Is it possible to exit the chroot(2) environment?

[Kyle Evans]

On Sun, Sep 27, 2020 at 2:07 PM Kyle Evans <kevans at freebsd.org> wrote:
>
> On Sun, Sep 27, 2020 at 2:03 PM Warner Losh <imp at bsdimp.com> wrote:
> >
> > On Sun, Sep 27, 2020 at 12:30 PM Yuri <yuri at rawbw.com> wrote:
> >
> > > This line
> > >
> > > https://github.com/rpm-software-management/rpm/blob/master/lib/rpmchroot.c#L155
> > > calls chroot(".") in order to exit from the chroot environment.
> > >
> >
> > Interesting. FreeBSD doesn't allow that.
> >
> >
> > > It apparently succeeds on Linux (this is rpm), but it fails on FreeBSD
> > > with "Operation not permitted", while executed under sudo.
> > >
> > > The chroot(2) man page doesn't mention anything about exiting the chroot
> > > environment.
> > >
> >
> > True. Such behavior is undefined. There's no defined notion of exiting a
> > chroot. It doesn't seem to be documented in the few examples of the
> > chroot(2) call linux man pages I've found. Do you have documentation on
> > what, exactly, it's supposed to do?
> >
>
> I'm almost certain they just aren't restricting you from chrooting to
> a directory out of the chroot if you have a reference to it, so it
> probably does something like:
>
> chdir("/");
> chroot("/some/root");
> /* Do stuff, but never chdir */
> chroot("."); /* Working directory is still the real root. */
>

I think the original report needs a ktrace to narrow down what's
really going on...

kevans at shiva:~/grep$ cat chr.c
#include <unistd.h>
#include <stdio.h>

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

    printf("chdir %d\n", chdir("/"));
    printf("chroot %d\n", chroot("/tmp"));
    printf("chroot %d\n", chroot("."));
    return (0);
}

kevans at shiva:~/grep$ sudo ./a.out
chdir 0
chroot 0
chroot 0

Sprinkling some stat() calls in between reveals that chroot(2) is
working in both cases and properly entering/exiting the chroot.