BSD Kernel Hacking - Part 2 (character pseudo-devices)

Continuing on from the last post, I now tackle character devices. It turns out that OpenBSD is quite a lot different than FreeBSD in the way that devices are registered. To begin, I need to set up a cdevsw entry for the new device. Rather than defining the struct directly in the implementation code, it is implemented via a macro which is called in the cdevsw struct definition in sys/arch/amd64/amd64/conf.c

As opposed to FreeBSD, the cdevsw structs are built out by a macro, this macro will take the number of devices being initialized, and the name of the module passed to it (e.g. "cdev_example_init(1, example)") and fill out struct cdevsw to point to the appropriate functions via name concatenation (e.g. the open function will be registered as exampleopen).

I begin by adding a cdev_example_init macro to sys/sys/conf.h, then call the macro within the cdevsw[] array in sys/arch/amd64/amd64/conf.c. Also, while I'm in here, I add a call to the cdev_decl() macro. This should take care of the cdevsw setup portion of the code.

From there, I add example.c under sys/dev/ and build out basic open, close, read, and write functions for it, and add it to files.conf. Much to my surprise, after fixing a few compile errors for missing headers, the kernel compiles and installs fine. Once the machine had booted into my new kernel, I ran a "doas mknod /dev/example c 98 0" to create a node for the device in /dev. In this case, 98 was the position in the cdevsw[] array where my call to cdev_exampl_init landed. In OpenBSD, the major node number is simply an index into the cdevsw array.

I had to switch from using copystr to uiomove in order to get I/O working properly, but once I did, the basic interface code worked fine.

Finally, I decided to work with the MAKEDEV script to ensure that my device would be present in a release, if I so chose to make one. The script required me to add a new case for "example)" with a call to the M function and a few parameters to set the device node name, Major/Minor numbers, permissions, and ownership. In order for etc/etc.amd64/MAKEDEV to be copied to /dev by sysmerge, I had to do a full build of the system. This is a bit time consuming, and there might be a better way, but it worked for my immediate purposes.

	Index: etc/etc.amd64/MAKEDEV
	===================================================================
	RCS file: /cvs/src/etc/etc.amd64/MAKEDEV,v
	retrieving revision 1.118
	diff -u -p -u -p -r1.118 MAKEDEV
	--- etc/etc.amd64/MAKEDEV 14 Nov 2017 20:22:04 -0000 1.118
	+++ etc/etc.amd64/MAKEDEV 5 Mar 2018 19:32:55 -0000
	@@ -251,6 +251,10 @@ switch*)
	M switch$U c 97 $U 600
	;;
	+example)
	+ M example c 98 0 660
	+ ;;
	+
	pvbus*)
	M pvbus$U c 95 $U 640
	;;
	@@ -575,7 +579,7 @@ all)
	R lpa2 lpt0 lpt1 lpt2 tty00 tty01 tty02 tty03 tty04 tty05
	R tty06 tty07 tty08 tty09 tty0a tty0b ttyc0 ttyc1 ttyc2 ttyc3
	R ttyc4 ttyc5 ttyc6 ttyc7 apm pf pctr wd0 wd1 wd2 wd3 std st0
	- R st1 fd
	+ R st1 fd example
	;;
	wd*|sd*)
	Index: sys/arch/amd64/amd64/conf.c
	===================================================================
	RCS file: /cvs/src/sys/arch/amd64/amd64/conf.c,v
	retrieving revision 1.60
	diff -u -p -u -p -r1.60 conf.c
	--- sys/arch/amd64/amd64/conf.c 4 Sep 2016 10:51:23 -0000 1.60
	+++ sys/arch/amd64/amd64/conf.c 5 Mar 2018 19:33:14 -0000
	@@ -116,6 +116,7 @@ int nblkdev = nitems(bdevsw);
	#define mmread mmrw
	#define mmwrite mmrw
	cdev_decl(mm);
	+cdev_decl(example);
	cdev_decl(wd);
	#include "bio.h"
	#include "pty.h"
	@@ -297,6 +298,7 @@ struct cdevsw cdevsw[] =
	cdev_pvbus_init(NPVBUS,pvbus), /* 95: pvbus(4) control interface */
	cdev_ipmi_init(NIPMI,ipmi), /* 96: ipmi */
	cdev_switch_init(NSWITCH,switch), /* 97: switch(4) control interface */
	+ cdev_example_init(1,example) /* 98: example character device */
	};
	int nchrdev = nitems(cdevsw);
	Index: sys/conf/files
	===================================================================
	RCS file: /cvs/src/sys/conf/files,v
	retrieving revision 1.660
	diff -u -p -u -p -r1.660 files
	--- sys/conf/files 14 Feb 2018 23:51:49 -0000 1.660
	+++ sys/conf/files 5 Mar 2018 19:33:14 -0000
	@@ -430,6 +430,9 @@ file dev/ic/malo.c malo
	device bwi: ether, ifnet, ifmedia, firmload, wlan
	file dev/ic/bwi.c bwi
	+# PHE Example Device
	+file dev/example.c
	+
	# Attributes which machine-independent bus support can be attached to.
	# These should be defined here, because some of these busses can have
	# devices which provide these attributes, and we'd like to avoid hairy
	Index: sys/dev/example.c
	===================================================================
	RCS file: sys/dev/example.c
	diff -N sys/dev/example.c
	--- /dev/null 1 Jan 1970 00:00:00 -0000
	+++ sys/dev/example.c 5 Mar 2018 19:33:14 -0000
	@@ -0,0 +1,34 @@
	+#include <sys/types.h>
	+#include <sys/time.h>
	+#include <sys/uio.h>
	+#include <sys/vnode.h>
	+#include <sys/systm.h>
	+
	+static char buf[512 + 1];
	+
	+int
	+exampleopen(dev_t dev, int flag, int mode, struct proc *p)
	+{
	+ memset(&buf, '\0', 513);
	+ return 0;
	+}
	+
	+int
	+exampleclose(dev_t dev, int flag, int mode, struct proc *p)
	+{
	+ return 0;
	+}
	+
	+int
	+exampleread(dev_t dev, struct uio *uio, int flags)
	+{
	+ uiomove(&buf, 513, uio);
	+ return 0;
	+}
	+
	+int
	+examplewrite(dev_t dev, struct uio *uio, int flags)
	+{
	+ uiomove(&buf, 513, uio);
	+ return 0;
	+}
	Index: sys/sys/conf.h
	===================================================================
	RCS file: /cvs/src/sys/sys/conf.h,v
	retrieving revision 1.144
	diff -u -p -u -p -r1.144 conf.h
	--- sys/sys/conf.h 13 Jan 2018 13:03:42 -0000 1.144
	+++ sys/sys/conf.h 5 Mar 2018 19:33:15 -0000
	@@ -475,6 +475,12 @@ extern struct cdevsw cdevsw[];
	(dev_type_stop((*))) enodev, 0, (dev_type_poll((*))) enodev, \
	(dev_type_mmap((*))) enodev, 0 }
	+/* open, close, read, write */
	+#define cdev_example_init(c,n) { \
	+ dev_init(c,n,open), dev_init(c,n,close), dev_init(c,n,read), \
	+ dev_init(c,n,write), (dev_type_ioctl((*))) enodev, \
	+ (dev_type_stop((*))) enodev, 0, seltrue, (dev_type_mmap((*))) enodev }
	+
	#endif
	/*

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	#include <fcntl.h>
	#include <stdio.h>
	#include <string.h>
	#include <unistd.h>
	#include <errno.h>
	#include <stdlib.h>
	int main(int argc, char *argv[]) {
	int kernel_fd;
	size_t len;
	static char buf[512 + 1];
	errno = 0;
	kernel_fd = open(argv[1], O_RDWR);
	if (kernel_fd == -1) {
	perror("open");
	exit(EXIT_FAILURE);
	}
	len = strlen(argv[2]);
	write(kernel_fd, argv[2], len + 1);
	printf("Wrote \"%s\" to %s\n", argv[2], argv[1]);
	read(kernel_fd, buf, len + 1);
	printf("Read \"%s\" from %s\n", buf, argv[1]);
	return 0;
	}

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