Mail Archives: cygwin/2012/11/23/09:20:26
On Nov 23 13:19, Corinna Vinschen wrote:
> On Nov 19 16:04, Lavrentiev, Anton (NIH/NLM/NCBI) [C] wrote:
> > Hi,
> >
> > As I previously reported, there is a weird behavior of CYGWIN implementation
> > of SYSV semaphores, and a bug exposition for one problem is attached below
> > (I'm still looking into some other issues in there).
> >
> > If the code is compiled with both BUG1 and BUG2 defined (as shown), it
> > will abort at iteration 16384 (just the default semaphore overflow).
> >
> > Undefining BUG2 causes the problem disappear (because there is no longer
> > UNDO on the 1st semaphore). Also, running the code with just one semaphore
> > (undef BUG1) causes no problem. Finally, replacing "#if 1" with "#if 0"
> > to unlock the semaphore allows to run indefinitely with any combination of
> > BUG1/2 (just remember to issue ipcs/ipcrm to start with a clean slate at
> > all times).
> >
> > Reviewing the code of CYGSERVER, there is an apparent bug in the semundo_clear()
> > routine (at around line 536, which looks like "i++, sunptr++;", and advances
> > both undo indexes even when "if (sunptr->un_id == semid)" (line 524)
> > failed to match semid. This means that for two (or more) semaphores, the
> > undo index "i" moves ahead even when nothing was done while still searching.
> > This causes the adjust pointer to miss the position to clear, and overflow
> > the semaphore adjust value (line 1207, semop(), by the virtue of
> > semundo_adjust()'s logic at about line 486).
>
> This is original FreeBSD code, so I have a hard time to follow the idea
> that it might be wrong. I stared a long while into the source now, and
> I compared that with the latest version of the upstream code.
>
> The i counter is in lockstep with the sunptr pointer. The only time
> something happens is if sunptr->un_num (== suptr->un_ent[i].un_num)
> equals semnum. In that case, suptr->un_ent[i] is overwritten with
> with the last element uptr->un_ent[suptr->un_cnt], and then the code
> calls continue, thus NOT incrementing i and sunptr. So the same
> element, now containing the contents of suptr->un_ent[suptr->un_cnt],
> is evaluated again.
>
> Am I missing something?
Yes, I do, and debugging as well as comparing my observations with the
NetBSD code of this function reveals a long-standing bug in the FreeBSD
code. This call:
if (semnum != -1)
break;
is outside of the `if (semnum == -1 || sunptr->un_num == semnum)'
condition, and that results in ignoring every member of the un_ent array
but the first one (i == 0).
Can you please test the below patch?
Thanks,
Corinna
* sysv_sem.cc (semundo_clear): Move condition to break from
inner loop to the right spot.
Index: sysv_sem.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygserver/sysv_sem.cc,v
retrieving revision 1.10
diff -u -p -r1.10 sysv_sem.cc
--- sysv_sem.cc 6 Feb 2008 22:30:38 -0000 1.10
+++ sysv_sem.cc 23 Nov 2012 14:18:21 -0000
@@ -529,9 +529,9 @@ semundo_clear(int semid, int semnum, str
suptr->un_ent[suptr->un_cnt];
continue;
}
+ if (semnum != -1)
+ break;
}
- if (semnum != -1)
- break;
}
i++, sunptr++;
}
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Project Co-Leader cygwin AT cygwin DOT com
Red Hat
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