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

[Warner Losh]

On Sun, Sep 27, 2020 at 1:02 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?
>
> Does chroot(2) behave differently on Linux and FreeBSD, and chroot(".")
>> is a valid way to exit on Linux and not on FreeBSD? Or what is going on
>> here?
>>
>
> Generally, one is not supposed to exit a chroot. :) Though jail(2) exists
> because it's trivially possible in most cases.
>
> I wish somebody familiar with chroot add this information into the
>> chroot(2) man page.
>>
>
> POSIX never defined the behavior (and it's been removed in newer versions
> of POSIX).
>

Also, what happens when you set kern.chroot_allow_open_directories=2 ?

Warner