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, 232 insertions, 0 deletions

diff --git a/drivers/char/adi.c b/drivers/char/adi.c
new file mode 100644
index 000000000..0849d933a
--- /dev/null
+++ b/drivers/char/adi.c
@@ -0,0 +1,232 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Privileged ADI driver for sparc64
+ *
+ * Author: Tom Hromatka <tom.hromatka@oracle.com>
+ */
+#include <linux/kernel.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/proc_fs.h>
+#include <linux/slab.h>
+#include <linux/uaccess.h>
+#include <asm/asi.h>
+
+#define MAX_BUF_SZ	PAGE_SIZE
+
+static int read_mcd_tag(unsigned long addr)
+{
+	long err;
+	int ver;
+
+	__asm__ __volatile__(
+		"1:	ldxa [%[addr]] %[asi], %[ver]\n"
+		"	mov 0, %[err]\n"
+		"2:\n"
+		"	.section .fixup,#alloc,#execinstr\n"
+		"	.align 4\n"
+		"3:	sethi %%hi(2b), %%g1\n"
+		"	jmpl  %%g1 + %%lo(2b), %%g0\n"
+		"	mov %[invalid], %[err]\n"
+		"	.previous\n"
+		"	.section __ex_table, \"a\"\n"
+		"	.align 4\n"
+		"	.word  1b, 3b\n"
+		"	.previous\n"
+		: [ver] "=r" (ver), [err] "=r" (err)
+		: [addr] "r"  (addr), [invalid] "i" (EFAULT),
+		  [asi] "i" (ASI_MCD_REAL)
+		: "memory", "g1"
+		);
+
+	if (err)
+		return -EFAULT;
+	else
+		return ver;
+}
+
+static ssize_t adi_read(struct file *file, char __user *buf,
+			size_t count, loff_t *offp)
+{
+	size_t ver_buf_sz, bytes_read = 0;
+	int ver_buf_idx = 0;
+	loff_t offset;
+	u8 *ver_buf;
+	ssize_t ret;
+
+	ver_buf_sz = min_t(size_t, count, MAX_BUF_SZ);
+	ver_buf = kmalloc(ver_buf_sz, GFP_KERNEL);
+	if (!ver_buf)
+		return -ENOMEM;
+
+	offset = (*offp) * adi_blksize();
+
+	while (bytes_read < count) {
+		ret = read_mcd_tag(offset);
+		if (ret < 0)
+			goto out;
+
+		ver_buf[ver_buf_idx] = (u8)ret;
+		ver_buf_idx++;
+		offset += adi_blksize();
+
+		if (ver_buf_idx >= ver_buf_sz) {
+			if (copy_to_user(buf + bytes_read, ver_buf,
+					 ver_buf_sz)) {
+				ret = -EFAULT;
+				goto out;
+			}
+
+			bytes_read += ver_buf_sz;
+			ver_buf_idx = 0;
+
+			ver_buf_sz = min_t(size_t, count - bytes_read,
+					   MAX_BUF_SZ);
+		}
+	}
+
+	(*offp) += bytes_read;
+	ret = bytes_read;
+out:
+	kfree(ver_buf);
+	return ret;
+}
+
+static int set_mcd_tag(unsigned long addr, u8 ver)
+{
+	long err;
+
+	__asm__ __volatile__(
+		"1:	stxa %[ver], [%[addr]] %[asi]\n"
+		"	mov 0, %[err]\n"
+		"2:\n"
+		"	.section .fixup,#alloc,#execinstr\n"
+		"	.align 4\n"
+		"3:	sethi %%hi(2b), %%g1\n"
+		"	jmpl %%g1 + %%lo(2b), %%g0\n"
+		"	mov %[invalid], %[err]\n"
+		"	.previous\n"
+		"	.section __ex_table, \"a\"\n"
+		"	.align 4\n"
+		"	.word 1b, 3b\n"
+		"	.previous\n"
+		: [err] "=r" (err)
+		: [ver] "r" (ver), [addr] "r"  (addr),
+		  [invalid] "i" (EFAULT), [asi] "i" (ASI_MCD_REAL)
+		: "memory", "g1"
+		);
+
+	if (err)
+		return -EFAULT;
+	else
+		return ver;
+}
+
+static ssize_t adi_write(struct file *file, const char __user *buf,
+			 size_t count, loff_t *offp)
+{
+	size_t ver_buf_sz, bytes_written = 0;
+	loff_t offset;
+	u8 *ver_buf;
+	ssize_t ret;
+	int i;
+
+	if (count == 0)
+		return -EINVAL;
+
+	ver_buf_sz = min_t(size_t, count, MAX_BUF_SZ);
+	ver_buf = kmalloc(ver_buf_sz, GFP_KERNEL);
+	if (!ver_buf)
+		return -ENOMEM;
+
+	offset = (*offp) * adi_blksize();
+
+	do {
+		if (copy_from_user(ver_buf, &buf[bytes_written],
+				   ver_buf_sz)) {
+			ret = -EFAULT;
+			goto out;
+		}
+
+		for (i = 0; i < ver_buf_sz; i++) {
+			ret = set_mcd_tag(offset, ver_buf[i]);
+			if (ret < 0)
+				goto out;
+
+			offset += adi_blksize();
+		}
+
+		bytes_written += ver_buf_sz;
+		ver_buf_sz = min_t(size_t, count - bytes_written, MAX_BUF_SZ);
+	} while (bytes_written < count);
+
+	(*offp) += bytes_written;
+	ret = bytes_written;
+out:
+	__asm__ __volatile__("membar #Sync");
+	kfree(ver_buf);
+	return ret;
+}
+
+static loff_t adi_llseek(struct file *file, loff_t offset, int whence)
+{
+	loff_t ret = -EINVAL;
+
+	switch (whence) {
+	case SEEK_END:
+	case SEEK_DATA:
+	case SEEK_HOLE:
+		/* unsupported */
+		return -EINVAL;
+	case SEEK_CUR:
+		if (offset == 0)
+			return file->f_pos;
+
+		offset += file->f_pos;
+		break;
+	case SEEK_SET:
+		break;
+	}
+
+	if (offset != file->f_pos) {
+		file->f_pos = offset;
+		ret = offset;
+	}
+
+	return ret;
+}
+
+static const struct file_operations adi_fops = {
+	.owner		= THIS_MODULE,
+	.llseek		= adi_llseek,
+	.read		= adi_read,
+	.write		= adi_write,
+	.fop_flags	= FOP_UNSIGNED_OFFSET,
+};
+
+static struct miscdevice adi_miscdev = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name = KBUILD_MODNAME,
+	.fops = &adi_fops,
+};
+
+static int __init adi_init(void)
+{
+	if (!adi_capable())
+		return -EPERM;
+
+	return misc_register(&adi_miscdev);
+}
+
+static void __exit adi_exit(void)
+{
+	misc_deregister(&adi_miscdev);
+}
+
+module_init(adi_init);
+module_exit(adi_exit);
+
+MODULE_AUTHOR("Tom Hromatka <tom.hromatka@oracle.com>");
+MODULE_DESCRIPTION("Privileged interface to ADI");
+MODULE_VERSION("1.0");
+MODULE_LICENSE("GPL v2");