[PATCH v2] um: Rewrite host RNG driver.

From: Alexander Neville
Date: Thu Aug 29 2019 - 13:57:00 EST

The old driver had a bug that would cause it to outright stop working if
the host's /dev/random were to block. Instead of trying to track down
the cause of said bug, rewriting it from scratch turned out to be a much
better option as it came with a few benefits:

- The new driver uses the hw_random framework.

- The code is simpler and therefore easier to maintain.

- It serves as a minimal example of writing a hardware RNG driver.

I also edited the Kconfig symbol to bring it up to more modern
standards.

Signed-off-by: Alexander Neville <dark@xxxxxxxxxxx>
---
v2:
* fix -EAGAIN handling

---
arch/um/drivers/Makefile | 3 +-
arch/um/drivers/random.c | 199 +++++++++------------------------
drivers/char/hw_random/Kconfig | 21 ++--
3 files changed, 66 insertions(+), 157 deletions(-)

diff --git a/arch/um/drivers/Makefile b/arch/um/drivers/Makefile
index 693319839f69..29b0364f267d 100644
--- a/arch/um/drivers/Makefile
+++ b/arch/um/drivers/Makefile
@@ -17,6 +17,7 @@ hostaudio-objs := hostaudio_kern.o
ubd-objs := ubd_kern.o ubd_user.o
port-objs := port_kern.o port_user.o
harddog-objs := harddog_kern.o harddog_user.o
+uml-rng-objs := random.o

LDFLAGS_pcap.o := -r $(shell $(CC) $(KBUILD_CFLAGS) -print-file-name=libpcap.a)

@@ -60,7 +61,7 @@ obj-$(CONFIG_TTY_CHAN) += tty.o
obj-$(CONFIG_XTERM_CHAN) += xterm.o xterm_kern.o
obj-$(CONFIG_UML_WATCHDOG) += harddog.o
obj-$(CONFIG_BLK_DEV_COW_COMMON) += cow_user.o
-obj-$(CONFIG_UML_RANDOM) += random.o
+obj-$(CONFIG_UML_RANDOM) += uml-rng.o

# pcap_user.o must be added explicitly.
USER_OBJS := fd.o null.o pty.o tty.o xterm.o slip_common.o pcap_user.o vde_user.o vector_user.o
diff --git a/arch/um/drivers/random.c b/arch/um/drivers/random.c
index 1d5d3057e6f1..cb8eb026df9c 100644
--- a/arch/um/drivers/random.c
+++ b/arch/um/drivers/random.c
@@ -1,175 +1,82 @@
-/* Copyright (C) 2005 - 2008 Jeff Dike <jdike@{linux.intel,addtoit}.com> */
-
-/* Much of this ripped from drivers/char/hw_random.c, see there for other
- * copyright.
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * UML Host RNG Driver
+ *
+ * (c) Copright 2019 Alexander Neville <dark@xxxxxxxxxxx>
*
- * This software may be used and distributed according to the terms
- * of the GNU General Public License, incorporated herein by reference.
+ * This file is licensed under the terms of the GNU General Public
+ * License version 2. This program is licensed "as is" without any
+ * warranty of any kind, whether express or implied.
*/
-#include <linux/sched/signal.h>
+
+#include <linux/errno.h>
+#include <linux/fcntl.h>
+#include <linux/hw_random.h>
+#include <linux/kernel.h>
#include <linux/module.h>
-#include <linux/fs.h>
-#include <linux/interrupt.h>
-#include <linux/miscdevice.h>
-#include <linux/delay.h>
-#include <linux/uaccess.h>
-#include <init.h>
-#include <irq_kern.h>
+#include <linux/types.h>
#include <os.h>

-/*
- * core module and version information
- */
-#define RNG_VERSION "1.0.0"
-#define RNG_MODULE_NAME "hw_random"
-
-#define RNG_MISCDEV_MINOR 183 /* official */
-
-/* Changed at init time, in the non-modular case, and at module load
- * time, in the module case. Presumably, the module subsystem
- * protects against a module being loaded twice at the same time.
- */
-static int random_fd = -1;
-static DECLARE_WAIT_QUEUE_HEAD(host_read_wait);
-
-static int rng_dev_open (struct inode *inode, struct file *filp)
+static int uml_rng_read(struct hwrng *rng, void *data, size_t bufsize,
+ bool wait)
{
- /* enforce read-only access to this chrdev */
- if ((filp->f_mode & FMODE_READ) == 0)
- return -EINVAL;
- if ((filp->f_mode & FMODE_WRITE) != 0)
- return -EINVAL;
+ int err = os_read_file(rng->priv, data, bufsize);

- return 0;
-}
+ // Returning -EAGAIN to userspace is not nice.
+ if (err == -EAGAIN)
+ return 0;

-static atomic_t host_sleep_count = ATOMIC_INIT(0);
+ return err;
+}

-static ssize_t rng_dev_read (struct file *filp, char __user *buf, size_t size,
- loff_t *offp)
+static int uml_rng_init(struct hwrng *rng)
{
- u32 data;
- int n, ret = 0, have_data;
-
- while (size) {
- n = os_read_file(random_fd, &data, sizeof(data));
- if (n > 0) {
- have_data = n;
- while (have_data && size) {
- if (put_user((u8) data, buf++)) {
- ret = ret ? : -EFAULT;
- break;
- }
- size--;
- ret++;
- have_data--;
- data >>= 8;
- }
- }
- else if (n == -EAGAIN) {
- DECLARE_WAITQUEUE(wait, current);
-
- if (filp->f_flags & O_NONBLOCK)
- return ret ? : -EAGAIN;
-
- atomic_inc(&host_sleep_count);
- add_sigio_fd(random_fd);
-
- add_wait_queue(&host_read_wait, &wait);
- set_current_state(TASK_INTERRUPTIBLE);
-
- schedule();
- remove_wait_queue(&host_read_wait, &wait);
-
- if (atomic_dec_and_test(&host_sleep_count)) {
- ignore_sigio_fd(random_fd);
- deactivate_fd(random_fd, RANDOM_IRQ);
- }
- }
- else
- return n;
-
- if (signal_pending (current))
- return ret ? : -ERESTARTSYS;
+ int fd = os_open_file("/dev/random", of_read(OPENFLAGS()), O_NONBLOCK);
+
+ if (fd < 0) {
+ pr_debug("uml-rng: failed to open /dev/random");
+ return fd;
}
- return ret;
-}

-static const struct file_operations rng_chrdev_ops = {
- .owner = THIS_MODULE,
- .open = rng_dev_open,
- .read = rng_dev_read,
- .llseek = noop_llseek,
-};
+ int err = os_set_fd_async(fd);

-/* rng_init shouldn't be called more than once at boot time */
-static struct miscdevice rng_miscdev = {
- RNG_MISCDEV_MINOR,
- RNG_MODULE_NAME,
- &rng_chrdev_ops,
-};
+ if (err < 0) {
+ os_close_file(fd);
+ return err;
+ }

-static irqreturn_t random_interrupt(int irq, void *data)
-{
- wake_up(&host_read_wait);
+ rng->priv = fd;

- return IRQ_HANDLED;
+ return 0;
}

-/*
- * rng_init - initialize RNG module
- */
-static int __init rng_init (void)
+static void uml_rng_cleanup(struct hwrng *rng)
{
- int err;
-
- err = os_open_file("/dev/random", of_read(OPENFLAGS()), 0);
- if (err < 0)
- goto out;
-
- random_fd = err;
-
- err = um_request_irq(RANDOM_IRQ, random_fd, IRQ_READ, random_interrupt,
- 0, "random", NULL);
- if (err)
- goto err_out_cleanup_hw;
-
- sigio_broken(random_fd, 1);
-
- err = misc_register (&rng_miscdev);
- if (err) {
- printk (KERN_ERR RNG_MODULE_NAME ": misc device register "
- "failed\n");
- goto err_out_cleanup_hw;
- }
-out:
- return err;
-
-err_out_cleanup_hw:
- os_close_file(random_fd);
- random_fd = -1;
- goto out;
+ os_close_file(rng->priv);
}

-/*
- * rng_cleanup - shutdown RNG module
- */

-static void cleanup(void)
+static struct hwrng uml_rng_ops = {
+ .name = "uml-rng",
+ .init = uml_rng_init,
+ .cleanup = uml_rng_cleanup,
+ .read = uml_rng_read,
+ .quality = 1024
+};
+
+static int __init uml_rng_mod_init(void)
{
- free_irq_by_fd(random_fd);
- os_close_file(random_fd);
+ return hwrng_register(&uml_rng_ops);
}

-static void __exit rng_cleanup(void)
+static void __exit uml_rng_mod_exit(void)
{
- os_close_file(random_fd);
- misc_deregister (&rng_miscdev);
+ hwrng_unregister(&uml_rng_ops);
}

-module_init (rng_init);
-module_exit (rng_cleanup);
-__uml_exitcall(cleanup);
+module_init(uml_rng_mod_init);
+module_exit(uml_rng_mod_exit);

-MODULE_DESCRIPTION("UML Host Random Number Generator (RNG) driver");
+MODULE_AUTHOR("Alexander Neville <dark@xxxxxxxxxxx>");
+MODULE_DESCRIPTION("UML Host RNG Driver");
MODULE_LICENSE("GPL");
diff --git a/drivers/char/hw_random/Kconfig b/drivers/char/hw_random/Kconfig
index 59f25286befe..a0781f74d4e4 100644
--- a/drivers/char/hw_random/Kconfig
+++ b/drivers/char/hw_random/Kconfig
@@ -440,22 +440,23 @@ config HW_RANDOM_OPTEE

If unsure, say Y.

-endif # HW_RANDOM
-
config UML_RANDOM
+ tristate "UML Host Random Number Generator Support"
depends on UML
- tristate "Hardware random number generator"
+ default HW_RANDOM
help
This option enables UML's "hardware" random number generator. It
attaches itself to the host's /dev/random, supplying as much entropy
as the host has, rather than the small amount the UML gets from its
- own drivers. It registers itself as a standard hardware random number
- generator, major 10, minor 183, and the canonical device name is
- /dev/hwrng.
- The way to make use of this is to install the rng-tools package
- (check your distro, or download from
- http://sourceforge.net/projects/gkernel/). rngd periodically reads
- /dev/hwrng and injects the entropy into /dev/random.
+ own drivers.
+
+ To compile this driver as a moudle, choose M here: the module
+ will be called uml-rng.
+
+ If unsure, say Y.
+
+endif # HW_RANDOM
+

config HW_RANDOM_KEYSTONE
depends on ARCH_KEYSTONE
--
2.23.0