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, 253 insertions, 0 deletions

diff --git a/drivers/mcb/mcb-parse.c b/drivers/mcb/mcb-parse.c
new file mode 100644
index 000000000..7e09a0ea8
--- /dev/null
+++ b/drivers/mcb/mcb-parse.c
@@ -0,0 +1,253 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/types.h>
+#include <linux/ioport.h>
+#include <linux/slab.h>
+#include <linux/export.h>
+#include <linux/io.h>
+#include <linux/mcb.h>
+
+#include "mcb-internal.h"
+
+#define for_each_chameleon_cell(dtype, p)	\
+	for ((dtype) = get_next_dtype((p));	\
+	     (dtype) != CHAMELEON_DTYPE_END;	\
+	     (dtype) = get_next_dtype((p)))
+
+static inline uint32_t get_next_dtype(void __iomem *p)
+{
+	uint32_t dtype;
+
+	dtype = readl(p);
+	return dtype >> 28;
+}
+
+static int chameleon_parse_bdd(struct mcb_bus *bus,
+			struct chameleon_bar *cb,
+			void __iomem *base)
+{
+	return 0;
+}
+
+static int chameleon_parse_gdd(struct mcb_bus *bus,
+			struct chameleon_bar *cb,
+			void __iomem *base, int bar_count)
+{
+	struct chameleon_gdd __iomem *gdd =
+		(struct chameleon_gdd __iomem *) base;
+	struct mcb_device *mdev;
+	u32 dev_mapbase;
+	u32 offset;
+	u32 size;
+	int ret;
+	__le32 reg1;
+	__le32 reg2;
+
+	mdev = mcb_alloc_dev(bus);
+	if (!mdev)
+		return -ENOMEM;
+
+	reg1 = readl(&gdd->reg1);
+	reg2 = readl(&gdd->reg2);
+	offset = readl(&gdd->offset);
+	size = readl(&gdd->size);
+
+	mdev->id = GDD_DEV(reg1);
+	mdev->rev = GDD_REV(reg1);
+	mdev->var = GDD_VAR(reg1);
+	mdev->bar = GDD_BAR(reg2);
+	mdev->group = GDD_GRP(reg2);
+	mdev->inst = GDD_INS(reg2);
+
+	/*
+	 * If the BAR is missing, dev_mapbase is zero, or if the
+	 * device is IO mapped we just print a warning and go on with the
+	 * next device, instead of completely stop the gdd parser
+	 */
+	if (mdev->bar > bar_count - 1) {
+		pr_info("No BAR for 16z%03d\n", mdev->id);
+		ret = 0;
+		goto err;
+	}
+
+	dev_mapbase = cb[mdev->bar].addr;
+	if (!dev_mapbase) {
+		pr_info("BAR not assigned for 16z%03d\n", mdev->id);
+		ret = 0;
+		goto err;
+	}
+
+	if (dev_mapbase & 0x01) {
+		pr_info("IO mapped Device (16z%03d) not yet supported\n",
+			mdev->id);
+		ret = 0;
+		goto err;
+	}
+
+	pr_debug("Found a 16z%03d\n", mdev->id);
+
+	mdev->irq.start = GDD_IRQ(reg1);
+	mdev->irq.end = GDD_IRQ(reg1);
+	mdev->irq.flags = IORESOURCE_IRQ;
+
+	mdev->mem.start = dev_mapbase + offset;
+
+	mdev->mem.end = mdev->mem.start + size - 1;
+	mdev->mem.flags = IORESOURCE_MEM;
+
+	ret = mcb_device_register(bus, mdev);
+	if (ret < 0)
+		return ret;
+
+	return 0;
+
+err:
+	mcb_free_dev(mdev);
+
+	return ret;
+}
+
+static void chameleon_parse_bar(void __iomem *base,
+				struct chameleon_bar *cb, int bar_count)
+{
+	char __iomem *p = base;
+	int i;
+
+	/* skip reg1 */
+	p += sizeof(__le32);
+
+	for (i = 0; i < bar_count; i++) {
+		cb[i].addr = readl(p);
+		cb[i].size = readl(p + 4);
+
+		p += sizeof(struct chameleon_bar);
+	}
+}
+
+static int chameleon_get_bar(void __iomem **base, phys_addr_t mapbase,
+			     struct chameleon_bar **cb)
+{
+	struct chameleon_bar *c;
+	int bar_count;
+	__le32 reg;
+	u32 dtype;
+
+	/*
+	 * For those devices which are not connected
+	 * to the PCI Bus (e.g. LPC) there is a bar
+	 * descriptor located directly after the
+	 * chameleon header. This header is comparable
+	 * to a PCI header.
+	 */
+	dtype = get_next_dtype(*base);
+	if (dtype == CHAMELEON_DTYPE_BAR) {
+		reg = readl(*base);
+
+		bar_count = BAR_CNT(reg);
+		if (bar_count <= 0 || bar_count > CHAMELEON_BAR_MAX)
+			return -ENODEV;
+
+		c = kzalloc_objs(struct chameleon_bar, bar_count);
+		if (!c)
+			return -ENOMEM;
+
+		chameleon_parse_bar(*base, c, bar_count);
+		*base += BAR_DESC_SIZE(bar_count);
+	} else {
+		c = kzalloc_obj(struct chameleon_bar);
+		if (!c)
+			return -ENOMEM;
+
+		bar_count = 1;
+		c->addr = mapbase;
+	}
+
+	*cb = c;
+
+	return bar_count;
+}
+
+int chameleon_parse_cells(struct mcb_bus *bus, phys_addr_t mapbase,
+			void __iomem *base)
+{
+	struct chameleon_fpga_header *header;
+	struct chameleon_bar *cb;
+	void __iomem *p = base;
+	int num_cells = 0;
+	uint32_t dtype;
+	int bar_count;
+	int ret;
+	u32 hsize;
+	u32 table_size;
+
+	hsize = sizeof(struct chameleon_fpga_header);
+
+	header = kzalloc(hsize, GFP_KERNEL);
+	if (!header)
+		return -ENOMEM;
+
+	/* Extract header information */
+	memcpy_fromio(header, p, hsize);
+	/* We only support chameleon v2 at the moment */
+	header->magic = le16_to_cpu(header->magic);
+	if (header->magic != CHAMELEONV2_MAGIC) {
+		pr_err("Unsupported chameleon version 0x%x\n",
+				header->magic);
+		ret = -ENODEV;
+		goto free_header;
+	}
+	p += hsize;
+
+	bus->revision = header->revision;
+	bus->model = header->model;
+	bus->minor = header->minor;
+	snprintf(bus->name, CHAMELEON_FILENAME_LEN + 1, "%s",
+		 header->filename);
+
+	bar_count = chameleon_get_bar(&p, mapbase, &cb);
+	if (bar_count < 0) {
+		ret = bar_count;
+		goto free_header;
+	}
+
+	for_each_chameleon_cell(dtype, p) {
+		switch (dtype) {
+		case CHAMELEON_DTYPE_GENERAL:
+			ret = chameleon_parse_gdd(bus, cb, p, bar_count);
+			if (ret < 0)
+				goto free_bar;
+			p += sizeof(struct chameleon_gdd);
+			break;
+		case CHAMELEON_DTYPE_BRIDGE:
+			chameleon_parse_bdd(bus, cb, p);
+			p += sizeof(struct chameleon_bdd);
+			break;
+		case CHAMELEON_DTYPE_END:
+			break;
+		default:
+			pr_err("Invalid chameleon descriptor type 0x%x\n",
+				dtype);
+			ret = -EINVAL;
+			goto free_bar;
+		}
+		num_cells++;
+	}
+
+	if (num_cells == 0) {
+		ret = -EINVAL;
+		goto free_bar;
+	}
+
+	table_size = p - base;
+	pr_debug("%d cell(s) found. Chameleon table size: 0x%04x bytes\n", num_cells, table_size);
+	kfree(cb);
+	kfree(header);
+	return table_size;
+
+free_bar:
+	kfree(cb);
+free_header:
+	kfree(header);
+
+	return ret;
+}
+EXPORT_SYMBOL_NS_GPL(chameleon_parse_cells, "MCB");