE_EVENT(ipi_send_cpumask,
+
+	TP_PROTO(const struct cpumask *cpumask, unsigned long callsite, void *callback),
+
+	TP_ARGS(cpumask, callsite, callback),
+
+	TP_STRUCT__entry(
+		__cpumask(cpumask)
+		__field(void *, callsite)
+		__field(void *, callback)
+	),
+
+	TP_fast_assign(
+		__assign_cpumask(cpumask, cpumask_bits(cpumask));
+		__entry->callsite = (void *)callsite;
+		__entry->callback = callback;
+	),
+
+	TP_printk("cpumask=%s callsite=%pS callback=%pS",
+		  __get_cpumask(cpumask), __entry->callsite, __entry->callback)
+);
+
+#ifdef CONFIG_HAVE_EXTRA_IPI_TRACEPOINTS
+/**
+ * ipi_raise - called when a smp cross call is made
+ *
+ * @mask: mask of recipient CPUs for the IPI
+ * @reason: string identifying the IPI purpose
+ *
+ * It is necessary for @reason to be a static string declared with
+ * __tracepoint_string.
+ */
+TRACE_EVENT(ipi_raise,
+
+	TP_PROTO(const struct cpumask *mask, const char *reason),
+
+	TP_ARGS(mask, reason),
+
+	TP_STRUCT__entry(
+		__cpumask(target_cpus)
+		__field(const char *, reason)
+	),
+
+	TP_fast_assign(
+		__assign_cpumask(target_cpus, cpumask_bits(mask));
+		__entry->reason = reason;
+	),
+
+	TP_printk("target_mask=%s (%s)", __get_cpumask(target_cpus), __entry->reason)
+);
+
+DECLARE_EVENT_CLASS(ipi_handler,
+
+	TP_PROTO(const char *reason),
+
+	TP_ARGS(reason),
+
+	TP_STRUCT__entry(
+		__field(const char *, reason)
+	),
+
+	TP_fast_assign(
+		__entry->reason = reason;
+	),
+
+	TP_printk("(%s)", __entry->reason)
+);
+
+/**
+ * ipi_entry - called immediately before the IPI handler
+ *
+ * @reason: string identifying the IPI purpose
+ *
+ * It is necessary for @reason to be a static string declared with
+ * __tracepoint_string, ideally the same as used with trace_ipi_raise
+ * for that IPI.
+ */
+DEFINE_EVENT(ipi_handler, ipi_entry,
+
+	TP_PROTO(const char *reason),
+
+	TP_ARGS(reason)
+);
+
+/**
+ * ipi_exit - called immediately after the IPI handler returns
+ *
+ * @reason: string identifying the IPI purpose
+ *
+ * It is necessary for @reason to be a static string declared with
+ * __tracepoint_string, ideally the same as used with trace_ipi_raise for
+ * that IPI.
+ */
+DEFINE_EVENT(ipi_handler, ipi_exit,
+
+	TP_PROTO(const char *reason),
+
+	TP_ARGS(reason)
+);
+#endif /* CONFIG_HAVE_EXTRA_IPI_TRACEPOINTS */
+
+#endif /* _TRACE_IPI_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>