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From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.bugs.4bsd
Subject: Re: VAX 4.2bsd /dev/kmem panic
Message-ID: <438@umcp-cs.UUCP>
Date: Sat, 22-Mar-86 13:30:27 EST
Article-I.D.: umcp-cs.438
Posted: Sat Mar 22 13:30:27 1986
Date-Received: Mon, 24-Mar-86 03:41:31 EST
References: <222@nyit.UUCP>
Distribution: net
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 26

My 4.3ish kernel does not crash with Rick's example, but the bug
is real.  As a quick fix, I would suggest just disallowing accesses
to discontiguous regions.  (The only one that makes even slight
sense is P1/S space, and no current programs cross the boundary in
a single read, or at least not intentionally.)

Change the very beginning of kernacc to read as follows (just add
the lines marked):

ENTRY(kernacc, 0)		# in 4.2 it was `_kernacc:', I think
	movl	4(ap),r0	# virtual address
+ 	bicl3	$0x3fffffff,r0,r1
+ 	addl3	8(ap),r0,r2	# ending virtual address
+ 	bicl2	$0x3fffffff,r2
+ 	cmpl	r1,r2		# same region?
+ 	bneq	kacerr		# no, disallow it
	bbcc	$31,r0,kacc1
	.
	.
	.

Warning: the above is untested.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 1415)
UUCP:	seismo!umcp-cs!chris
CSNet:	chris@umcp-cs		ARPA:	chris@mimsy.umd.edu