Would it be worth adding this somewhere as a sample piece of
	code people could reference?

		David/absolute

                  -=-  Resentment is a renewable resource  -=-

On Tue, 12 Jan 1999, Michael L. Hitch wrote:

> On Tue, 12 Jan 1999, David Maxwell wrote:
> 
> > On Tue, Jan 12, 1999 at 12:08:25AM -0800, dustin sallings wrote:
> > > 
> > > 	For a kernel modification I'm doing, I need to be able to build
> > > and maintain a simple array of (arrays|hashes|linked lists) from a
> > > userland process.
> > > 
> > > 	I don't have the slightest idea where to put this in the kernel,
> > > what to name it, or how to interface it.  Can someone give me an idea?
> > 
> > I'll tag onto this message and ask... I'd like to write a userland program
> > which can read the kernel disklist linked list. I've looked into the 
> > kernel memory syscalls in a fair amount of detail, and looked at some
> > examples in the tree, but I can't find anything in the tree that
> > reads _structures_ from the kernel, just simple variables. I haven't
> > been able to make my code work.
> 
>   Here's a simple example.  I don't remember where I got this - it doesn't
> look like I wrote it, although I may have modified it somewhat.  [The
> program is almost 3 years old.]  I have written a number of little
> kernel-groveling programs like this to make it easier to see what's going
> on in a live kernel.
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <unistd.h>
> 
> #include <sys/dkstat.h>
> #include <sys/time.h>
> #include <sys/disk.h>
> 
> #include <err.h>
> #include <fcntl.h>
> #include <kvm.h>
> #include <limits.h>
> #include <nlist.h>
> 
> static struct nlist namelist[] = {
> #define	X_TK_NIN	0
> 	{ "_tk_nin" },		/* tty characters in */
> #define	X_TK_NOUT	1
> 	{ "_tk_nout" },		/* tty characters out */
> #define	X_CP_TIME	2
> 	{ "_cp_time" },		/* system timer ticks */
> #define	X_HZ		3
> 	{ "_hz" },		/* ticks per second */
> #define	X_STATHZ	4
> 	{ "_stathz" },
> #define X_DISK_COUNT	5
> 	{ "_disk_count" },	/* number of disks */
> #define X_DISKLIST	6
> 	{ "_disklist" },	/* TAILQ of disks */
> 	{ NULL },
> };
> 
> /* Kernel pointers: nlistf and memf defined in calling program. */
> static kvm_t	*kd = NULL;
> char	*nlistf;
> char	*memf;
> 
> /* Pointer to list of disks. */
> static struct disk	*dk_drivehead = NULL;
> 
> /* Backward compatibility references. */
> int	  	dk_ndrive = 0;
> 
> #define	KVM_ERROR(_string) {						\
> 	warnx((_string));						\
> 	errx(1, kvm_geterr(kd));					\
> }
> 
> /*
>  * Dereference the namelist pointer `v' and fill in the local copy 
>  * 'p' which is of size 's'.
>  */
> #define deref_nl(v, p, s) deref_kptr((void *)namelist[(v)].n_value, (p), (s));
> 
> static void deref_kptr __P((void *, void *, size_t));
> 
> main(int argc, char *argv[])
> {
> 	int i;
> 	struct disklist_head disk_head;
> 	struct disk	cur_disk, *p;
>         char		errbuf[_POSIX2_LINE_MAX];
> 
>         if ((kd = kvm_openfiles(nlistf, memf, NULL, O_RDONLY, errbuf)) == NULL)
> 		errx(1, "kvm_openfiles: %s", errbuf);
> 
> 	/* Obtain the namelist symbols from the kernel. */
> 	if (kvm_nlist(kd, namelist))
> 		KVM_ERROR("kvm_nlist failed to read symbols.");
> 
> 	/* Get the number of attached drives. */
> 	deref_nl(X_DISK_COUNT, &dk_ndrive, sizeof(dk_ndrive));
> 
> 	if (dk_ndrive < 0)
> 		errx(1, "invalid _disk_count %d.", dk_ndrive);
> 	else if (dk_ndrive == 0) {
> 		warnx("No drives attached.");
> 	}
> 	else {
> 		/* Get a pointer to the first disk. */
> 		deref_nl(X_DISKLIST, &disk_head, sizeof(disk_head));
> 		dk_drivehead = disk_head.tqh_first;
> 	}
> 
> 	printf("%d drives, header %x\n", dk_ndrive, dk_drivehead);
> 
> 	p = dk_drivehead;
> 	printf("Address  Name     Transfers   Seeks     Bytes\n");
> 	for (i = 0; i < dk_ndrive; ++i) {
> 		deref_kptr(p, &cur_disk, sizeof(cur_disk));
> 		deref_kptr(cur_disk.dk_name, errbuf, 10);
> 		printf("%08x %-10s%8lld%8lld%10lld\n", p, errbuf,
> 		    cur_disk.dk_xfer, cur_disk.dk_seek, cur_disk.dk_bytes);
> 		p = cur_disk.dk_link.tqe_next;
> 	}
> }
> 
> /*
>  * Dereference the kernel pointer `kptr' and fill in the local copy 
>  * pointed to by `ptr'.  The storage space must be pre-allocated,
>  * and the size of the copy passed in `len'.
>  */
> static void
> deref_kptr(kptr, ptr, len)
> 	void *kptr, *ptr;
> 	size_t len;
> {
> 	char buf[128];
> 
> 	if (kvm_read(kd, (u_long)kptr, (char *)ptr, len) != len) {
> 		bzero(buf, sizeof(buf));
> 		snprintf(buf, (sizeof(buf) - 1),
> 		     "can't dereference kptr 0x%lx", (u_long)kptr);
> 		KVM_ERROR(buf);
> 	}
> }
> 
> ---
> Michael L. Hitch			mhitch@montana.edu
> Computer Consultant
> Information Technology Center
> Montana State University	Bozeman, MT	USA
>