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, 173 insertions, 0 deletions

diff --git a/drivers/misc/ibmasm/command.c b/drivers/misc/ibmasm/command.c
new file mode 100644
index 000000000..6b037675f
--- /dev/null
+++ b/drivers/misc/ibmasm/command.c
@@ -0,0 +1,173 @@
+// 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 <linux/sched.h>
+#include <linux/slab.h>
+#include "ibmasm.h"
+#include "lowlevel.h"
+
+static void exec_next_command(struct service_processor *sp);
+
+static atomic_t command_count = ATOMIC_INIT(0);
+
+struct command *ibmasm_new_command(struct service_processor *sp, size_t buffer_size)
+{
+	struct command *cmd;
+
+	if (buffer_size > IBMASM_CMD_MAX_BUFFER_SIZE)
+		return NULL;
+
+	cmd = kzalloc_obj(struct command);
+	if (cmd == NULL)
+		return NULL;
+
+
+	cmd->buffer = kzalloc(buffer_size, GFP_KERNEL);
+	if (cmd->buffer == NULL) {
+		kfree(cmd);
+		return NULL;
+	}
+	cmd->buffer_size = buffer_size;
+
+	kref_init(&cmd->kref);
+	cmd->lock = &sp->lock;
+
+	cmd->status = IBMASM_CMD_PENDING;
+	init_waitqueue_head(&cmd->wait);
+	INIT_LIST_HEAD(&cmd->queue_node);
+
+	atomic_inc(&command_count);
+	dbg("command count: %d\n", atomic_read(&command_count));
+
+	return cmd;
+}
+
+void ibmasm_free_command(struct kref *kref)
+{
+	struct command *cmd = to_command(kref);
+
+	list_del(&cmd->queue_node);
+	atomic_dec(&command_count);
+	dbg("command count: %d\n", atomic_read(&command_count));
+	kfree(cmd->buffer);
+	kfree(cmd);
+}
+
+static void enqueue_command(struct service_processor *sp, struct command *cmd)
+{
+	list_add_tail(&cmd->queue_node, &sp->command_queue);
+}
+
+static struct command *dequeue_command(struct service_processor *sp)
+{
+	struct command *cmd;
+	struct list_head *next;
+
+	if (list_empty(&sp->command_queue))
+		return NULL;
+
+	next = sp->command_queue.next;
+	list_del_init(next);
+	cmd = list_entry(next, struct command, queue_node);
+
+	return cmd;
+}
+
+static inline void do_exec_command(struct service_processor *sp)
+{
+	char tsbuf[32];
+
+	dbg("%s:%d at %s\n", __func__, __LINE__, get_timestamp(tsbuf));
+
+	if (ibmasm_send_i2o_message(sp)) {
+		sp->current_command->status = IBMASM_CMD_FAILED;
+		wake_up(&sp->current_command->wait);
+		command_put(sp->current_command);
+		exec_next_command(sp);
+	}
+}
+
+/*
+ * exec_command
+ * send a command to a service processor
+ * Commands are executed sequentially. One command (sp->current_command)
+ * is sent to the service processor. Once the interrupt handler gets a
+ * message of type command_response, the message is copied into
+ * the current commands buffer,
+ */
+void ibmasm_exec_command(struct service_processor *sp, struct command *cmd)
+{
+	unsigned long flags;
+	char tsbuf[32];
+
+	dbg("%s:%d at %s\n", __func__, __LINE__, get_timestamp(tsbuf));
+
+	spin_lock_irqsave(&sp->lock, flags);
+
+	if (!sp->current_command) {
+		sp->current_command = cmd;
+		command_get(sp->current_command);
+		spin_unlock_irqrestore(&sp->lock, flags);
+		do_exec_command(sp);
+	} else {
+		enqueue_command(sp, cmd);
+		spin_unlock_irqrestore(&sp->lock, flags);
+	}
+}
+
+static void exec_next_command(struct service_processor *sp)
+{
+	unsigned long flags;
+	char tsbuf[32];
+
+	dbg("%s:%d at %s\n", __func__, __LINE__, get_timestamp(tsbuf));
+
+	spin_lock_irqsave(&sp->lock, flags);
+	sp->current_command = dequeue_command(sp);
+	if (sp->current_command) {
+		command_get(sp->current_command);
+		spin_unlock_irqrestore(&sp->lock, flags);
+		do_exec_command(sp);
+	} else {
+		spin_unlock_irqrestore(&sp->lock, flags);
+	}
+}
+
+/*
+ * Sleep until a command has failed or a response has been received
+ * and the command status been updated by the interrupt handler.
+ * (see receive_response).
+ */
+void ibmasm_wait_for_response(struct command *cmd, int timeout)
+{
+	wait_event_interruptible_timeout(cmd->wait,
+				cmd->status == IBMASM_CMD_COMPLETE ||
+				cmd->status == IBMASM_CMD_FAILED,
+				timeout * HZ);
+}
+
+/*
+ * receive_command_response
+ * called by the interrupt handler when a dot command of type command_response
+ * was received.
+ */
+void ibmasm_receive_command_response(struct service_processor *sp, void *response, size_t size)
+{
+	struct command *cmd = sp->current_command;
+
+	if (!sp->current_command)
+		return;
+
+	memcpy_fromio(cmd->buffer, response, min(size, cmd->buffer_size));
+	cmd->status = IBMASM_CMD_COMPLETE;
+	wake_up(&sp->current_command->wait);
+	command_put(sp->current_command);
+	exec_next_command(sp);
+}