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, 288 insertions, 0 deletions

diff --git a/drivers/tty/rpmsg_tty.c b/drivers/tty/rpmsg_tty.c
new file mode 100644
index 000000000..c5fd6d9b3
--- /dev/null
+++ b/drivers/tty/rpmsg_tty.c
@@ -0,0 +1,288 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2021 STMicroelectronics - All Rights Reserved
+ *
+ * The rpmsg tty driver implements serial communication on the RPMsg bus to makes
+ * possible for user-space programs to send and receive rpmsg messages as a standard
+ * tty protocol.
+ *
+ * The remote processor can instantiate a new tty by requesting a "rpmsg-tty" RPMsg service.
+ * The "rpmsg-tty" service is directly used for data exchange. No flow control is implemented yet.
+ */
+
+#define pr_fmt(fmt)		KBUILD_MODNAME ": " fmt
+
+#include <linux/module.h>
+#include <linux/rpmsg.h>
+#include <linux/slab.h>
+#include <linux/tty.h>
+#include <linux/tty_flip.h>
+
+#define RPMSG_TTY_NAME	"ttyRPMSG"
+#define MAX_TTY_RPMSG	32
+
+static DEFINE_IDR(tty_idr);	/* tty instance id */
+static DEFINE_MUTEX(idr_lock);	/* protects tty_idr */
+
+static struct tty_driver *rpmsg_tty_driver;
+
+struct rpmsg_tty_port {
+	struct tty_port		port;	 /* TTY port data */
+	int			id;	 /* TTY rpmsg index */
+	struct rpmsg_device	*rpdev;	 /* rpmsg device */
+};
+
+static int rpmsg_tty_cb(struct rpmsg_device *rpdev, void *data, int len, void *priv, u32 src)
+{
+	struct rpmsg_tty_port *cport = dev_get_drvdata(&rpdev->dev);
+	int copied;
+
+	if (!len)
+		return -EINVAL;
+	copied = tty_insert_flip_string(&cport->port, data, len);
+	if (copied != len)
+		dev_err_ratelimited(&rpdev->dev, "Trunc buffer: available space is %d\n", copied);
+	tty_flip_buffer_push(&cport->port);
+
+	return 0;
+}
+
+static int rpmsg_tty_install(struct tty_driver *driver, struct tty_struct *tty)
+{
+	struct rpmsg_tty_port *cport = idr_find(&tty_idr, tty->index);
+	struct tty_port *port;
+
+	tty->driver_data = cport;
+
+	port = tty_port_get(&cport->port);
+	return tty_port_install(port, driver, tty);
+}
+
+static void rpmsg_tty_cleanup(struct tty_struct *tty)
+{
+	tty_port_put(tty->port);
+}
+
+static int rpmsg_tty_open(struct tty_struct *tty, struct file *filp)
+{
+	return tty_port_open(tty->port, tty, filp);
+}
+
+static void rpmsg_tty_close(struct tty_struct *tty, struct file *filp)
+{
+	return tty_port_close(tty->port, tty, filp);
+}
+
+static ssize_t rpmsg_tty_write(struct tty_struct *tty, const u8 *buf,
+			       size_t len)
+{
+	struct rpmsg_tty_port *cport = tty->driver_data;
+	struct rpmsg_device *rpdev;
+	int msg_max_size, msg_size;
+	int ret;
+
+	rpdev = cport->rpdev;
+
+	msg_max_size = rpmsg_get_mtu(rpdev->ept);
+	if (msg_max_size < 0)
+		return msg_max_size;
+
+	msg_size = min_t(unsigned int, len, msg_max_size);
+
+	/*
+	 * Use rpmsg_trysend instead of rpmsg_send to send the message so the caller is not
+	 * hung until a rpmsg buffer is available. In such case rpmsg_trysend returns -ENOMEM.
+	 */
+	ret = rpmsg_trysend(rpdev->ept, (void *)buf, msg_size);
+	if (ret) {
+		dev_dbg_ratelimited(&rpdev->dev, "rpmsg_send failed: %d\n", ret);
+		return ret;
+	}
+
+	return msg_size;
+}
+
+static unsigned int rpmsg_tty_write_room(struct tty_struct *tty)
+{
+	struct rpmsg_tty_port *cport = tty->driver_data;
+	int size;
+
+	size = rpmsg_get_mtu(cport->rpdev->ept);
+	if (size < 0)
+		return 0;
+
+	return size;
+}
+
+static void rpmsg_tty_hangup(struct tty_struct *tty)
+{
+	tty_port_hangup(tty->port);
+}
+
+static const struct tty_operations rpmsg_tty_ops = {
+	.install	= rpmsg_tty_install,
+	.open		= rpmsg_tty_open,
+	.close		= rpmsg_tty_close,
+	.write		= rpmsg_tty_write,
+	.write_room	= rpmsg_tty_write_room,
+	.hangup		= rpmsg_tty_hangup,
+	.cleanup	= rpmsg_tty_cleanup,
+};
+
+static struct rpmsg_tty_port *rpmsg_tty_alloc_cport(void)
+{
+	struct rpmsg_tty_port *cport;
+	int ret;
+
+	cport = kzalloc_obj(*cport);
+	if (!cport)
+		return ERR_PTR(-ENOMEM);
+
+	mutex_lock(&idr_lock);
+	ret = idr_alloc(&tty_idr, cport, 0, MAX_TTY_RPMSG, GFP_KERNEL);
+	mutex_unlock(&idr_lock);
+
+	if (ret < 0) {
+		kfree(cport);
+		return ERR_PTR(ret);
+	}
+
+	cport->id = ret;
+
+	return cport;
+}
+
+static void rpmsg_tty_destruct_port(struct tty_port *port)
+{
+	struct rpmsg_tty_port *cport = container_of(port, struct rpmsg_tty_port, port);
+
+	mutex_lock(&idr_lock);
+	idr_remove(&tty_idr, cport->id);
+	mutex_unlock(&idr_lock);
+
+	kfree(cport);
+}
+
+static const struct tty_port_operations rpmsg_tty_port_ops = {
+	.destruct = rpmsg_tty_destruct_port,
+};
+
+
+static int rpmsg_tty_probe(struct rpmsg_device *rpdev)
+{
+	struct rpmsg_tty_port *cport;
+	struct device *dev = &rpdev->dev;
+	struct device *tty_dev;
+	int ret;
+
+	cport = rpmsg_tty_alloc_cport();
+	if (IS_ERR(cport))
+		return dev_err_probe(dev, PTR_ERR(cport), "Failed to alloc tty port\n");
+
+	tty_port_init(&cport->port);
+	cport->port.ops = &rpmsg_tty_port_ops;
+
+	tty_dev = tty_port_register_device(&cport->port, rpmsg_tty_driver,
+					   cport->id, dev);
+	if (IS_ERR(tty_dev)) {
+		ret = dev_err_probe(dev, PTR_ERR(tty_dev), "Failed to register tty port\n");
+		tty_port_put(&cport->port);
+		return ret;
+	}
+
+	cport->rpdev = rpdev;
+
+	dev_set_drvdata(dev, cport);
+
+	dev_dbg(dev, "New channel: 0x%x -> 0x%x: " RPMSG_TTY_NAME "%d\n",
+		rpdev->src, rpdev->dst, cport->id);
+
+	return 0;
+}
+
+static void rpmsg_tty_remove(struct rpmsg_device *rpdev)
+{
+	struct rpmsg_tty_port *cport = dev_get_drvdata(&rpdev->dev);
+
+	dev_dbg(&rpdev->dev, "Removing rpmsg tty device %d\n", cport->id);
+
+	/* User hang up to release the tty */
+	tty_port_tty_hangup(&cport->port, false);
+
+	tty_unregister_device(rpmsg_tty_driver, cport->id);
+
+	tty_port_put(&cport->port);
+}
+
+static struct rpmsg_device_id rpmsg_driver_tty_id_table[] = {
+	{ .name	= "rpmsg-tty" },
+	{ },
+};
+MODULE_DEVICE_TABLE(rpmsg, rpmsg_driver_tty_id_table);
+
+static struct rpmsg_driver rpmsg_tty_rpmsg_drv = {
+	.drv.name	= KBUILD_MODNAME,
+	.id_table	= rpmsg_driver_tty_id_table,
+	.probe		= rpmsg_tty_probe,
+	.callback	= rpmsg_tty_cb,
+	.remove		= rpmsg_tty_remove,
+};
+
+static int __init rpmsg_tty_init(void)
+{
+	int ret;
+
+	rpmsg_tty_driver = tty_alloc_driver(MAX_TTY_RPMSG, TTY_DRIVER_REAL_RAW |
+					    TTY_DRIVER_DYNAMIC_DEV);
+	if (IS_ERR(rpmsg_tty_driver))
+		return PTR_ERR(rpmsg_tty_driver);
+
+	rpmsg_tty_driver->driver_name = "rpmsg_tty";
+	rpmsg_tty_driver->name = RPMSG_TTY_NAME;
+	rpmsg_tty_driver->major = 0;
+	rpmsg_tty_driver->type = TTY_DRIVER_TYPE_CONSOLE;
+
+	/* Disable unused mode by default */
+	rpmsg_tty_driver->init_termios = tty_std_termios;
+	rpmsg_tty_driver->init_termios.c_lflag &= ~(ECHO | ICANON);
+	rpmsg_tty_driver->init_termios.c_oflag &= ~(OPOST | ONLCR);
+
+	tty_set_operations(rpmsg_tty_driver, &rpmsg_tty_ops);
+
+	ret = tty_register_driver(rpmsg_tty_driver);
+	if (ret < 0) {
+		pr_err("Couldn't install driver: %d\n", ret);
+		goto error_put;
+	}
+
+	ret = register_rpmsg_driver(&rpmsg_tty_rpmsg_drv);
+	if (ret < 0) {
+		pr_err("Couldn't register driver: %d\n", ret);
+		goto error_unregister;
+	}
+
+	return 0;
+
+error_unregister:
+	tty_unregister_driver(rpmsg_tty_driver);
+
+error_put:
+	tty_driver_kref_put(rpmsg_tty_driver);
+
+	return ret;
+}
+
+static void __exit rpmsg_tty_exit(void)
+{
+	unregister_rpmsg_driver(&rpmsg_tty_rpmsg_drv);
+	tty_unregister_driver(rpmsg_tty_driver);
+	tty_driver_kref_put(rpmsg_tty_driver);
+	idr_destroy(&tty_idr);
+}
+
+module_init(rpmsg_tty_init);
+module_exit(rpmsg_tty_exit);
+
+MODULE_AUTHOR("Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>");
+MODULE_DESCRIPTION("remote processor messaging tty driver");
+MODULE_LICENSE("GPL v2");