[Libguestfs] Weird results from g.sh()

[Sam Eiderman]

On Thu, Aug 27, 2020 at 11:08 AM Richard W.M. Jones <rjones at redhat.com> wrote:
>
> On Thu, Aug 27, 2020 at 10:50:12AM +0300, Sam Eiderman wrote:
> > Hi,
> >
> > I'm using libguestfs 1.42,
> >
> > When I run the following python3 commands on a rhel7.8:
> >
> > print(g.ls('/sys'))
> > []
>
> This lists the files in /sys in the filesystem, which in most disk
> images is likely to be an empty directory.
>
> > print(g.sh('ls /sys'))
> > block
> > bus
> > class
> > dev
> > devices
> > firmware
> > fs
> > hypervisor
> > kernel
> > module
> > power
>
> sh bind-mounts /sys from the appliance into the filesystem temporarily
> in order to make it possible to run some commands:
>
> https://github.com/libguestfs/libguestfs/blob/fce82fe55a2b64a1a7e494858aa272d608e5e54e/daemon/sh.c#L60
>
> However this /sys has nothing to do with /sys as it may appear in the
> guest when it is running.  Libguestfs edits filesystems, it doesn't
> run the guest.

Yes, I understand that, I am just surprised to know that we bind /sys
for g.sh() commands.

However, "g.command([self.mkconfig, '-o', self.config])" shouldn't
have the appliances /sys available (I looked briefly in command.c and
I don't see the same bind-mount code), or maybe I'm missing something.

>
> > It seems that g.ls('/sys') is chrooted correctly.
> > But g.sh('ls /sys') isn't.
> >
> > I came across this behavior when I used g.command(['grub2-mkconfig', '-o',
> > '....']) on rhel7.8 and it didn't produce the correct linuxefi/initrdefi
> > entries in grub, since in rhel7.8 the uefi detection algorithm is checking
> > in bash '[ -d /sys/firmware/efi ]'.
> > https://src.fedoraproject.org/rpms/grub2/blob/71e0fb4ea646071ec86e4a65bb24241e31c156bb/f/0106-Make-10_linux-work-with-our-changes-for-linux16-and-.patch
>
> It's pretty unlikely that running grub can work.  The libguestfs
> appliance is sufficiently different from how the guest boots that
> things like BIOS mappings are not related, and libguestfs doesn't use
> EFI at all.
>
> There's unfortunately no good way to run grub.  syslinux/extlinux is
> the only option since it is designed to make modifications to offline
> filesystems, and grub is not.
>
> Rich.
>

Well, at the moment in my code in and in virt-v2v, that's what we do:

https://github.com/libguestfs/virt-v2v/blob/master/v2v/linux_bootloaders.ml#L336

I thought I could use my g.mkdir_p('/sys/firmware/efi') hack, but
since it didn't work, I'm now resort to explicitly handling this
failure after running the mkconfig command:

def _fix_grub2_config_efi(self, g):
        # On rhel7.8 the grub config is set to non efi commands due to
        # a weak uefi detection algorithm which doesn't work from a chrooted
        # environment (checks for [ -d /sys/firmware/efi ])
        if (self.root.family, self.boot_type) != (LinuxFamily.RHEL,
                                                  BootType.UEFI):
            return
        grub_config = g.read_file(self.config).decode('utf-8')
        write = False
        for old, new in [('linux16', 'linuxefi'),
                         ('initrd16', 'initrdefi')]:
            if f'\t{old} ' in grub_config:
                debug(f'Replacing {old} with {new} in {self.config}')
                grub_config = grub_config.replace(f'\t{old} ', f'\t{new} ')
                write = True
        if write:
            g.write(self.config, grub_config.encode('utf-8'))

The problem is easily reproduced by using the 7.8 iso available now
from rhel, installed on VMware as UEFI.
Should probably be reproduced in virt-v2v too.
The problematic file is 10_linux which belongs to:
grub2-tools-2.02-0.81.el8.x86_64

Sam

> > I thought I could simply g.mkdir_p('/sys/firmware/efi') - but that created
> > a fake dir in my chrooted environment, where grub2-mkconfig somehow was
> > inspecting the real /sys as demonstrated in the above 'ls' example.
> >
> > Any ideas on why sh jailbreaks the chroot? is /sys set through some
> > environment variable?
> >
> > Thanks!
>
> > _______________________________________________
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> > Libguestfs at redhat.com
> > https://www.redhat.com/mailman/listinfo/libguestfs
>
>
> --
> Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> Read my programming and virtualization blog: http://rwmj.wordpress.com
> virt-p2v converts physical machines to virtual machines.  Boot with a
> live CD or over the network (PXE) and turn machines into KVM guests.
> http://libguestfs.org/virt-v2v
>