Subject: Re: [RFC][PATCH] ns: Syscalls for better namespace sharing control.

From: Daniel Lezcano <[email-masked]>
Date: Tue, 09 Mar 2010 11:03:53 +0100

Eric W. Biederman wrote:

[ ... ]
> I guess my meaning is I was expecting.
> child = fork();
> if (child == 0) {
> 	execve(...);
> }
> waitpid(child);
>
> This puts /bin/sh in the container as well.
>   
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <syscall.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/param.h>

#define __NR_setns 300

int setns(int nstype, int fd)
{
    return syscall (__NR_setns, nstype, fd);
}

int main(int argc, char *argv[])
{
    char path[MAXPATHLEN];
    char *ns[] = { "pid", "mnt", "net", "pid", "uts" };
    const int size = sizeof(ns) / sizeof(char *);
    int fd[size];
    int i;
    pid_t pid;
   
    if (argc != 3) {
        fprintf(stderr, "mynsenter <pid> <command>\n");
        exit(1);
    }

    for (i = 0; i < size; i++) {
       
        sprintf(path, "/proc/%s/ns/%s", argv[1], ns[i]);

        fd[i] = open(path, O_RDONLY| FD_CLOEXEC);
        if (fd[i] < 0) {
            perror("open");
            return -1;
        }

    }
   
    for (i = 0; i < size; i++)
        if (setns(0, fd[i])) {
            perror("setns");
            return -1;
        }

    pid = fork();
    if (!pid) {

        fprintf(stderr, "mypid is %d\n", syscall(__NR_getpid));

        execve(argv[2], &argv[2], NULL);
        perror("execve");

    }

    if (pid < 0) {
        perror("fork");
        return -1;
    }

    if (waitpid(&pid, NULL, 0) < 0) {
        perror("waitpid");
    }

    return 0;
}

Waitpid returns an error:

waitpid: No child processes

The pid number returned by fork is the pid from the init pid namespace 
but it seems waitpid is waiting for a pid belonging to the child pid 
namespace.

waitpid
 -> wait4
   -> find_get_pid
     -> find_vpid
       -> find_pid_ns(nr, current->nsproxy->pid_ns);

The current->nsproxy->pid_ns is the one of the namespace we attached to. 
So the real pid returned by the fork does not exist in this pid namespace.
Maybe fork should return a pid number belonging to the current pid 
namespace we are attached no ?