Introduce new example kernel module: panic_string

The panic_string module accepts input from a device file over the write(2)
system call.

It prints panic(9) message passed from user space, e.g.:

   echo "Hello world" > /dev/panic

If a string is empty or contain only whitespaces it's ignored and panic(9)
is not triggered. If a non printable character is passed within the input
buffer, the rest of the string is ignored.

This module performs the sync(8) operation before panic(9) to flush cache.

Submitted by Harry Pantazis.
Simplified and cleaned up by myself.

sys/modules/examples/Makefile

@@ -1,11 +1,12 @@
-#	$NetBSD: Makefile,v 1.5 2018/04/20 00:06:45 kamil Exp $
+#	$NetBSD: Makefile,v 1.6 2018/05/29 16:53:56 kamil Exp $
 
 .include <bsd.own.mk>
 
 SUBDIR+=	executor
 SUBDIR+=	hello
 #SUBDIR+=	luahello		# Nothing to build here, only text files
 SUBDIR+=	luareadhappy		# Needs an additional Lua script
+SUBDIR+=	panic_string		# Crashes the system
 SUBDIR+=	ping			# Needs an additional helper program
 SUBDIR+=	properties
 SUBDIR+=	readhappy

sys/modules/examples/README

@@ -1,4 +1,4 @@
-	$NetBSD: README,v 1.7 2018/04/20 00:06:45 kamil Exp $
+	$NetBSD: README,v 1.8 2018/05/29 16:53:56 kamil Exp $
 
                            Kernel Developer's Manual
 
@@ -10,8 +10,9 @@ DESCRIPTION
      * hello           - the simplest `hello world' module
      * luahello        - the simplest `hello world' Lua module
      * luareadhappy    - demonstrates calling Lua code from C
-     * properties      - handle incoming properties during the module load
+     * panic_string    - shows how panic is being called through a device
      * ping            - basic ioctl(9)
+     * properties      - handle incoming properties during the module load
      * readhappy       - basic implementation of read(9) with happy numbers
      * readhappy_mpsafe- demonstrates how to make a module MPSAFE
      * sysctl          - demonstrates adding a sysctl handle dynamically
@@ -62,5 +63,8 @@ HISTORY
      The readhappy_mpsafe, executor and sysctls modules first appeared in NetBSD
      9.0 and were authored by Siddharth Muralee.
 
+     The panic_string module first appeared in NetBSD 9.0 and was authored by
+     Harry Pantazis.
+
 AUTHORS
      This document was written by Kamil Rytarowski.

sys/modules/examples/panic_string/Makefile

@@ -0,0 +1,9 @@
+#	$NetBSD: Makefile,v 1.1 2018/05/29 16:53:56 kamil Exp $
+
+.include "../Makefile.inc"
+
+#S?=	/usr/src/sys
+KMOD=	panic_string
+SRCS=	panic_string.c
+
+.include <bsd.kmodule.mk>

sys/modules/examples/panic_string/panic_string.c

@@ -0,0 +1,196 @@
+/*	$NetBSD: panic_string.c,v 1.1 2018/05/29 16:53:56 kamil Exp $	*/
+
+/*-
+ * Copyright (c) 2018 The NetBSD Foundation, Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+ * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+ * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+
+#include <sys/cdefs.h>
+__KERNEL_RCSID(0, "$NetBSD: panic_string.c,v 1.1 2018/05/29 16:53:56 kamil Exp $");
+
+#include <sys/param.h>
+#include <sys/types.h>
+#include <sys/conf.h>
+#include <sys/device.h>
+#include <sys/filedesc.h>
+#include <sys/kernel.h>
+#include <sys/kmem.h>
+#include <sys/lwp.h>
+#include <sys/module.h>
+#include <sys/vfs_syscalls.h>
+
+/*
+ * Create a device /dev/panic from which you can read sequential
+ * user input.
+ *
+ * To use this device you need to do:
+ *      mknod /dev/panic c 210 0
+ *
+ * To write to the device you might need:
+ *      chmod 666 /dev/panic
+ *
+ * Commentary:
+ * This module manages the device /dev/panic,
+ * tranfers a string from userspace to kernel space
+ * and calls kernel panic with the passed string.
+ *
+ *  echo 'string' > /dev/panic
+ * will do the trick after loading the module.
+ */
+
+dev_type_open(panic_string_open);
+dev_type_close(panic_string_close);
+dev_type_write(panic_string_write);
+
+static struct cdevsw panic_string_cdevsw = {
+	.d_open = panic_string_open,
+	.d_close = panic_string_close,
+	.d_read = noread,
+	.d_write = panic_string_write,
+	.d_ioctl = noioctl,
+	.d_stop = nostop,
+	.d_tty = notty,
+	.d_poll = nopoll,
+	.d_mmap = nommap,
+	.d_kqfilter = nokqfilter,
+	.d_discard = nodiscard,
+	.d_flag = D_OTHER
+};
+
+static struct panic_string_softc {
+	int refcnt;
+} sc;
+
+/*
+ * A function similar to strnlen + isprint
+ *
+ * Detect length of the printable and non-whitespace string in the buffer.
+ * A string is accepted if it contains any non-space character.
+ */
+
+static size_t
+printable_length(const char *str, size_t len)
+{
+	size_t n;	
+	bool accepted;
+
+	n = 0;
+	accepted = false;
+
+	while (len > n) {
+		if (str[n] >= 0x20 && str[n] <= 0x7e) {
+			if (str[n] != 0x20 /* space */)
+				accepted = true;
+			n++;
+		} else
+			break;
+	}
+
+	if (accepted)
+		return n;
+	else
+		return 0;
+}
+
+int
+panic_string_open(dev_t self __unused, int flag __unused, int mod __unused, struct lwp *l)
+{
+
+	/* Make sure the device is opened once at a time */
+	if (sc.refcnt > 0)
+		return EBUSY;
+
+	++sc.refcnt;
+
+	return 0;
+}
+
+int
+panic_string_close(dev_t self __unused, int flag __unused, int mod __unused, struct lwp *l __unused)
+{
+
+	--sc.refcnt;
+	return 0;
+}
+
+int
+panic_string_write(dev_t self, struct uio *uio, int flags)
+{
+	size_t len, printlen;
+	char *buffer;
+
+	/* Buffer length */
+	len = uio->uio_iov->iov_len;
+
+	/* Allocate a local buffer to store the string */
+	buffer = (char *)kmem_alloc(len, KM_SLEEP);
+
+	/* Move the string from user to kernel space and store it locally */
+	uiomove(buffer, len, uio);
+
+	printlen = printable_length(buffer, len);
+
+	if (printlen > 0) {
+		/* Flushing disk changes */
+		do_sys_sync(curlwp);
+
+		panic("panic string: %.*s\n", (int)printlen, buffer);
+//		printf("panic string: %.*s\n", (int)printlen, buffer);
+
+		/* NOTREACHED */
+	}
+
+	kmem_free(buffer, len);
+	return 0;
+}
+
+MODULE(MODULE_CLASS_MISC, panic_string, NULL);
+
+static int
+panic_string_modcmd(modcmd_t cmd, void *arg __unused)
+{
+	/* The major should be verified and changed if needed to avoid
+	 * conflicts with other devices. */
+	int cmajor = 210, bmajor = -1;
+
+	switch (cmd) {
+	case MODULE_CMD_INIT:
+		printf("Panic String module loaded.\n");
+		if (devsw_attach("panic", NULL, &bmajor, &panic_string_cdevsw,
+						 &cmajor))
+			return ENXIO;
+		return 0;
+
+	case MODULE_CMD_FINI:
+		printf("Panic String module unloaded.\n");
+		if (sc.refcnt > 0)
+			return EBUSY;
+
+		devsw_detach(NULL, &panic_string_cdevsw);
+		return 0;
+	default:
+		return ENOTTY;
+	}
+}