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, 351 insertions, 0 deletions

diff --git a/io_uring/mock_file.c b/io_uring/mock_file.c
new file mode 100644
index 000000000..b318ed697
--- /dev/null
+++ b/io_uring/mock_file.c
@@ -0,0 +1,351 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/device.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/miscdevice.h>
+#include <linux/module.h>
+#include <linux/anon_inodes.h>
+#include <linux/ktime.h>
+#include <linux/hrtimer.h>
+#include <linux/poll.h>
+
+#include <linux/io_uring/cmd.h>
+#include <linux/io_uring_types.h>
+#include <uapi/linux/io_uring/mock_file.h>
+
+struct io_mock_iocb {
+	struct kiocb		*iocb;
+	struct hrtimer		timer;
+	int			res;
+};
+
+struct io_mock_file {
+	size_t			size;
+	u64			rw_delay_ns;
+	bool			pollable;
+	struct wait_queue_head	poll_wq;
+};
+
+#define IO_VALID_COPY_CMD_FLAGS		IORING_MOCK_COPY_FROM
+
+static int io_copy_regbuf(struct iov_iter *reg_iter, void __user *ubuf)
+{
+	size_t ret, copied = 0;
+	size_t buflen = PAGE_SIZE;
+	void *tmp_buf;
+
+	tmp_buf = kzalloc(buflen, GFP_KERNEL);
+	if (!tmp_buf)
+		return -ENOMEM;
+
+	while (iov_iter_count(reg_iter)) {
+		size_t len = min(iov_iter_count(reg_iter), buflen);
+
+		if (iov_iter_rw(reg_iter) == ITER_SOURCE) {
+			ret = copy_from_iter(tmp_buf, len, reg_iter);
+			if (ret <= 0)
+				break;
+			if (copy_to_user(ubuf, tmp_buf, ret))
+				break;
+		} else {
+			if (copy_from_user(tmp_buf, ubuf, len))
+				break;
+			ret = copy_to_iter(tmp_buf, len, reg_iter);
+			if (ret <= 0)
+				break;
+		}
+		ubuf += ret;
+		copied += ret;
+	}
+
+	kfree(tmp_buf);
+	return copied;
+}
+
+static int io_cmd_copy_regbuf(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	const struct io_uring_sqe *sqe = cmd->sqe;
+	const struct iovec __user *iovec;
+	unsigned flags, iovec_len;
+	struct iov_iter iter;
+	void __user *ubuf;
+	int dir, ret;
+
+	ubuf = u64_to_user_ptr(READ_ONCE(sqe->addr3));
+	iovec = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	iovec_len = READ_ONCE(sqe->len);
+	flags = READ_ONCE(sqe->file_index);
+
+	if (unlikely(sqe->ioprio || sqe->__pad1))
+		return -EINVAL;
+	if (flags & ~IO_VALID_COPY_CMD_FLAGS)
+		return -EINVAL;
+
+	dir = (flags & IORING_MOCK_COPY_FROM) ? ITER_SOURCE : ITER_DEST;
+	ret = io_uring_cmd_import_fixed_vec(cmd, iovec, iovec_len, dir, &iter,
+					    issue_flags);
+	if (ret)
+		return ret;
+	ret = io_copy_regbuf(&iter, ubuf);
+	return ret ? ret : -EFAULT;
+}
+
+static int io_mock_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	switch (cmd->cmd_op) {
+	case IORING_MOCK_CMD_COPY_REGBUF:
+		return io_cmd_copy_regbuf(cmd, issue_flags);
+	}
+	return -ENOTSUPP;
+}
+
+static enum hrtimer_restart io_mock_rw_timer_expired(struct hrtimer *timer)
+{
+	struct io_mock_iocb *mio = container_of(timer, struct io_mock_iocb, timer);
+	struct kiocb *iocb = mio->iocb;
+
+	WRITE_ONCE(iocb->private, NULL);
+	iocb->ki_complete(iocb, mio->res);
+	kfree(mio);
+	return HRTIMER_NORESTART;
+}
+
+static ssize_t io_mock_delay_rw(struct kiocb *iocb, size_t len)
+{
+	struct io_mock_file *mf = iocb->ki_filp->private_data;
+	struct io_mock_iocb *mio;
+
+	mio = kzalloc_obj(*mio);
+	if (!mio)
+		return -ENOMEM;
+
+	mio->iocb = iocb;
+	mio->res = len;
+	hrtimer_setup(&mio->timer, io_mock_rw_timer_expired,
+		      CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_start(&mio->timer, ns_to_ktime(mf->rw_delay_ns),
+		      HRTIMER_MODE_REL);
+	return -EIOCBQUEUED;
+}
+
+static ssize_t io_mock_read_iter(struct kiocb *iocb, struct iov_iter *to)
+{
+	struct io_mock_file *mf = iocb->ki_filp->private_data;
+	size_t len = iov_iter_count(to);
+	size_t nr_zeroed;
+
+	if (iocb->ki_pos + len > mf->size)
+		return -EINVAL;
+	nr_zeroed = iov_iter_zero(len, to);
+	if (!mf->rw_delay_ns || nr_zeroed != len)
+		return nr_zeroed;
+
+	return io_mock_delay_rw(iocb, len);
+}
+
+static ssize_t io_mock_write_iter(struct kiocb *iocb, struct iov_iter *from)
+{
+	struct io_mock_file *mf = iocb->ki_filp->private_data;
+	size_t len = iov_iter_count(from);
+
+	if (iocb->ki_pos + len > mf->size)
+		return -EINVAL;
+	if (!mf->rw_delay_ns) {
+		iov_iter_advance(from, len);
+		return len;
+	}
+
+	return io_mock_delay_rw(iocb, len);
+}
+
+static loff_t io_mock_llseek(struct file *file, loff_t offset, int whence)
+{
+	struct io_mock_file *mf = file->private_data;
+
+	return fixed_size_llseek(file, offset, whence, mf->size);
+}
+
+static __poll_t io_mock_poll(struct file *file, struct poll_table_struct *pt)
+{
+	struct io_mock_file *mf = file->private_data;
+	__poll_t mask = 0;
+
+	poll_wait(file, &mf->poll_wq, pt);
+
+	mask |= EPOLLOUT | EPOLLWRNORM;
+	mask |= EPOLLIN | EPOLLRDNORM;
+	return mask;
+}
+
+static int io_mock_release(struct inode *inode, struct file *file)
+{
+	struct io_mock_file *mf = file->private_data;
+
+	kfree(mf);
+	return 0;
+}
+
+static const struct file_operations io_mock_fops = {
+	.owner		= THIS_MODULE,
+	.release	= io_mock_release,
+	.uring_cmd	= io_mock_cmd,
+	.read_iter	= io_mock_read_iter,
+	.write_iter	= io_mock_write_iter,
+	.llseek		= io_mock_llseek,
+};
+
+static const struct file_operations io_mock_poll_fops = {
+	.owner		= THIS_MODULE,
+	.release	= io_mock_release,
+	.uring_cmd	= io_mock_cmd,
+	.read_iter	= io_mock_read_iter,
+	.write_iter	= io_mock_write_iter,
+	.llseek		= io_mock_llseek,
+	.poll		= io_mock_poll,
+};
+
+#define IO_VALID_CREATE_FLAGS (IORING_MOCK_CREATE_F_SUPPORT_NOWAIT | \
+				IORING_MOCK_CREATE_F_POLL)
+
+static int io_create_mock_file(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	const struct file_operations *fops = &io_mock_fops;
+	const struct io_uring_sqe *sqe = cmd->sqe;
+	struct io_uring_mock_create mc, __user *uarg;
+	struct file *file;
+	struct io_mock_file *mf __free(kfree) = NULL;
+	size_t uarg_size;
+
+	/*
+	 * It's a testing only driver that allows exercising edge cases
+	 * that wouldn't be possible to hit otherwise.
+	 */
+	add_taint(TAINT_TEST, LOCKDEP_STILL_OK);
+
+	uarg = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	uarg_size = READ_ONCE(sqe->len);
+
+	if (sqe->ioprio || sqe->__pad1 || sqe->addr3 || sqe->file_index)
+		return -EINVAL;
+	if (uarg_size != sizeof(mc))
+		return -EINVAL;
+
+	memset(&mc, 0, sizeof(mc));
+	if (copy_from_user(&mc, uarg, uarg_size))
+		return -EFAULT;
+	if (!mem_is_zero(mc.__resv, sizeof(mc.__resv)))
+		return -EINVAL;
+	if (mc.flags & ~IO_VALID_CREATE_FLAGS)
+		return -EINVAL;
+	if (mc.file_size > SZ_1G)
+		return -EINVAL;
+	if (mc.rw_delay_ns > NSEC_PER_SEC)
+		return -EINVAL;
+
+	mf = kzalloc_obj(*mf, GFP_KERNEL_ACCOUNT);
+	if (!mf)
+		return -ENOMEM;
+
+	init_waitqueue_head(&mf->poll_wq);
+	mf->size = mc.file_size;
+	mf->rw_delay_ns = mc.rw_delay_ns;
+	if (mc.flags & IORING_MOCK_CREATE_F_POLL) {
+		fops = &io_mock_poll_fops;
+		mf->pollable = true;
+	}
+
+	FD_PREPARE(fdf, O_RDWR | O_CLOEXEC,
+		   anon_inode_create_getfile("[io_uring_mock]", fops, mf,
+					     O_RDWR | O_CLOEXEC, NULL));
+	if (fdf.err)
+		return fdf.err;
+
+	retain_and_null_ptr(mf);
+	file = fd_prepare_file(fdf);
+	file->f_mode |= FMODE_READ | FMODE_CAN_READ | FMODE_WRITE |
+			FMODE_CAN_WRITE | FMODE_LSEEK;
+	if (mc.flags & IORING_MOCK_CREATE_F_SUPPORT_NOWAIT)
+		file->f_mode |= FMODE_NOWAIT;
+
+	mc.out_fd = fd_prepare_fd(fdf);
+	if (copy_to_user(uarg, &mc, uarg_size))
+		return -EFAULT;
+
+	fd_publish(fdf);
+	return 0;
+}
+
+static int io_probe_mock(struct io_uring_cmd *cmd)
+{
+	const struct io_uring_sqe *sqe = cmd->sqe;
+	struct io_uring_mock_probe mp, __user *uarg;
+	size_t uarg_size;
+
+	uarg = u64_to_user_ptr(READ_ONCE(sqe->addr));
+	uarg_size = READ_ONCE(sqe->len);
+
+	if (sqe->ioprio || sqe->__pad1 || sqe->addr3 || sqe->file_index ||
+	    uarg_size != sizeof(mp))
+		return -EINVAL;
+
+	memset(&mp, 0, sizeof(mp));
+	if (copy_from_user(&mp, uarg, uarg_size))
+		return -EFAULT;
+	if (!mem_is_zero(&mp, sizeof(mp)))
+		return -EINVAL;
+
+	mp.features = IORING_MOCK_FEAT_END;
+
+	if (copy_to_user(uarg, &mp, uarg_size))
+		return -EFAULT;
+	return 0;
+}
+
+static int iou_mock_mgr_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	switch (cmd->cmd_op) {
+	case IORING_MOCK_MGR_CMD_PROBE:
+		return io_probe_mock(cmd);
+	case IORING_MOCK_MGR_CMD_CREATE:
+		return io_create_mock_file(cmd, issue_flags);
+	}
+	return -EOPNOTSUPP;
+}
+
+static const struct file_operations iou_mock_dev_fops = {
+	.owner		= THIS_MODULE,
+	.uring_cmd	= iou_mock_mgr_cmd,
+};
+
+static struct miscdevice iou_mock_miscdev = {
+	.minor			= MISC_DYNAMIC_MINOR,
+	.name			= "io_uring_mock",
+	.fops			= &iou_mock_dev_fops,
+};
+
+static int __init io_mock_init(void)
+{
+	int ret;
+
+	ret = misc_register(&iou_mock_miscdev);
+	if (ret < 0) {
+		pr_err("Could not initialize io_uring mock device\n");
+		return ret;
+	}
+	return 0;
+}
+
+static void __exit io_mock_exit(void)
+{
+	misc_deregister(&iou_mock_miscdev);
+}
+
+module_init(io_mock_init)
+module_exit(io_mock_exit)
+
+MODULE_AUTHOR("Pavel Begunkov <asml.silence@gmail.com>");
+MODULE_DESCRIPTION("io_uring mock file");
+MODULE_LICENSE("GPL");