xp.c driver fix for BSD2_10

[Jack Nelson]

Index:	/usr/src/sys/pdpuba/xp.c  2.10BSD
Index:	/usr/src/sys/pdpuba/hk.c  2.10BSD
Description:
	BADSECT handling is wrong in xp.c and hk.c of BSD2_10
Repeat-By:
	Do dd if=/dev/rxp0c of=/dev/null bs=10k on a partition
	that has badblocks and watch the system hang up or crash with
	"kernel red stack violation" panic.  In some cases, the file
	system gets munged up, giving a spurious "file system full"
	message.  Usually when trying to backup with tar in my experience.
	  Put a printf in xpecc() code where calculations being done
        of "ndone" and "wc" on CONT return from revectoring; watch how
	the wrong numbers get generated:
	    The main error is division by NBPW in line 857 of xp.c; since
	    that is defined as sizeof(int), and sizeof is an unsigned operator,
	    the division is promoted to unsigned, and the result is a nonsense
	    number in the 32000's range, i.e. the unsigned value shifted right
	    one bit as an unsigned number.
	  Probably NBPW was "#define NBPW 2" at some time in the past,
	  and was redefined "sizeof(int)" without considering this unsigned
	  property of the sizeof operator.  A simple C program will prove
	  the point: 
		printf("%d\n",-10/2);
			output: -5
		printf("%d\n",-10/sizeof(int));
			output: 32763
Fix:
	apply the patch below, which casts NBPG to (int) and does a bit
	of cosmetic rearrangement of the code to express its purpose better.
	"wc" is supposed to be the negative count of words to be read,
	as fed into the controller register.

UUCP: { akgua | allegra | cmcl2 | idis | ihnp4 | mi-cec | pitt |
	psuvax1 | sun | sunrise | vax135 } ! cadre ! nelslab ! jack
ARPA: jack at cadre.dsl.pittsburgh.edu
John P. Nelson, M.D., 3811 O'Hara St, Pittsburgh, PA 15213, t:412-624-1769
Dept. of Psychiatry, U. of Pittsburgh
Cut:----------

*** ../orgs/xp.c	Sun Aug 30 22:47:05 1987
--- xp.c.badfix	Sun Apr 24 20:57:20 1988
***************
*** 854,865 ****
  		npx = bp->b_error;
  		bp->b_error = 0;
  		ndone = npx * NBPG;
! 		wc = ((int)(ndone - bp->b_bcount)) / NBPW;
  	}
  	else {
  #endif
  		wc = xpaddr->hpwc;
! 		ndone = (wc * NBPW) + bp->b_bcount;
  		npx = ndone / NBPG;
  #ifdef BADSECT
  	}
--- 854,865 ----
  		npx = bp->b_error;
  		bp->b_error = 0;
  		ndone = npx * NBPG;
! 		wc = -((short)(bp->b_bcount - ndone) / (short)NBPW);
  	}
  	else {
  #endif
  		wc = xpaddr->hpwc;
! 		ndone = bp->b_bcount - ((unsigned)(-wc) * NBPW) ;
  		npx = ndone / NBPG;
  #ifdef BADSECT
  	}
***************
*** 929,935 ****
  #ifdef DEBUG
  			printf("revector to cn %d tn %d sn %d\n", cn, tn, sn);
  #endif
! 			wc = -(512 / NBPW);
  			break;
  		case CONT:
  			bp->b_flags &= ~B_BAD;
--- 929,935 ----
  #ifdef DEBUG
  			printf("revector to cn %d tn %d sn %d\n", cn, tn, sn);
  #endif
! 			wc = -(512 / (short)NBPW);
  			break;
  		case CONT:
  			bp->b_flags &= ~B_BAD;
-- 
John P. Nelson, M.D., 3811 O'Hara St, Pittsburgh, PA 15213, t:412-624-1769 Dept. of Psychiatry, U. of Pittsburgh
UUCP: { akgua | allegra | cmcl2 | idis | ihnp4 | mi-cec | pitt | psuvax1 | sun | sunrise | vax135 } ! cadre ! jack
ARPA: jack at cadre.dsl.pittsburgh.edu