[Pkg-sysvinit-devel] Bug#626263: Clarification of §10.5 symlink wording needed

Roger Leigh rleigh at codelibre.net
Sun May 15 17:57:52 UTC 2011

On Wed, May 11, 2011 at 10:14:16AM -0700, Russ Allbery wrote:
> Roger Leigh <rleigh at codelibre.net> writes:
> The "solution" to the chroot problem is to always manipulate the chroot
> via chroot, which ensures that the view of everything in the chroot is
> consistent.  But I realize that's not always feasible or obvious.  (Again,
> something that I'm used to doing due to other issues over the years, but
> which may no longer be "current".)
> I would say, though, that I consider it pretty dangerous to manipulate a
> chroot environment without using chroot.  It's way too easy to
> accidentally ascend out of the chroot and do things you didn't intend to
> do, and if you're already doing things as root, there's absolutely no
> reason not to just stick a chroot invocation in front of your command.

> > From the POV of packaging, I'd like symlinks to point to a specific
> > place, without ambiguity, and in the context of chroots, a relative link
> > is unambiguous whereas an absolute link changes depending on where we
> > are rooted.
> Yeah, this makes sense.  Maybe it's time to reconsider this policy.

I'd just like to bring a related bug to your attention: 626826

Here, sbuild creates a lockfile inside the chroot (/var/lock/sbuild).
If, in the chroot, /var/lock is a symlink to /run/lock then when we
create the lockfile we actually create it on the host.  When the
symlink is relative this doesn't occur.

One could argue that such a thing should be done via the chroot
program (or schroot, in our case).  However, given that we are
doing an open(2) with O_WRONLY|O_CREAT|O_TRUNC|O_EXCL and then
writing job-specific information into the lockfile and doing other
stuff like finding the process owning the lock, this is not ideal-
doing it via a long shell command is not easy or robust, and
installing additional code into the chroot is also not ideal.

Tools like sbuild, which by their nature need to copy files in and
out of the chroot (packages, stuff to set up the environment, apt
configuration and all sorts) could in theory do all this inside the
chroot.  But in reality some of this stuff needs doing outside as
well due to lack of certain tools inside and the huge increase in
complexity to do so.


  .''`.  Roger Leigh
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