Date: Fri, 06 Apr 2001 18:46:11 -0700 From: Dima Dorfman <dima@unixfreak.org> To: current@freebsd.org Cc: jkh@freebsd.org Subject: Obscure sysinstall null dereference bug Message-ID: <20010407014612.0AA963E09@bazooka.unixfreak.org>
next in thread | raw e-mail | index | archive | help
(Excuse the size of the e-mail; small, one-line patch is at the end.)
On some occasions, sysinstall can be persuaded to follow a NULL
pointer. This particular bug is actually not so straight-forward that
many people would run into it, but it seems I always do :-/ (I've had
this bite me for just about every release since 4.0 or so).
The problem is that a lot of code in sysinstall (dist.c, particuarly)
expects the mediaDevice global variable to be non-NULL. When some
operation fails, you might see code like:
mediaDevice->shutdown(mediaDevice);
if (!mediaDevice->init(mediaDevice))
...
The problem is that sometimes the operation that fails sets
mediaDevice to NULL, causing a nice, annoying NULL dereference. The
simplest way to reproduce this that I've found is as follows:
1. Run sysinstall in whatever mode you want; doesn't matter if it's
on an existing system or not. Somehow get to the part where you
choose which distributions you want, choose at least one, then somehow
get to the part where it will extract the distributions
2. Choose an FTP install. Select ftp.freebsd.org (you can use any
server you want; I'm just using that as an example).
3. Let it start extracting distributions. Allow it to extract at
least one file (i.e., let it do something). At this point, cut off
its link to the FTP server. If this is an existing system,
`ipfw add 1 reset tcp from any to ftp.freebsd.org` works nicely. In
practice, it can be any reason (I've had this happen when I chose "FTP"
instead of "FTP passive" or when the Internet link died).
4. Wait for it to figure out the server is dead. It should prompt
you to select another one. At the "Please select a FreeBSD FTP
distribution site" menu, select Cancel (you'd do this if, e.g., you
wanted to switch from "FTP" to "FTP passive").
5. Push "OK" or "yes" on the dialog boxes it presents.
6. Watch it crash (if this is a new install where sysinstall is
acting as init, it will display a nice "I got sigsegv and that's bad"
message).
As you can deduce from the size of the instructions, few people
probably run into this. Nevertheless, I think that for sysinstall,
anything short of corruption is better than crashing; the latter is
very annoying, esp. after you've spent some time deciding on the
filesystem layout, distributions, packages, etc.
The patch below corrects the problem by removing the
"mediaDevice = NULL" assignment from the mediaClose routine. It's not the most
correct or elegant fix in the world, but too much code assumes
mediaDevice is non-NULL to simply go through and fix all of it.
Thanks,
Dima Dorfman
dima@unixfreak.org
Index: media.c
===================================================================
RCS file: /st/src/FreeBSD/src/usr.sbin/sysinstall/media.c,v
retrieving revision 1.112
diff -u -r1.112 media.c
--- media.c 2000/09/25 20:19:43 1.112
+++ media.c 2001/04/07 01:19:01
@@ -126,7 +126,6 @@
{
if (mediaDevice)
mediaDevice->shutdown(mediaDevice);
- mediaDevice = NULL;
}
/*
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-current" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20010407014612.0AA963E09>