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, 208 insertions, 0 deletions

diff --git a/drivers/accel/drm_accel.c b/drivers/accel/drm_accel.c
new file mode 100644
index 000000000..ca3357acd
--- /dev/null
+++ b/drivers/accel/drm_accel.c
@@ -0,0 +1,208 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2022 HabanaLabs, Ltd.
+ * All Rights Reserved.
+ *
+ */
+
+#include <linux/debugfs.h>
+#include <linux/device.h>
+#include <linux/xarray.h>
+
+#include <drm/drm_accel.h>
+#include <drm/drm_auth.h>
+#include <drm/drm_debugfs.h>
+#include <drm/drm_drv.h>
+#include <drm/drm_file.h>
+#include <drm/drm_ioctl.h>
+#include <drm/drm_print.h>
+
+DEFINE_XARRAY_ALLOC(accel_minors_xa);
+
+static const struct device_type accel_sysfs_device_minor = {
+	.name = "accel_minor"
+};
+
+static char *accel_devnode(const struct device *dev, umode_t *mode)
+{
+	return kasprintf(GFP_KERNEL, "accel/%s", dev_name(dev));
+}
+
+static const struct class accel_class = {
+	.name = "accel",
+	.devnode = accel_devnode,
+};
+
+static int accel_sysfs_init(void)
+{
+	return class_register(&accel_class);
+}
+
+static void accel_sysfs_destroy(void)
+{
+	class_unregister(&accel_class);
+}
+
+static int accel_name_info(struct seq_file *m, void *data)
+{
+	struct drm_info_node *node = (struct drm_info_node *) m->private;
+	struct drm_minor *minor = node->minor;
+	struct drm_device *dev = minor->dev;
+	struct drm_master *master;
+
+	mutex_lock(&dev->master_mutex);
+	master = dev->master;
+	seq_printf(m, "%s", dev->driver->name);
+	if (dev->dev)
+		seq_printf(m, " dev=%s", dev_name(dev->dev));
+	if (master && master->unique)
+		seq_printf(m, " master=%s", master->unique);
+	if (dev->unique)
+		seq_printf(m, " unique=%s", dev->unique);
+	seq_puts(m, "\n");
+	mutex_unlock(&dev->master_mutex);
+
+	return 0;
+}
+
+static const struct drm_info_list accel_debugfs_list[] = {
+	{"name", accel_name_info, 0}
+};
+#define ACCEL_DEBUGFS_ENTRIES ARRAY_SIZE(accel_debugfs_list)
+
+/**
+ * accel_debugfs_register() - Register debugfs for device
+ * @dev: Pointer to the device instance.
+ *
+ * Creates common files for accelerators.
+ */
+void accel_debugfs_register(struct drm_device *dev)
+{
+	struct drm_minor *minor = dev->accel;
+
+	minor->debugfs_root = dev->debugfs_root;
+
+	drm_debugfs_create_files(accel_debugfs_list, ACCEL_DEBUGFS_ENTRIES,
+				 dev->debugfs_root, minor);
+}
+
+/**
+ * accel_set_device_instance_params() - Set some device parameters for accel device
+ * @kdev: Pointer to the device instance.
+ * @index: The minor's index
+ *
+ * This function creates the dev_t of the device using the accel major and
+ * the device's minor number. In addition, it sets the class and type of the
+ * device instance to the accel sysfs class and device type, respectively.
+ */
+void accel_set_device_instance_params(struct device *kdev, int index)
+{
+	kdev->devt = MKDEV(ACCEL_MAJOR, index);
+	kdev->class = &accel_class;
+	kdev->type = &accel_sysfs_device_minor;
+}
+
+/**
+ * accel_open - open method for ACCEL file
+ * @inode: device inode
+ * @filp: file pointer.
+ *
+ * This function must be used by drivers as their &file_operations.open method.
+ * It looks up the correct ACCEL device and instantiates all the per-file
+ * resources for it. It also calls the &drm_driver.open driver callback.
+ *
+ * Return: 0 on success or negative errno value on failure.
+ */
+int accel_open(struct inode *inode, struct file *filp)
+{
+	struct drm_device *dev;
+	struct drm_minor *minor;
+	int retcode;
+
+	minor = drm_minor_acquire(&accel_minors_xa, iminor(inode));
+	if (IS_ERR(minor))
+		return PTR_ERR(minor);
+
+	dev = minor->dev;
+
+	atomic_fetch_inc(&dev->open_count);
+
+	/* share address_space across all char-devs of a single device */
+	filp->f_mapping = dev->anon_inode->i_mapping;
+
+	retcode = drm_open_helper(filp, minor);
+	if (retcode)
+		goto err_undo;
+
+	return 0;
+
+err_undo:
+	atomic_dec(&dev->open_count);
+	drm_minor_release(minor);
+	return retcode;
+}
+EXPORT_SYMBOL_GPL(accel_open);
+
+static int accel_stub_open(struct inode *inode, struct file *filp)
+{
+	const struct file_operations *new_fops;
+	struct drm_minor *minor;
+	int err;
+
+	minor = drm_minor_acquire(&accel_minors_xa, iminor(inode));
+	if (IS_ERR(minor))
+		return PTR_ERR(minor);
+
+	new_fops = fops_get(minor->dev->driver->fops);
+	if (!new_fops) {
+		err = -ENODEV;
+		goto out;
+	}
+
+	replace_fops(filp, new_fops);
+	if (filp->f_op->open)
+		err = filp->f_op->open(inode, filp);
+	else
+		err = 0;
+
+out:
+	drm_minor_release(minor);
+
+	return err;
+}
+
+static const struct file_operations accel_stub_fops = {
+	.owner = THIS_MODULE,
+	.open = accel_stub_open,
+	.llseek = noop_llseek,
+};
+
+void accel_core_exit(void)
+{
+	unregister_chrdev(ACCEL_MAJOR, "accel");
+	accel_sysfs_destroy();
+	WARN_ON(!xa_empty(&accel_minors_xa));
+}
+
+int __init accel_core_init(void)
+{
+	int ret;
+
+	ret = accel_sysfs_init();
+	if (ret < 0) {
+		DRM_ERROR("Cannot create ACCEL class: %d\n", ret);
+		goto error;
+	}
+
+	ret = register_chrdev(ACCEL_MAJOR, "accel", &accel_stub_fops);
+	if (ret < 0)
+		DRM_ERROR("Cannot register ACCEL major: %d\n", ret);
+
+error:
+	/*
+	 * Any cleanup due to errors will be done in drm_core_exit() that
+	 * will call accel_core_exit()
+	 */
+	return ret;
+}