Re: Local DoS (was: Strange 'zombie' problem both in 2.4 and 2.6)

[David Lang]

I think I may be running into the same (or a similar) issue with a 
workload that forks heavily (~3500 forks/sec). What can I do to let the 
system survive this sort of load?

David Lang

On Tue, 13 Apr 2004, Marcelo Tosatti wrote:

> Date: Tue, 13 Apr 2004 10:10:17 -0300
> From: Marcelo Tosatti <marcelo.tosatti@xxxxxxxxxxxx>
> To: Nikita V. Youshchenko <yoush@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: Local DoS (was: Strange 'zombie' problem both in 2.4 and 2.6)
>
> On Fri, Apr 09, 2004 at 01:11:50PM +0400, Nikita V. Youshchenko wrote:
> > Hello.
> >
> > Several days ago I've posted to linux-kernel describing "zombie problem"
> > related to sigqueue overflow.
> >
> > Futher exploration of the problem showed that the reason of the described
> > behaviour is in user-space. There is a process that blocks a signal and
> > later receives tons of such signals. This effectively causes sigqueue
> > overflow.
> >
> > The following program gives the same effect:
> >
> > #include <signal.h>
> > #include <unistd.h>
> > #include <stdlib.h>
> >
> > int main()
> > {
> >         sigset_t set;
> >         int i;
> >         pid_t pid;
> >
> >         sigemptyset(&set);
> >         sigaddset(&set, 40);
> >         sigprocmask(SIG_BLOCK, &set, 0);
> >
> >         pid = getpid();
> >         for (i = 0; i < 1024; i++)
> >                 kill(pid, 40);
> >
> >         while (1)
> >                 sleep(1);
> > }
> >
> > Running this program on 2.4 or 2.6 kernel with
> > default /proc/sys/kernel/rtsig-max value will cause sigqueue overflow, and
> > all linuxthreads-based programs, INCLUDING DAEMONS RUNNING AS ROOT, will
> > stop receiving notifications about thread exits, so all completed threads
> > will become zombies. Exact reason why this is hapenning is described in
> > detail in my previous postings.
> >
> > This is a local DoS.
> >
> > Affected system services include (but are not limited to) mysql and clamav.
> > In fact, any linuxthreads application will be affected.
> >
> > The problem is not that bad on 2.6, since NPTL is used instead of
> > linuxthreads, so there are no zombies from system daemons. However, bad
> > things still happen: when sigqueue is overflown, all processes get zeroed
> > siginfo, which causes random application misbehaviours (like hangs in
> > pthread_cancel()).
> >
> > I don't know what is the correct solution for this issue. Probably there
> > should be per-process or per-user (but not systemwide) limits on number of
> > pending signals.
>
> Indeed, per-user sigqueue limit is the way to fix this.
>
> Anyone willing to implement it ?
>
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