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From: stevesu@copper.TEK.COM (Steve Summit)
Newsgroups: comp.unix.wizards
Subject: Re: Debugging the kernel: proper methods?
Message-ID: <1168@copper.TEK.COM>
Date: Thu, 25-Jun-87 23:58:35 EDT
Article-I.D.: copper.1168
Posted: Thu Jun 25 23:58:35 1987
Date-Received: Sat, 27-Jun-87 05:30:44 EDT
References: <2713@uw-june.UUCP> <479@winchester.UUCP>
Organization: Tektronix Inc., Beaverton, Or.
Lines: 64

Nobody has mentioned this yet, so I'll toss it in, although I
don't know the full details: someone once built adb into the
kernel, so it could "debug itself," so to speak.  You typed some
magic command, and the console terminal started talking adb.
You could set breakpoints and everything.

Building adb into a program is actually easier than it sounds.
The only real problem is uniqueifying all of its global variables
so they don't clash with those in your program.  You also need a
special version of ptrace that can examine and modify locations
in your own process instead of another one.

I built a copy of adb into a window manager I was working on --
invoking a special window manager command would cause a new
window to open up with adb "running" in it.  This was handy
because using conventional adb on a full-screen, interactive
process like a window manager is tricky (and requires two
terminals), and because most of the problems this window manager
had were hard to reproduce, but with adb "already there," you
could track down a bug when it appeared, rather than having to
recapture it in a later run under adb.

(Actually, I lied: it's only easy to build adb in if you leave
out breakpoints, which I did, because all I really wanted to do
was examine data structures.  Getting breakpoints to work would
require a writable text segment and a _r_e_a_l clever SIGTRAP handler.)

					Steve Summit
					stevesu@copper.tek.com

P.S.  Here, for your amusement, is the "special version of ptrace
that can examine and modify locations in your own process."  As
you can see, it's not rippingly difficult to write.  (Of course,
it ignores the requests having to do with running the other
process.  It also doesn't handle the u-area stuff)

	ptrace(request, pid, addr, data)
	int request;
	int pid;
	int *addr;
	int data;
	{
	switch(request)
		{
		case 0:
			return(0);

		case 1:
		case 2:			/* sorry, no split I&D */
			return(*addr);

		case 3:
			return(*addr);		/* ??? */

		case 4:
		case 5:			/* sorry, no split I&D */
			*addr = data;
			return(0);

		case 6:
			*addr = data;		/* ??? */
			return(0);
		}
	}