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, 178 insertions, 0 deletions

diff --git a/drivers/nvmem/rmem.c b/drivers/nvmem/rmem.c
new file mode 100644
index 000000000..b39d628cb
--- /dev/null
+++ b/drivers/nvmem/rmem.c
@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (C) 2020 Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
+ */
+
+#include <linux/crc32.h>
+#include <linux/io.h>
+#include <linux/module.h>
+#include <linux/nvmem-provider.h>
+#include <linux/of_reserved_mem.h>
+#include <linux/platform_device.h>
+#include <linux/slab.h>
+
+struct rmem {
+	struct device *dev;
+	struct nvmem_device *nvmem;
+	struct reserved_mem *mem;
+};
+
+struct rmem_match_data {
+	int (*checksum)(struct rmem *priv);
+};
+
+struct __packed rmem_eyeq5_header {
+	u32 magic;
+	u32 version;
+	u32 size;
+};
+
+#define RMEM_EYEQ5_MAGIC	((u32)0xDABBAD00)
+
+static int rmem_read(void *context, unsigned int offset,
+		     void *val, size_t bytes)
+{
+	struct rmem *priv = context;
+	void *addr;
+
+	if ((phys_addr_t)offset + bytes > priv->mem->size)
+		return -EIO;
+
+	/*
+	 * Only map the reserved memory at this point to avoid potential rogue
+	 * kernel threads inadvertently modifying it. Based on the current
+	 * uses-cases for this driver, the performance hit isn't a concern.
+	 * Nor is likely to be, given the nature of the subsystem. Most nvmem
+	 * devices operate over slow buses to begin with.
+	 *
+	 * An alternative would be setting the memory as RO, set_memory_ro(),
+	 * but as of Dec 2020 this isn't possible on arm64.
+	 */
+	addr = memremap(priv->mem->base, priv->mem->size, MEMREMAP_WB);
+	if (!addr) {
+		dev_err(priv->dev, "Failed to remap memory region\n");
+		return -ENOMEM;
+	}
+
+	memcpy(val, addr + offset, bytes);
+
+	memunmap(addr);
+
+	return 0;
+}
+
+static int rmem_eyeq5_checksum(struct rmem *priv)
+{
+	void *buf __free(kfree) = NULL;
+	struct rmem_eyeq5_header header;
+	u32 computed_crc, *target_crc;
+	size_t data_size;
+	int ret;
+
+	ret = rmem_read(priv, 0, &header, sizeof(header));
+	if (ret)
+		return ret;
+
+	if (header.magic != RMEM_EYEQ5_MAGIC)
+		return -EINVAL;
+
+	/*
+	 * Avoid massive kmalloc() if header read is invalid;
+	 * the check would be done by the next rmem_read() anyway.
+	 */
+	if (header.size > priv->mem->size)
+		return -EINVAL;
+
+	/*
+	 *           0 +-------------------+
+	 *             | Header (12 bytes) | \
+	 *             +-------------------+ |
+	 *             |                   | | data to be CRCed
+	 *             |        ...        | |
+	 *             |                   | /
+	 *   data_size +-------------------+
+	 *             |   CRC (4 bytes)   |
+	 * header.size +-------------------+
+	 */
+
+	buf = kmalloc(header.size, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	ret = rmem_read(priv, 0, buf, header.size);
+	if (ret)
+		return ret;
+
+	data_size = header.size - sizeof(*target_crc);
+	target_crc = buf + data_size;
+	computed_crc = crc32(U32_MAX, buf, data_size) ^ U32_MAX;
+
+	if (computed_crc == *target_crc)
+		return 0;
+
+	dev_err(priv->dev,
+		"checksum failed: computed %#x, expected %#x, header (%#x, %#x, %#x)\n",
+		computed_crc, *target_crc, header.magic, header.version, header.size);
+	return -EINVAL;
+}
+
+static int rmem_probe(struct platform_device *pdev)
+{
+	struct nvmem_config config = { };
+	struct device *dev = &pdev->dev;
+	const struct rmem_match_data *match_data = device_get_match_data(dev);
+	struct reserved_mem *mem;
+	struct rmem *priv;
+
+	priv = devm_kzalloc(dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+	priv->dev = dev;
+
+	mem = of_reserved_mem_lookup(dev->of_node);
+	if (!mem) {
+		dev_err(dev, "Failed to lookup reserved memory\n");
+		return -EINVAL;
+	}
+	priv->mem = mem;
+
+	config.dev = dev;
+	config.priv = priv;
+	config.name = "rmem";
+	config.id = NVMEM_DEVID_AUTO;
+	config.size = mem->size;
+	config.reg_read = rmem_read;
+
+	if (match_data && match_data->checksum) {
+		int ret = match_data->checksum(priv);
+
+		if (ret)
+			return ret;
+	}
+
+	return PTR_ERR_OR_ZERO(devm_nvmem_register(dev, &config));
+}
+
+static const struct rmem_match_data rmem_eyeq5_match_data = {
+	.checksum = rmem_eyeq5_checksum,
+};
+
+static const struct of_device_id rmem_match[] = {
+	{ .compatible = "mobileye,eyeq5-bootloader-config", .data = &rmem_eyeq5_match_data },
+	{ .compatible = "nvmem-rmem", },
+	{ /* sentinel */ },
+};
+MODULE_DEVICE_TABLE(of, rmem_match);
+
+static struct platform_driver rmem_driver = {
+	.probe = rmem_probe,
+	.driver = {
+		.name = "rmem",
+		.of_match_table = rmem_match,
+	},
+};
+module_platform_driver(rmem_driver);
+
+MODULE_AUTHOR("Nicolas Saenz Julienne <nsaenzjulienne@suse.de>");
+MODULE_DESCRIPTION("Reserved Memory Based nvmem Driver");
+MODULE_LICENSE("GPL");