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, 209 insertions, 0 deletions

diff --git a/drivers/misc/open-dice.c b/drivers/misc/open-dice.c
new file mode 100644
index 000000000..45060fb4e
--- /dev/null
+++ b/drivers/misc/open-dice.c
@@ -0,0 +1,209 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2021 - Google LLC
+ * Author: David Brazdil <dbrazdil@google.com>
+ *
+ * Driver for Open Profile for DICE.
+ *
+ * This driver takes ownership of a reserved memory region containing data
+ * generated by the Open Profile for DICE measured boot protocol. The memory
+ * contents are not interpreted by the kernel but can be mapped into a userspace
+ * process via a misc device. Userspace can also request a wipe of the memory.
+ *
+ * Userspace can access the data with (w/o error handling):
+ *
+ *     fd = open("/dev/open-dice0", O_RDWR);
+ *     read(fd, &size, sizeof(unsigned long));
+ *     data = mmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+ *     write(fd, NULL, 0); // wipe
+ *     close(fd);
+ */
+
+#include <linux/io.h>
+#include <linux/miscdevice.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/of_reserved_mem.h>
+#include <linux/platform_device.h>
+
+#define DRIVER_NAME "open-dice"
+
+struct open_dice_drvdata {
+	struct mutex lock;
+	char name[16];
+	struct reserved_mem *rmem;
+	struct miscdevice misc;
+};
+
+static inline struct open_dice_drvdata *to_open_dice_drvdata(struct file *filp)
+{
+	return container_of(filp->private_data, struct open_dice_drvdata, misc);
+}
+
+static int open_dice_wipe(struct open_dice_drvdata *drvdata)
+{
+	void *kaddr;
+
+	mutex_lock(&drvdata->lock);
+	kaddr = devm_memremap(drvdata->misc.this_device, drvdata->rmem->base,
+			      drvdata->rmem->size, MEMREMAP_WC);
+	if (IS_ERR(kaddr)) {
+		mutex_unlock(&drvdata->lock);
+		return PTR_ERR(kaddr);
+	}
+
+	memset(kaddr, 0, drvdata->rmem->size);
+	devm_memunmap(drvdata->misc.this_device, kaddr);
+	mutex_unlock(&drvdata->lock);
+	return 0;
+}
+
+/*
+ * Copies the size of the reserved memory region to the user-provided buffer.
+ */
+static ssize_t open_dice_read(struct file *filp, char __user *ptr, size_t len,
+			      loff_t *off)
+{
+	unsigned long val = to_open_dice_drvdata(filp)->rmem->size;
+
+	return simple_read_from_buffer(ptr, len, off, &val, sizeof(val));
+}
+
+/*
+ * Triggers a wipe of the reserved memory region. The user-provided pointer
+ * is never dereferenced.
+ */
+static ssize_t open_dice_write(struct file *filp, const char __user *ptr,
+			       size_t len, loff_t *off)
+{
+	if (open_dice_wipe(to_open_dice_drvdata(filp)))
+		return -EIO;
+
+	/* Consume the input buffer. */
+	return len;
+}
+
+/*
+ * Creates a mapping of the reserved memory region in user address space.
+ */
+static int open_dice_mmap_prepare(struct vm_area_desc *desc)
+{
+	struct file *filp = desc->file;
+	struct open_dice_drvdata *drvdata = to_open_dice_drvdata(filp);
+
+	if (vma_desc_test(desc, VMA_MAYSHARE_BIT)) {
+		/* Do not allow userspace to modify the underlying data. */
+		if (vma_desc_test(desc, VMA_WRITE_BIT))
+			return -EPERM;
+		/* Ensure userspace cannot acquire VM_WRITE later. */
+		vma_desc_clear_flags(desc, VMA_MAYWRITE_BIT);
+	}
+
+	/* Create write-combine mapping so all clients observe a wipe. */
+	desc->page_prot = pgprot_writecombine(desc->page_prot);
+	vma_desc_set_flags(desc, VMA_DONTCOPY_BIT, VMA_DONTDUMP_BIT);
+	mmap_action_simple_ioremap(desc, drvdata->rmem->base,
+				   drvdata->rmem->size);
+	return 0;
+}
+
+static const struct file_operations open_dice_fops = {
+	.owner = THIS_MODULE,
+	.read = open_dice_read,
+	.write = open_dice_write,
+	.mmap_prepare = open_dice_mmap_prepare,
+};
+
+static int __init open_dice_probe(struct platform_device *pdev)
+{
+	static unsigned int dev_idx;
+	struct device *dev = &pdev->dev;
+	struct reserved_mem *rmem;
+	struct open_dice_drvdata *drvdata;
+	int ret;
+
+	rmem = of_reserved_mem_lookup(dev->of_node);
+	if (!rmem) {
+		dev_err(dev, "failed to lookup reserved memory\n");
+		return -EINVAL;
+	}
+
+	if (!rmem->size || (rmem->size > ULONG_MAX)) {
+		dev_err(dev, "invalid memory region size\n");
+		return -EINVAL;
+	}
+
+	if (!PAGE_ALIGNED(rmem->base) || !PAGE_ALIGNED(rmem->size)) {
+		dev_err(dev, "memory region must be page-aligned\n");
+		return -EINVAL;
+	}
+
+	drvdata = devm_kmalloc(dev, sizeof(*drvdata), GFP_KERNEL);
+	if (!drvdata)
+		return -ENOMEM;
+
+	*drvdata = (struct open_dice_drvdata){
+		.rmem = rmem,
+		.misc = (struct miscdevice){
+			.parent	= dev,
+			.name	= drvdata->name,
+			.minor	= MISC_DYNAMIC_MINOR,
+			.fops	= &open_dice_fops,
+			.mode	= 0600,
+		},
+	};
+	mutex_init(&drvdata->lock);
+
+	/* Index overflow check not needed, misc_register() will fail. */
+	snprintf(drvdata->name, sizeof(drvdata->name), DRIVER_NAME"%u", dev_idx++);
+
+	ret = misc_register(&drvdata->misc);
+	if (ret) {
+		dev_err(dev, "failed to register misc device '%s': %d\n",
+			drvdata->name, ret);
+		return ret;
+	}
+
+	platform_set_drvdata(pdev, drvdata);
+	return 0;
+}
+
+static void open_dice_remove(struct platform_device *pdev)
+{
+	struct open_dice_drvdata *drvdata = platform_get_drvdata(pdev);
+
+	misc_deregister(&drvdata->misc);
+}
+
+static const struct of_device_id open_dice_of_match[] = {
+	{ .compatible = "google,open-dice" },
+	{},
+};
+
+static struct platform_driver open_dice_driver = {
+	.remove = open_dice_remove,
+	.driver = {
+		.name = DRIVER_NAME,
+		.of_match_table = open_dice_of_match,
+	},
+};
+
+static int __init open_dice_init(void)
+{
+	int ret = platform_driver_probe(&open_dice_driver, open_dice_probe);
+
+	/* DICE regions are optional. Succeed even with zero instances. */
+	return (ret == -ENODEV) ? 0 : ret;
+}
+
+static void __exit open_dice_exit(void)
+{
+	platform_driver_unregister(&open_dice_driver);
+}
+
+module_init(open_dice_init);
+module_exit(open_dice_exit);
+
+MODULE_DESCRIPTION("Driver for Open Profile for DICE.");
+MODULE_LICENSE("GPL v2");
+MODULE_AUTHOR("David Brazdil <dbrazdil@google.com>");