Pid restore - Revision history

Kir: minor typos

2019-01-16T20:27:32Z

minor typos

Efiop: Colorize example by using `tag.`


2014-08-06T18:21:58Z
Colorize example by using <code lang="c"> tag.

				
				
				
				
				
				← Older revision
				Revision as of 18:21, 6 August 2014
				
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  '''BEWARE! This program requires root. I don't take any responsibility for what this code might do to your system.'''( tested though =) )   '''BEWARE! This program requires root. I don't take any responsibility for what this code might do to your system.'''( tested though =) )
    
− <pre> + <source lang="c">
  #include <sys/stat.h>   #include <sys/stat.h>
  #include <fcntl.h>   #include <fcntl.h>
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       return 0;        return 0;
  }   }
− </pre> + </source>
    
  [[Category:Under the hood]]   [[Category:Under the hood]]



Xemul: Categorized
2014-08-05T11:27:45Z
Categorized

				
				
				
				
				
				← Older revision
				Revision as of 11:27, 5 August 2014
				
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  }   }
  </pre>   </pre>
  + 
  + [[Category:Under the hood]]



Efiop: Explain how PID restore works
2014-08-05T10:47:24Z
Explain how PID restore works
New page
== ns_last_pid ==

In order to restore PID, CRIU uses /proc/sys/kernel/ns_last_pid, which is available in kernel since v3.3(according to [[Upstream_kernel_commits]]). It requires CONFIG_CHECKPOINT_RESTORE to be set and it's enabled in the vast majority of distros. ns_last_pid contains the last pid that was assigned by the kernel. So, when kernel needs to assign a new one, it looks into ns_last_pid, gets last_pid and assigns last_pid+1. To restore PID, criu locks ns_last_pid, writes PID-1 and calls clone().

== Example ==

Here is a simple program that shows how to set PID for forked child.



'''BEWARE! This program requires root. I don't take any responsibility for what this code might do to your system.'''( tested though =) )



<pre>

#include <sys/stat.h>

#include <fcntl.h>

#include <stdio.h>

#include <string.h>

#include <stdlib.h>



int main(int argc, char *argv[])

{

    int fd, pid;

    char buf[32];



    if (argc != 2)

     return 1;



    printf("Opening ns_last_pid...\n");

    fd = open("/proc/sys/kernel/ns_last_pid", O_RDWR | O_CREAT, 0644);

    if (fd < 0) {

        perror("Can't open ns_last_pid");

        return 1;

    }

    printf("Done\n");



    printf("Locking ns_last_pid...\n");

    if (flock(fd, LOCK_EX)) {

        close(fd);

        printf("Can't lock ns_last_pid\n");

        return 1;

    }

    printf("Done\n");



    pid = atoi(argv[1]);

    snprintf(buf, sizeof(buf), "%d", pid - 1);



    printf("Writing pid-1 to ns_last_pid...\n");

    if (write(fd, buf, strlen(buf)) != strlen(buf)) {

        printf("Can't write to buf\n");

        return 1;

    }

    printf("Done\n");



    printf("Forking...\n");

    int new_pid;

    new_pid = fork();

    if (new_pid == 0) {

        printf("I'm child!\n");

        exit(0);

    } else if (new_pid == pid) {

        printf("I'm parent. My child got right pid!\n");

    } else {

        printf("pid does not match expected one\n");

    }

    printf("Done\n");



    printf("Cleaning up...");

    if (flock(fd, LOCK_UN)) {

        printf("Can't unlock");

    }



    close(fd);



    printf("Done\n");



    return 0;

}

</pre>

← Older revision		Revision as of 11:27, 5 August 2014
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	}		}
	</pre>		</pre>
		+
		+	[[Category:Under the hood]]

Pid restore - Revision history

Kir: minor typos

Efiop: Colorize example by using tag.

Xemul: Categorized

Efiop: Explain how PID restore works

Efiop: Colorize example by using `tag.`