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Subjectunkillable process
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It's really bad when a task group leader exits.
The process becomes unkillable.

This is with the 2.6.8-rc1 kernel. I haven't seen
any mention of this getting fixed since then.
Here's the top of the /proc/*/status file:
Name: a.out
State: Z (zombie)
SleepAVG: 59%
Tgid: 9662
Pid: 9662
PPid: 1
TracerPid: 0
Uid: 1000 1000 1000 1000
Gid: 1000 1000 1000 1000
FDSize: 0
Groups: 500 1000
Threads: 9

Here's the code:

///////////////////////////////////////////////////////////////
#include <sys/types.h>
#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <sched.h>
#ifndef CLONE_THREAD
#define CLONE_THREAD 0x00010000
#endif
#ifndef CLONE_DETACHED
#define CLONE_DETACHED 0x00400000
#endif
#ifndef CLONE_STOPPED
#define CLONE_STOPPED 0x02000000
#endif
#define FLAGS (CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_VM|CLONE_THREAD|CLONE_DETACHED)
static pid_t one;

static void die(int signo){
(void)signo;
_exit(0);
}
static void hang(void){
for(;;) pause();
}
static int clone_fn(void *vp){
(void)vp;
hang();
return 0; // keep gcc happy
}
static long clone_stack_data[2048];
#ifdef __hppa__
static long *clone_stack = &clone_stack_data[0];
#else
static long *clone_stack = &clone_stack_data[2048];
#endif

int main(int argc, char *argv[]){
pid_t minime;
int i = 8;
(void)argc;
(void)argv;
one = getpid();
signal(SIGHUP,die);
if(fork()) hang(); // parent later killed as readyness signal
while(i--){
// better be stopped... they share a stack
minime = clone(clone_fn, clone_stack, FLAGS | CLONE_STOPPED, NULL);
if(minime==-1){
perror("no clone");
kill(one,SIGKILL);
_exit(8);
}
}
kill(one,SIGHUP); // let the shell know we're ready
_exit(0); // make task group leader a zombie
return 0; // keep gcc happy
}
/////////////////////////////////////////////////////////////////////

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