On Thu, Jul 27 2017, Klemens Nanni <k...@posteo.org> wrote:
> Only main() calls pr_args() in L330 with ep->kp as argument which in
> turn is set in L257 or L266 for every utmp entry. kp is checked against
> NULL already in L229.
>
> Even if kp was somehow NULL chances are high we'd fail before pr_args()
> was called anyway since L244, L256 and L265 would then cause a NULL pointer
> dereference. In fact, proc_compare() even expects its second argument to
> to be not NULL (only the first one is checked explicitly).
>
> Not that crashing is guaranteed upon undefined behaviour but noone
> seems to have reported failure within the last 13 years, so I think it's
> safe to remove that check.
>
> Feedback? Comments?
I don't think this is correct. ep->kp is set only if we find a matching
live process. If we don't, the diff results in a crash. I don't know
all the details of utmp handling, but I really doubt that we can assume
a 1->1 mapping between utmp entries and live processes (stale entries,
race conditions, etc).
Here's an example with a stale ssh entry:
ritchie /usr/src/usr.bin/w$ w
1:59PM up 58 mins, 2 users, load averages: 0.27, 0.16, 0.14
USER TTY FROM LOGIN@ IDLE WHAT
jca p0 :0 1:03PM 0 tmux: client (/tmp/tmux-1000/default)
jca pg 127.0.0.1 1:57PM 1 -
ritchie /usr/src/usr.bin/w$ ./obj/w
1:59PM up 58 mins, 2 users, load averages: 0.21, 0.16, 0.13
USER TTY FROM LOGIN@ IDLE WHAT
jca p0 :0 1:03PM 0 tmux: client (/tmp/tmux-1000/default)
Segmentation fault (core dumped)
So the answer is "yes, this can happen".
Index: w.c
===================================================================
RCS file: /d/cvs/src/usr.bin/w/w.c,v
retrieving revision 1.62
diff -u -p -p -u -r1.62 w.c
--- w.c 30 May 2017 15:10:48 -0000 1.62
+++ w.c 27 Jul 2017 11:59:27 -0000
@@ -384,7 +384,7 @@ pr_args(struct kinfo_proc *kp)
int left;
if (kp == NULL)
- goto nothing; /* XXX - can this happen? */
+ goto nothing; /* no matching process found */
left = argwidth;
argv = kvm_getargv(kd, kp, argwidth+60); /* +60 for ftpd snip */
if (argv == NULL)
--
jca | PGP : 0x1524E7EE / 5135 92C1 AD36 5293 2BDF DDCC 0DFA 74AE 1524 E7EE
