From: ebiederm@xmission.com (Eric W. Biederman)
To: "Michael Kerrisk \(man-pages\)" <mtk.manpages@gmail.com>
Cc: Philipp Wendler <ml@philippwendler.de>,
linux-man <linux-man@vger.kernel.org>,
Containers <containers@lists.linux-foundation.org>,
lkml <linux-kernel@vger.kernel.org>,
Andy Lutomirski <luto@amacapital.net>,
Jordan Ogas <jogas@lanl.gov>,
werner@almesberger.net, Al Viro <viro@ftp.linux.org.uk>
Subject: Re: pivot_root(".", ".") and the fchdir() dance
Date: Mon, 09 Sep 2019 05:40:05 -0500 [thread overview]
Message-ID: <87r24piwhm.fsf@x220.int.ebiederm.org> (raw)
In-Reply-To: <CAKgNAkjS+x7aMVUiVSgCRwgi8rnukqJv=svtTARE-tt-oxQxWw@mail.gmail.com> (Michael Kerrisk's message of "Tue, 6 Aug 2019 14:03:13 +0200")
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> writes:
> Hello Philipp,
>
> On Tue, 6 Aug 2019 at 10:12, Philipp Wendler <ml@philippwendler.de> wrote:
>>
>> Hello Michael, hello Aleksa,
>>
>> Am 05.08.19 um 14:29 schrieb Michael Kerrisk (man-pages):
>>
>> > On 8/5/19 12:36 PM, Aleksa Sarai wrote:
>> >> On 2019-08-01, Michael Kerrisk (man-pages) <mtk.manpages@gmail.com> wrote:
>> >>> I'd like to add some documentation about the pivot_root(".", ".")
>> >>> idea, but I have a doubt/question. In the lxc_pivot_root() code we
>> >>> have these steps
>> >>>
>> >>> oldroot = open("/", O_DIRECTORY | O_RDONLY | O_CLOEXEC);
>> >>> newroot = open(rootfs, O_DIRECTORY | O_RDONLY | O_CLOEXEC);
>> >>>
>> >>> fchdir(newroot);
>> >>> pivot_root(".", ".");
>> >>>
>> >>> fchdir(oldroot); // ****
>> >>>
>> >>> mount("", ".", "", MS_SLAVE | MS_REC, NULL);
>> >>> umount2(".", MNT_DETACH);
>> >>
>> >>> fchdir(newroot); // ****
>> >>
>> >> And this one is required because we are in @oldroot at this point, due
>> >> to the first fchdir(2). If we don't have the first one, then switching
>> >> from "." to "/" in the mount/umount2 calls should fix the issue.
>> >
>> > See my notes above for why I therefore think that the second fchdir()
>> > is also not needed (and therefore why switching from "." to "/" in the
>> > mount()/umount2() calls is unnecessary.
>> >
>> > Do you agree with my analysis?
>>
>> If both the second and third fchdir are not required,
>> then we do not need to bother with file descriptors at all, right?
>
> Exactly.
>
>> Indeed, my tests show that the following seems to work fine:
>>
>> chdir(rootfs)
>> pivot_root(".", ".")
>> umount2(".", MNT_DETACH)
>
> Thanks for the confirmation, That's also exactly what I tested.
>
>> I tested that with my own tool[1] that uses user namespaces and marks
>> everything MS_PRIVATE before, so I do not need the mount(MS_SLAVE) here.
>>
>> And it works the same with both umount2("/") and umount2(".").
>
> Yes.
>
>> Did I overlook something that makes the file descriptors required?
>
> No.
>
>> If not, wouldn't the above snippet make sense as example in the man page?
>
> I have exactly that snippet in a pending change for the manual page :-).
I have just spotted this conversation and I expect if you are going
to use this example it is probably good to document what is going
on so that people can follow along.
>> chdir(rootfs)
>> pivot_root(".", ".")
At this point the mount stack should be:
old_root
new_root
rootfs
With "." and "/" pointing to new_root.
>> umount2(".", MNT_DETACH)
At this point resolving "." starts with new_root and follows up the
mount stack to old-root.
Ordinarily if you unmount "/" as is happening above you then need to
call chroot and possibly chdir to ensure neither "/" nor "." point to
somewhere other than the unmounted root filesystem. In this specific
case because "/" and "." resolve to new_root under the filesystem that is
being unmounted that all is well.
Eric
next prev parent reply other threads:[~2019-09-09 11:16 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 13:38 pivot_root(".", ".") and the fchdir() dance Michael Kerrisk (man-pages)
2019-08-05 10:36 ` Aleksa Sarai
2019-08-05 12:29 ` Michael Kerrisk (man-pages)
2019-08-05 13:37 ` Aleksa Sarai
2019-08-06 19:35 ` Michael Kerrisk (man-pages)
2019-08-06 8:12 ` Philipp Wendler
2019-08-06 12:03 ` Michael Kerrisk (man-pages)
2019-09-09 10:40 ` Eric W. Biederman [this message]
2019-09-09 14:48 ` Michael Kerrisk (man-pages)
2019-09-09 23:40 ` Eric W. Biederman
2019-09-10 10:27 ` Michael Kerrisk (man-pages)
2019-09-10 11:15 ` Christian Brauner
2019-09-10 11:21 ` Michael Kerrisk (man-pages)
2019-09-10 23:06 ` Eric W. Biederman
2019-09-15 8:12 ` Michael Kerrisk (man-pages)
2019-09-15 18:17 ` Eric W. Biederman
2019-09-23 11:10 ` Michael Kerrisk (man-pages)
2019-09-28 15:05 ` Michael Kerrisk (man-pages)
2019-09-30 11:42 ` Eric W. Biederman
2019-10-07 11:02 ` Michael Kerrisk (man-pages)
2019-10-07 15:46 ` Eric W. Biederman
2019-10-08 14:27 ` Michael Kerrisk (man-pages)
2019-10-08 19:40 ` Eric W. Biederman
2019-10-08 21:40 ` Michael Kerrisk (man-pages)
2019-10-08 22:16 ` Eric W. Biederman
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