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, 105 insertions, 0 deletions

diff --git a/drivers/parisc/hppb.c b/drivers/parisc/hppb.c
new file mode 100644
index 000000000..33709eafe
--- /dev/null
+++ b/drivers/parisc/hppb.c
@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+** hppb.c:
+**      HP-PB bus driver for the NOVA and K-Class systems.
+**
+**      (c) Copyright 2002 Ryan Bradetich
+**      (c) Copyright 2002 Hewlett-Packard Company
+**
+**
+*/
+
+#include <linux/types.h>
+#include <linux/init.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+#include <linux/dma-mapping.h>
+#include <linux/ioport.h>
+
+#include <asm/io.h>
+#include <asm/hardware.h>
+#include <asm/parisc-device.h>
+
+#include "iommu.h"
+
+struct hppb_card {
+	unsigned long hpa;
+	struct resource mmio_region;
+	struct hppb_card *next;
+};
+
+static struct hppb_card hppb_card_head = {
+	.hpa = 0,
+	.next = NULL,
+};
+
+#define IO_IO_LOW  offsetof(struct bc_module, io_io_low)
+#define IO_IO_HIGH offsetof(struct bc_module, io_io_high)
+
+/**
+ * hppb_probe - Determine if the hppb driver should claim this device.
+ * @dev: The device which has been found
+ *
+ * Determine if hppb driver should claim this chip (return 0) or not 
+ * (return 1). If so, initialize the chip and tell other partners in crime 
+ * they have work to do.
+ */
+static int __init hppb_probe(struct parisc_device *dev)
+{
+	int status;
+	struct hppb_card *card = &hppb_card_head;
+
+	while(card->next) {
+		card = card->next;
+	}
+
+	if(card->hpa) {
+		card->next = kzalloc_obj(struct hppb_card);
+		if(!card->next) {
+			printk(KERN_ERR "HP-PB: Unable to allocate memory.\n");
+			return 1;
+		}
+		card = card->next;
+	}
+
+	card->hpa = dev->hpa.start;
+	card->mmio_region.name = "HP-PB Bus";
+	card->mmio_region.flags = IORESOURCE_MEM;
+
+	card->mmio_region.start = gsc_readl(dev->hpa.start + IO_IO_LOW);
+	card->mmio_region.end = gsc_readl(dev->hpa.start + IO_IO_HIGH) - 1;
+
+	status = ccio_request_resource(dev, &card->mmio_region);
+
+	pr_info("Found GeckoBoa at %pap, bus space %pR,%s claimed.\n",
+			&dev->hpa.start,
+			&card->mmio_region,
+			(status < 0) ? " not":"" );
+
+        return 0;
+}
+
+static const struct parisc_device_id hppb_tbl[] __initconst = {
+        { HPHW_BCPORT, HVERSION_REV_ANY_ID, 0x500, 0xc }, /* E25 and K */
+        { HPHW_BCPORT, 0x0, 0x501, 0xc }, /* E35 */
+        { HPHW_BCPORT, 0x0, 0x502, 0xc }, /* E45 */
+        { HPHW_BCPORT, 0x0, 0x503, 0xc }, /* E55 */
+        { 0, }
+};
+
+static struct parisc_driver hppb_driver __refdata = {
+        .name =         "gecko_boa",
+        .id_table =     hppb_tbl,
+	.probe =        hppb_probe,
+};
+
+/**
+ * hppb_init - HP-PB bus initialization procedure.
+ *
+ * Register this driver.
+ */
+static int __init hppb_init(void)
+{
+        return register_parisc_driver(&hppb_driver);
+}
+arch_initcall(hppb_init);