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

From: [email-masked] (Eric W. Biederman)
Date: Tue, 09 Mar 2010 02:13:08 -0800

Daniel Lezcano <[email-masked]> writes:

> 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;
> }

&pid ???  Isn't that a type error?

> 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);

But it isn't.  It is.
           find_pid_ns(nr, task_active_pid_ns(current));
Which is:
           find_pid_ns(nr, ns_of_pid(task_pid(current)));
           
Which is a value that doesn't change.  When we attach to a pid namespace.

> 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 ?

Do you not have my patch that changed that?

Eric