pivot_root(".", ".") and the fchdir() dance
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Tue Aug 6 12:03:13 UTC 2019
On Tue, 6 Aug 2019 at 10:12, Philipp Wendler <ml at 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 at 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?
> Indeed, my tests show that the following seems to work fine:
> pivot_root(".", ".")
> umount2(".", MNT_DETACH)
Thanks for the confirmation, That's also exactly what I tested.
> I tested that with my own tool 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(".").
> Did I overlook something that makes the file descriptors required?
> 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 :-).
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