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Explain how PID restore works
== 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>
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>