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Re: [PATCH] polystrap-binfmt for preparing the build host

On Thu, Jun 30, 2011 at 08:02:04AM +0200, Johannes Schauer wrote:
> > > I'm not sure whether you dont understand the problem that is to be
> > > solved or whether I understand how you suggest something I dont see
> > > a need to be fixed - please explain.
> > > 
> > > The $ARCH variable is being set in the config file and determines for
> > > what debian architecture a multistrap is done. To figure out which
> > > qemu-*-static binary to copy in the root directory, the above lines are
> > > evaluated - how do you think that can be improved?
> > 
> > As seen above, there is no bijection between ARCH and BINFMT_ARCH, and
> > several ARCH can map to a single /etc/qemu-binfmt/$BINFMT_ARCH/.
> > 
> > OTOH, when we're at the point of running qemu-*-static, we have
> > already produced a rootfs that can be used by qemu to locate the libs,
> > and which holds precisely those libs that would be used on the target
> > system.  So why would we want to use another tree forced onto all
> > users ?
> > 
> > Incidentally, I was given a suggestion to solve the problem, that
> > should prove easier to implement that the ptrace-based approach, and
> > whose description will hopefully make things more clear:
> > 
> > Instead of having qemu-*-static directly called by binfmt, we could
> > call a wrapper script.  That wrapper would be able to pass the -L flag
> > to qemu, depending on caller's choice, eg. through the use of an
> > envvar.  If no envvar was set, then it could fall back to the
> > system-default tree if any.
> > 
> > What annoys me in this approach, are the possible security
> > implications of letting a user influence running binaries.  I suspect
> > this could be applied to attack the system via setuid foreign
> > binaries.  The user mechanism should at least be ignored in that case,
> > much like LD_* envvars in the same case.
> > 
> > And that mechanism would be in conflict with the current
> > qemu-user-static package, so coordination with the qemu maints will be
> > required.
> ah okay I understand now what you mean and I would also totally like a
> solution to the /etc/qemu-binfmt/ problem. Using shared libraries from
> the just extracted rootfs with the -L option would be an idea but
> wouldnt it also be possible to let multiarch handle it?

Multiarch will still require root permissions for package
installation, and will not solve the qemu-arm issue.

> Do you have a proof of concept for your idea inside a fakechroot?

Not yet - I've had a long AFK period shortly after this discussion,
and I'm still handling backlogs :)

> That there is no bijection between ARCH and BINFMT_ARCH has also proven
> to be a problem for me [1]. Now every time I build for armel or armhf I
> have to change a symlink in /etc/qemu-binfmt which is very annoying.
> josch
> [1] http://lists.debian.org/debian-embedded/2011/06/msg00045.html
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