Timed action after fork() in parent process Post: 302511008

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Top Forums Programming Timed action after fork() in parent process Post 302511008 by gaurav1086 on Tuesday 5th of April 2011 01:37:07 PM

04-05-2011

Registered User

Quote:

Originally Posted by disaster

Assume you have such a piece of (more or less pseudo-)code:

Code:

if(fork() == 0) {// childprocess
     chmod(someProgram, 00777);
     exec(someProgram);
} else { // assume it never fails and this is the parent
     chmod(someProgram, 00000); // should be executed as soon as possible after the exec call of the child
}

What do I want to achieve: Assume a non executable program someProgram. I want to execute this program in such a way, that it gets executable rights, execute it in a child process via fork, and then take away the executable rights as soon as possible after the exec call.
Is there any way to do this?

If you want the child process to execute first, the parent process should wait() for the child before changing the permission like this ->

Code:

switch(pid = fork())
{
case -1: perror("fork error");
break;

case 0 : chmod(); //do the process with the file
break;

default :wait(); //wait for the child to finish. claim child's              // resources
             chmod() //change back the permission

break;
}
return 0;
}

But this has a security hole as when the child is executing, another malicious process may try to modify the file so you can employ a locking mechanism so that its only accessed by the particular process i.e the child .

Thanks,
Gaurav.

gaurav1086

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LEARN ABOUT DEBIAN

pthread_atfork

PTHREAD_ATFORK(3)					     Library Functions Manual						 PTHREAD_ATFORK(3)

NAME

       pthread_atfork - register handlers to be called at fork(2) time

SYNOPSIS

       #include <pthread.h>

       int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void));

DESCRIPTION

       pthread_atfork  registers handler functions to be called just before and just after a new process is created with fork(2). The prepare han-
       dler will be called from the parent process, just before the new process is created. The parent handler will  be  called  from  the  parent
       process, just before fork(2) returns. The child handler will be called from the child process, just before fork(2) returns.

       One or several of the three handlers prepare, parent and child can be given as NULL, meaning that no handler needs to be called at the cor-
       responding point.

       pthread_atfork can be called several times to install several sets of handlers. At fork(2) time, the prepare handlers are  called  in  LIFO
       order  (last  added  with  pthread_atfork,  first  called before fork), while the parent and child handlers are called in FIFO order (first
       added, first called).

       To understand the purpose of pthread_atfork, recall that fork(2) duplicates the whole memory space,  including  mutexes	in  their  current
       locking	state, but only the calling thread: other threads are not running in the child process.  The mutexes are not usable after the fork
       and must be initialized with pthread_mutex_init in the child process.  This is a limitation of the  current  implementation  and  might	or
       might not be present in future versions.

RETURN VALUE

       pthread_atfork returns 0 on success and a non-zero error code on error.

ERRORS

       ENOMEM insufficient memory available to register the handlers.

AUTHOR

       Xavier Leroy <Xavier.Leroy@inria.fr>

SEE ALSO

       fork(2), pthread_mutex_lock(3), pthread_mutex_unlock(3).

								   LinuxThreads 						 PTHREAD_ATFORK(3)

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