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, 75 insertions, 0 deletions

diff --git a/drivers/misc/ibmasm/lowlevel.c b/drivers/misc/ibmasm/lowlevel.c
new file mode 100644
index 000000000..5313230f3
--- /dev/null
+++ b/drivers/misc/ibmasm/lowlevel.c
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * IBM ASM Service Processor Device Driver
+ *
+ * Copyright (C) IBM Corporation, 2004
+ *
+ * Author: Max Asböck <amax@us.ibm.com>
+ */
+
+#include "ibmasm.h"
+#include "lowlevel.h"
+#include "i2o.h"
+#include "dot_command.h"
+#include "remote.h"
+
+static struct i2o_header header = I2O_HEADER_TEMPLATE;
+
+
+int ibmasm_send_i2o_message(struct service_processor *sp)
+{
+	u32 mfa;
+	size_t command_size;
+	struct i2o_message *message;
+	struct command *command = sp->current_command;
+
+	command_size = get_dot_command_size(command->buffer);
+	if (command_size > command->buffer_size)
+		return 1;
+	if (command_size > I2O_COMMAND_SIZE)
+		command_size = I2O_COMMAND_SIZE;
+
+	mfa = get_mfa_inbound(sp->base_address);
+	if (!mfa)
+		return 1;
+
+	header.message_size = outgoing_message_size((unsigned int)command_size);
+	message = get_i2o_message(sp->base_address, mfa);
+
+	memcpy_toio(&message->header, &header, sizeof(struct i2o_header));
+	memcpy_toio(&message->data, command->buffer, command_size);
+
+	set_mfa_inbound(sp->base_address, mfa);
+
+	return 0;
+}
+
+irqreturn_t ibmasm_interrupt_handler(int irq, void * dev_id)
+{
+	u32	mfa;
+	struct service_processor *sp = (struct service_processor *)dev_id;
+	void __iomem *base_address = sp->base_address;
+	char tsbuf[32];
+
+	if (!sp_interrupt_pending(base_address))
+		return IRQ_NONE;
+
+	dbg("respond to interrupt at %s\n", get_timestamp(tsbuf));
+
+	if (mouse_interrupt_pending(sp)) {
+		ibmasm_handle_mouse_interrupt(sp);
+		clear_mouse_interrupt(sp);
+	}
+
+	mfa = get_mfa_outbound(base_address);
+	if (valid_mfa(mfa)) {
+		struct i2o_message *msg = get_i2o_message(base_address, mfa);
+		ibmasm_receive_message(sp, &msg->data, incoming_data_size(msg));
+	} else
+		dbg("didn't get a valid MFA\n");
+
+	set_mfa_outbound(base_address, mfa);
+	dbg("finished interrupt at   %s\n", get_timestamp(tsbuf));
+
+	return IRQ_HANDLED;
+}