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, 220 insertions, 0 deletions

diff --git a/drivers/char/uv_mmtimer.c b/drivers/char/uv_mmtimer.c
new file mode 100644
index 000000000..956ebe208
--- /dev/null
+++ b/drivers/char/uv_mmtimer.c
@@ -0,0 +1,220 @@
+/*
+ * Timer device implementation for SGI UV platform.
+ *
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License.  See the file "COPYING" in the main directory of this archive
+ * for more details.
+ *
+ * Copyright (c) 2009 Silicon Graphics, Inc.  All rights reserved.
+ *
+ */
+
+#include <linux/types.h>
+#include <linux/kernel.h>
+#include <linux/ioctl.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/errno.h>
+#include <linux/mm.h>
+#include <linux/fs.h>
+#include <linux/mmtimer.h>
+#include <linux/miscdevice.h>
+#include <linux/posix-timers.h>
+#include <linux/interrupt.h>
+#include <linux/time.h>
+#include <linux/math64.h>
+
+#include <asm/genapic.h>
+#include <asm/uv/uv_hub.h>
+#include <asm/uv/bios.h>
+#include <asm/uv/uv.h>
+
+MODULE_AUTHOR("Dimitri Sivanich <sivanich@sgi.com>");
+MODULE_DESCRIPTION("SGI UV Memory Mapped RTC Timer");
+MODULE_LICENSE("GPL");
+
+/* name of the device, usually in /dev */
+#define UV_MMTIMER_NAME "mmtimer"
+#define UV_MMTIMER_DESC "SGI UV Memory Mapped RTC Timer"
+#define UV_MMTIMER_VERSION "1.0"
+
+static long uv_mmtimer_ioctl(struct file *file, unsigned int cmd,
+						unsigned long arg);
+static int uv_mmtimer_mmap(struct file *file, struct vm_area_struct *vma);
+
+/*
+ * Period in femtoseconds (10^-15 s)
+ */
+static unsigned long uv_mmtimer_femtoperiod;
+
+static const struct file_operations uv_mmtimer_fops = {
+	.owner = THIS_MODULE,
+	.mmap =	uv_mmtimer_mmap,
+	.unlocked_ioctl = uv_mmtimer_ioctl,
+	.llseek = noop_llseek,
+};
+
+/**
+ * uv_mmtimer_ioctl - ioctl interface for /dev/uv_mmtimer
+ * @file: file structure for the device
+ * @cmd: command to execute
+ * @arg: optional argument to command
+ *
+ * Executes the command specified by @cmd.  Returns 0 for success, < 0 for
+ * failure.
+ *
+ * Valid commands:
+ *
+ * %MMTIMER_GETOFFSET - Should return the offset (relative to the start
+ * of the page where the registers are mapped) for the counter in question.
+ *
+ * %MMTIMER_GETRES - Returns the resolution of the clock in femto (10^-15)
+ * seconds
+ *
+ * %MMTIMER_GETFREQ - Copies the frequency of the clock in Hz to the address
+ * specified by @arg
+ *
+ * %MMTIMER_GETBITS - Returns the number of bits in the clock's counter
+ *
+ * %MMTIMER_MMAPAVAIL - Returns 1 if registers can be mmap'd into userspace
+ *
+ * %MMTIMER_GETCOUNTER - Gets the current value in the counter and places it
+ * in the address specified by @arg.
+ */
+static long uv_mmtimer_ioctl(struct file *file, unsigned int cmd,
+						unsigned long arg)
+{
+	int ret = 0;
+
+	switch (cmd) {
+	case MMTIMER_GETOFFSET:	/* offset of the counter */
+		/*
+		 * Starting with HUB rev 2.0, the UV RTC register is
+		 * replicated across all cachelines of it's own page.
+		 * This allows faster simultaneous reads from a given socket.
+		 *
+		 * The offset returned is in 64 bit units.
+		 */
+		if (uv_get_min_hub_revision_id() == 1)
+			ret = 0;
+		else
+			ret = ((uv_blade_processor_id() * L1_CACHE_BYTES) %
+					PAGE_SIZE) / 8;
+		break;
+
+	case MMTIMER_GETRES: /* resolution of the clock in 10^-15 s */
+		if (copy_to_user((unsigned long __user *)arg,
+				&uv_mmtimer_femtoperiod, sizeof(unsigned long)))
+			ret = -EFAULT;
+		break;
+
+	case MMTIMER_GETFREQ: /* frequency in Hz */
+		if (copy_to_user((unsigned long __user *)arg,
+				&sn_rtc_cycles_per_second,
+				sizeof(unsigned long)))
+			ret = -EFAULT;
+		break;
+
+	case MMTIMER_GETBITS: /* number of bits in the clock */
+		ret = hweight64(UVH_RTC_REAL_TIME_CLOCK_MASK);
+		break;
+
+	case MMTIMER_MMAPAVAIL:
+		ret = 1;
+		break;
+
+	case MMTIMER_GETCOUNTER:
+		if (copy_to_user((unsigned long __user *)arg,
+				(unsigned long *)uv_local_mmr_address(UVH_RTC),
+				sizeof(unsigned long)))
+			ret = -EFAULT;
+		break;
+	default:
+		ret = -ENOTTY;
+		break;
+	}
+	return ret;
+}
+
+/**
+ * uv_mmtimer_mmap - maps the clock's registers into userspace
+ * @file: file structure for the device
+ * @vma: VMA to map the registers into
+ *
+ * Calls remap_pfn_range() to map the clock's registers into
+ * the calling process' address space.
+ */
+static int uv_mmtimer_mmap(struct file *file, struct vm_area_struct *vma)
+{
+	unsigned long uv_mmtimer_addr;
+
+	if (vma->vm_end - vma->vm_start != PAGE_SIZE)
+		return -EINVAL;
+
+	if (vma->vm_flags & VM_WRITE)
+		return -EPERM;
+
+	if (PAGE_SIZE > (1 << 16))
+		return -ENOSYS;
+
+	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+
+	uv_mmtimer_addr = UV_LOCAL_MMR_BASE | UVH_RTC;
+	uv_mmtimer_addr &= ~(PAGE_SIZE - 1);
+	uv_mmtimer_addr &= 0xfffffffffffffffUL;
+
+	if (remap_pfn_range(vma, vma->vm_start, uv_mmtimer_addr >> PAGE_SHIFT,
+					PAGE_SIZE, vma->vm_page_prot)) {
+		printk(KERN_ERR "remap_pfn_range failed in uv_mmtimer_mmap\n");
+		return -EAGAIN;
+	}
+
+	return 0;
+}
+
+static struct miscdevice uv_mmtimer_miscdev = {
+	MISC_DYNAMIC_MINOR,
+	UV_MMTIMER_NAME,
+	&uv_mmtimer_fops
+};
+
+
+/**
+ * uv_mmtimer_init - device initialization routine
+ *
+ * Does initial setup for the uv_mmtimer device.
+ */
+static int __init uv_mmtimer_init(void)
+{
+	if (!is_uv_system()) {
+		printk(KERN_ERR "%s: Hardware unsupported\n", UV_MMTIMER_NAME);
+		return -1;
+	}
+
+	/*
+	 * Sanity check the cycles/sec variable
+	 */
+	if (sn_rtc_cycles_per_second < 100000) {
+		printk(KERN_ERR "%s: unable to determine clock frequency\n",
+		       UV_MMTIMER_NAME);
+		return -1;
+	}
+
+	uv_mmtimer_femtoperiod = ((unsigned long)1E15 +
+				sn_rtc_cycles_per_second / 2) /
+				sn_rtc_cycles_per_second;
+
+	if (misc_register(&uv_mmtimer_miscdev)) {
+		printk(KERN_ERR "%s: failed to register device\n",
+		       UV_MMTIMER_NAME);
+		return -1;
+	}
+
+	printk(KERN_INFO "%s: v%s, %ld MHz\n", UV_MMTIMER_DESC,
+		UV_MMTIMER_VERSION,
+		sn_rtc_cycles_per_second/(unsigned long)1E6);
+
+	return 0;
+}
+
+module_init(uv_mmtimer_init);