discolo@ucsbcsl.UUCP (Anthony V. Discolo) (07/18/84)
> We are experiencing a semi-persistant and annoying (at the very least) > problem with getting "panic: munhash" when debugging (usually large programs). > Upon examination of the vmcore file generated it would seem the problem > comes when the system wants to change the protection on a text page of > the child process, probably for the purpose of altering the instruction > where a breakpoint existed, however it isn't necessarily clear. We are > running 4.2BSD on a 750 with 2Mbytes. The programs being debugged are on > the order of at least 1/2Mbyte in size. For anyone who would like to > tackle this problem, I am providing the (short) traceback below. This is a pretty common 4.xBSD bug. The following is a BSD bug report that I've had around for a while: %%% BEGIN BUG REPORT %%% Return-Path: <mjb%Brown@UDel-Relay> Received: from ucbvax.ARPA by ucbarpa.ARPA (4.10/4.7) id AA23877; Thu, 6 Oct 83 15:13:57 PDT Received: from udel-relay.ARPA by ucbvax.ARPA (4.12/4.7) id AA17869; Mon, 3 Oct 83 11:42:44 PDT Message-Id: <8310031842.AA17869@ucbvax.ARPA> Date: 3 Oct 83 (Monday) 03:06 EDT From: Mike Braca <mjb%Brown@UDel-Relay> Return-Path: <mjb%Brown@UDel-Relay> Subject: Using sdb or dbx can crash the system (panic: munhash) To: 4bsd-bugs@BERKELEY Via: Brown; 3 Oct 83 13:21-EDT Index: sys/vax/vm_machdep.c 4.1c Fix Description: You can crash the system using breakpoints in sdb or dbx (causing "panic: munhash"). Because of a bug in the compiler, incorrect code is generated in vax/vm_machdep.c. The compiler error is that a bit field which is declared as "unsigned int" gets its sign extended when it is cast to "long". The code affected is in routine chgprot() and is a parameter passed in a call to munhash() which reads: (daddr_t)c->c_blkno where "daddr_t" is typedef'ed to "long", "c" is a pointer to a "cmap" structure, and "c_blkno" is a 20-bit "unsigned int" field. If the file system is > 256MB, then the MSB of the bit field can be on, and what is passed to munhash() is a negative block number which it won't find, so it panics. Repeat-By: This is real tough to repeat. First you need a file system that's larger than 256MB. Then you need an executable binary file that resides in blocks numbered higher than 256MB on that fs. Run sdb or dbx on the file and use breakpoints. The system will crash when it's trying to make one of the text blocks writeable in order to set the breakpoint. Just look at the assembly language produced by compiling vm_machdep.c , or disassemble the code using adb. Notice that an "extv" instruction is generated to extract c->c_blkno in the mentioned case, rather than "extzv". Fix: Fix the compiler! Or do this: *** vm_machdep.c.ucb Fri Dec 17 14:57:33 1982 --- vm_machdep.c Wed Aug 24 03:33:08 1983 *************** *** 117,123 if (pte->pg_fod == 0 && pte->pg_pfnum) { c = &cmap[pgtocm(pte->pg_pfnum)]; if (c->c_blkno && c->c_mdev != MSWAPX) ! munhash(mount[c->c_mdev].m_dev, (daddr_t)c->c_blkno); } *(int *)pte &= ~PG_PROT; *(int *)pte |= tprot; --- 117,124 ----- if (pte->pg_fod == 0 && pte->pg_pfnum) { c = &cmap[pgtocm(pte->pg_pfnum)]; if (c->c_blkno && c->c_mdev != MSWAPX) ! munhash(mount[c->c_mdev].m_dev, ! (daddr_t)((u_long) 0 + c->c_blkno)); } *(int *)pte &= ~PG_PROT; *(int *)pte |= tprot; ====END OF FIX==== %%% END BUG REPORT %%% This fix does seem to work, as we made this fix on our machines a while back and haven't seen any panics of the munhash flavor since then. -- uucp: ucbvax!ucsbcsl!discolo arpa: ucsbcsl!discolo@berkeley csnet: discolo@ucsb USMail: Computer Systems Lab U.C. Santa Barbara Santa Barbara, CA 93106
barry@muddcs.UUCP (07/19/84)
We also got the the same munhash error when trying to run the maketest
script, that came with Mark Linton's update to dbx (the one on the net
and with DEC modula-2). Unfortunately, we did not have a uda driver
with code to handle a dump running at the time?
We are running 4.2BSD on a 750 with 5Mbytes of memory. Anybody else
seen this problem.
--
Barry Lustig 621-8000 x3447
Harvey Mudd College {ihnp4,allegra,seismo}!scgvaxd!muddcs!barry