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, 187 insertions, 0 deletions

diff --git a/drivers/mcb/mcb-lpc.c b/drivers/mcb/mcb-lpc.c
new file mode 100644
index 000000000..070aa787a
--- /dev/null
+++ b/drivers/mcb/mcb-lpc.c
@@ -0,0 +1,187 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * MEN Chameleon Bus.
+ *
+ * Copyright (C) 2014 MEN Mikroelektronik GmbH (www.men.de)
+ * Author: Andreas Werner <andreas.werner@men.de>
+ */
+
+#include <linux/platform_device.h>
+#include <linux/module.h>
+#include <linux/dmi.h>
+#include <linux/mcb.h>
+#include <linux/io.h>
+#include "mcb-internal.h"
+
+struct priv {
+	struct mcb_bus *bus;
+	struct resource *mem;
+	void __iomem *base;
+};
+
+static int mcb_lpc_probe(struct platform_device *pdev)
+{
+	struct resource *res;
+	struct priv *priv;
+	int ret = 0, table_size;
+
+	priv = devm_kzalloc(&pdev->dev, sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return -ENOMEM;
+
+	priv->mem = platform_get_resource(pdev, IORESOURCE_MEM, 0);
+	if (!priv->mem) {
+		dev_err(&pdev->dev, "No Memory resource\n");
+		return -ENODEV;
+	}
+
+	res = devm_request_mem_region(&pdev->dev, priv->mem->start,
+				      resource_size(priv->mem),
+				      KBUILD_MODNAME);
+	if (!res) {
+		dev_err(&pdev->dev, "Failed to request IO memory\n");
+		return -EBUSY;
+	}
+
+	priv->base = devm_ioremap(&pdev->dev, priv->mem->start,
+				  resource_size(priv->mem));
+	if (!priv->base) {
+		dev_err(&pdev->dev, "Cannot ioremap\n");
+		return -ENOMEM;
+	}
+
+	platform_set_drvdata(pdev, priv);
+
+	priv->bus = mcb_alloc_bus(&pdev->dev);
+	if (IS_ERR(priv->bus))
+		return PTR_ERR(priv->bus);
+
+	ret = chameleon_parse_cells(priv->bus, priv->mem->start, priv->base);
+	if (ret < 0) {
+		goto out_mcb_bus;
+	}
+
+	table_size = ret;
+
+	if (table_size < CHAM_HEADER_SIZE) {
+		/* Release the previous resources */
+		devm_iounmap(&pdev->dev, priv->base);
+		devm_release_mem_region(&pdev->dev, priv->mem->start, resource_size(priv->mem));
+
+		/* Then, allocate it again with the actual chameleon table size */
+		res = devm_request_mem_region(&pdev->dev, priv->mem->start,
+					      table_size,
+					      KBUILD_MODNAME);
+		if (!res) {
+			dev_err(&pdev->dev, "Failed to request PCI memory\n");
+			ret = -EBUSY;
+			goto out_mcb_bus;
+		}
+
+		priv->base = devm_ioremap(&pdev->dev, priv->mem->start, table_size);
+		if (!priv->base) {
+			dev_err(&pdev->dev, "Cannot ioremap\n");
+			ret = -ENOMEM;
+			goto out_mcb_bus;
+		}
+
+		platform_set_drvdata(pdev, priv);
+	}
+
+	mcb_bus_add_devices(priv->bus);
+
+	return 0;
+
+out_mcb_bus:
+	mcb_release_bus(priv->bus);
+	return ret;
+}
+
+static void mcb_lpc_remove(struct platform_device *pdev)
+{
+	struct priv *priv = platform_get_drvdata(pdev);
+
+	mcb_release_bus(priv->bus);
+}
+
+static struct platform_device *mcb_lpc_pdev;
+
+static int mcb_lpc_create_platform_device(const struct dmi_system_id *id)
+{
+	struct resource *res = id->driver_data;
+	int ret;
+
+	mcb_lpc_pdev = platform_device_alloc("mcb-lpc", -1);
+	if (!mcb_lpc_pdev)
+		return -ENOMEM;
+
+	ret = platform_device_add_resources(mcb_lpc_pdev, res, 1);
+	if (ret)
+		goto out_put;
+
+	ret = platform_device_add(mcb_lpc_pdev);
+	if (ret)
+		goto out_put;
+
+	return 0;
+
+out_put:
+	platform_device_put(mcb_lpc_pdev);
+	return ret;
+}
+
+static struct resource sc24_fpga_resource = DEFINE_RES_MEM(0xe000e000, CHAM_HEADER_SIZE);
+static struct resource sc31_fpga_resource = DEFINE_RES_MEM(0xf000e000, CHAM_HEADER_SIZE);
+
+static struct platform_driver mcb_lpc_driver = {
+	.driver		= {
+		.name = "mcb-lpc",
+	},
+	.probe		= mcb_lpc_probe,
+	.remove		= mcb_lpc_remove,
+};
+
+static const struct dmi_system_id mcb_lpc_dmi_table[] = {
+	{
+		.ident = "SC24",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "MEN"),
+			DMI_MATCH(DMI_PRODUCT_VERSION, "14SC24"),
+		},
+		.driver_data = (void *)&sc24_fpga_resource,
+		.callback = mcb_lpc_create_platform_device,
+	},
+	{
+		.ident = "SC31",
+		.matches = {
+			DMI_MATCH(DMI_SYS_VENDOR, "MEN"),
+			DMI_MATCH(DMI_PRODUCT_VERSION, "14SC31"),
+		},
+		.driver_data = (void *)&sc31_fpga_resource,
+		.callback = mcb_lpc_create_platform_device,
+	},
+	{}
+};
+MODULE_DEVICE_TABLE(dmi, mcb_lpc_dmi_table);
+
+static int __init mcb_lpc_init(void)
+{
+	if (!dmi_check_system(mcb_lpc_dmi_table))
+		return -ENODEV;
+
+	return platform_driver_register(&mcb_lpc_driver);
+}
+
+static void __exit mcb_lpc_exit(void)
+{
+	platform_device_unregister(mcb_lpc_pdev);
+	platform_driver_unregister(&mcb_lpc_driver);
+}
+
+module_init(mcb_lpc_init);
+module_exit(mcb_lpc_exit);
+
+MODULE_AUTHOR("Andreas Werner <andreas.werner@men.de>");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("MCB over LPC support");
+MODULE_IMPORT_NS("MCB");