sched_ext: Maintain an online cid mask in the scheduler arenagrafted

Schedulers on the default cid mapping treat [0, nr_online_cids) as the
online set and restart on hotplug. Schedulers that install their own mapping
with scx_bpf_cid_override() have no way to learn which cids are online: the
count no longer identifies members and the CPU-form cpumask is unusable from
cid programs. This is an obvious hole in the cid API.

Add scx_bpf_online_cmask(), a kernel-maintained cmask in the scheduler's
arena, allocated alongside the per-CPU scratch masks and populated after the
cid mapping is finalized and before ops.init(), for child schedulers too.
The pointer stays valid through ops.exit() with no reference to take. It is
the arena offset as a void pointer, the same form struct_ops arena arguments
arrive in. The verifier types the void return as a scalar for the program's
arena cast.

The mask follows the SCX hotplug notifications: seeded from cpu_active_mask
and updated before ops.cid_online/offline() runs, so it lags cpu_online_mask
only inside a hotplug transition. Updates walk the scheduler list under the
lock that also serializes unlinking. Reads are live, not atomic snapshots.
Root initialization excludes hotplug.

v2: Reworded the getter kerneldoc (Andrea Righi).

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Andrea Righi <arighi@nvidia.com>

1 files changed, 212 insertions, 0 deletions

diff --git a/drivers/nfc/virtual_ncidev.c b/drivers/nfc/virtual_ncidev.c
new file mode 100644
index 000000000..8eeb447ac
--- /dev/null
+++ b/drivers/nfc/virtual_ncidev.c
@@ -0,0 +1,212 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Virtual NCI device simulation driver
+ *
+ * Copyright (C) 2020 Samsung Electronics
+ * Bongsu Jeon <bongsu.jeon@samsung.com>
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/miscdevice.h>
+#include <linux/mutex.h>
+#include <linux/wait.h>
+#include <net/nfc/nci_core.h>
+
+#define IOCTL_GET_NCIDEV_IDX    0
+#define VIRTUAL_NFC_PROTOCOLS	(NFC_PROTO_JEWEL_MASK | \
+				 NFC_PROTO_MIFARE_MASK | \
+				 NFC_PROTO_FELICA_MASK | \
+				 NFC_PROTO_ISO14443_MASK | \
+				 NFC_PROTO_ISO14443_B_MASK | \
+				 NFC_PROTO_ISO15693_MASK)
+
+struct virtual_nci_dev {
+	struct nci_dev *ndev;
+	struct mutex mtx;
+	struct sk_buff *send_buff;
+	struct wait_queue_head wq;
+	bool running;
+};
+
+static int virtual_nci_open(struct nci_dev *ndev)
+{
+	struct virtual_nci_dev *vdev = nci_get_drvdata(ndev);
+
+	vdev->running = true;
+	return 0;
+}
+
+static int virtual_nci_close(struct nci_dev *ndev)
+{
+	struct virtual_nci_dev *vdev = nci_get_drvdata(ndev);
+
+	mutex_lock(&vdev->mtx);
+	kfree_skb(vdev->send_buff);
+	vdev->send_buff = NULL;
+	vdev->running = false;
+	mutex_unlock(&vdev->mtx);
+
+	return 0;
+}
+
+static int virtual_nci_send(struct nci_dev *ndev, struct sk_buff *skb)
+{
+	struct virtual_nci_dev *vdev = nci_get_drvdata(ndev);
+
+	mutex_lock(&vdev->mtx);
+	if (vdev->send_buff || !vdev->running) {
+		mutex_unlock(&vdev->mtx);
+		kfree_skb(skb);
+		return -1;
+	}
+	vdev->send_buff = skb_copy(skb, GFP_KERNEL);
+	if (!vdev->send_buff) {
+		mutex_unlock(&vdev->mtx);
+		kfree_skb(skb);
+		return -1;
+	}
+	mutex_unlock(&vdev->mtx);
+	wake_up_interruptible(&vdev->wq);
+	consume_skb(skb);
+
+	return 0;
+}
+
+static const struct nci_ops virtual_nci_ops = {
+	.open = virtual_nci_open,
+	.close = virtual_nci_close,
+	.send = virtual_nci_send
+};
+
+static ssize_t virtual_ncidev_read(struct file *file, char __user *buf,
+				   size_t count, loff_t *ppos)
+{
+	struct virtual_nci_dev *vdev = file->private_data;
+	size_t actual_len;
+
+	mutex_lock(&vdev->mtx);
+	while (!vdev->send_buff) {
+		mutex_unlock(&vdev->mtx);
+		if (wait_event_interruptible(vdev->wq, vdev->send_buff))
+			return -EFAULT;
+		mutex_lock(&vdev->mtx);
+	}
+
+	actual_len = min_t(size_t, count, vdev->send_buff->len);
+
+	if (copy_to_user(buf, vdev->send_buff->data, actual_len)) {
+		mutex_unlock(&vdev->mtx);
+		return -EFAULT;
+	}
+
+	skb_pull(vdev->send_buff, actual_len);
+	if (vdev->send_buff->len == 0) {
+		consume_skb(vdev->send_buff);
+		vdev->send_buff = NULL;
+	}
+	mutex_unlock(&vdev->mtx);
+
+	return actual_len;
+}
+
+static ssize_t virtual_ncidev_write(struct file *file,
+				    const char __user *buf,
+				    size_t count, loff_t *ppos)
+{
+	struct virtual_nci_dev *vdev = file->private_data;
+	struct sk_buff *skb;
+
+	skb = alloc_skb(count, GFP_KERNEL);
+	if (!skb)
+		return -ENOMEM;
+
+	if (copy_from_user(skb_put(skb, count), buf, count)) {
+		kfree_skb(skb);
+		return -EFAULT;
+	}
+
+	nci_recv_frame(vdev->ndev, skb);
+	return count;
+}
+
+static int virtual_ncidev_open(struct inode *inode, struct file *file)
+{
+	int ret = 0;
+	struct virtual_nci_dev *vdev;
+
+	vdev = kzalloc_obj(*vdev);
+	if (!vdev)
+		return -ENOMEM;
+	vdev->ndev = nci_allocate_device(&virtual_nci_ops,
+		VIRTUAL_NFC_PROTOCOLS, 0, 0);
+	if (!vdev->ndev) {
+		kfree(vdev);
+		return -ENOMEM;
+	}
+
+	mutex_init(&vdev->mtx);
+	init_waitqueue_head(&vdev->wq);
+	file->private_data = vdev;
+	nci_set_drvdata(vdev->ndev, vdev);
+
+	ret = nci_register_device(vdev->ndev);
+	if (ret < 0) {
+		nci_free_device(vdev->ndev);
+		mutex_destroy(&vdev->mtx);
+		kfree(vdev);
+		return ret;
+	}
+
+	return 0;
+}
+
+static int virtual_ncidev_close(struct inode *inode, struct file *file)
+{
+	struct virtual_nci_dev *vdev = file->private_data;
+
+	nci_unregister_device(vdev->ndev);
+	nci_free_device(vdev->ndev);
+	mutex_destroy(&vdev->mtx);
+	kfree(vdev);
+
+	return 0;
+}
+
+static long virtual_ncidev_ioctl(struct file *file, unsigned int cmd,
+				 unsigned long arg)
+{
+	struct virtual_nci_dev *vdev = file->private_data;
+	const struct nfc_dev *nfc_dev = vdev->ndev->nfc_dev;
+	void __user *p = (void __user *)arg;
+
+	if (cmd != IOCTL_GET_NCIDEV_IDX)
+		return -ENOTTY;
+
+	if (copy_to_user(p, &nfc_dev->idx, sizeof(nfc_dev->idx)))
+		return -EFAULT;
+
+	return 0;
+}
+
+static const struct file_operations virtual_ncidev_fops = {
+	.owner = THIS_MODULE,
+	.read = virtual_ncidev_read,
+	.write = virtual_ncidev_write,
+	.open = virtual_ncidev_open,
+	.release = virtual_ncidev_close,
+	.unlocked_ioctl = virtual_ncidev_ioctl
+};
+
+static struct miscdevice miscdev = {
+	.minor = MISC_DYNAMIC_MINOR,
+	.name = "virtual_nci",
+	.fops = &virtual_ncidev_fops,
+	.mode = 0600,
+};
+
+module_misc_device(miscdev);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Virtual NCI device simulation driver");
+MODULE_AUTHOR("Bongsu Jeon <bongsu.jeon@samsung.com>");