[Libguestfs] guestmount issue with change directory (cd)

[Richard W.M. Jones]

On Fri, Oct 14, 2016 at 07:25:42PM +0200, Pierre Neyron wrote:
> Hello,
> 
> I've got a strange behavior with guestmount: I've a root directory with
> mod/owner = drwx------ root root. I cannot change directory to it as a
> regular user (as normal), but I can list its content (not normally
> normal) and I can change directory to a subdirectory in it which is mod
> drwxr-xr-x.
> 
> E.g:
> $ mkdir /tmp/mnt
> $ guestmount --ro -i -a test.qcow2 /tmp/mnt/
> $ cd /tmp/mnt
> $ ls -ld root/
> drwx------ 4 root root 4096 Oct 14 19:21 root/
> $ ls -l root/
> total 4
> drwxr-xr-x 2 root root 4096 Oct 14 19:21 subdir
> $ cd root
> bash: cd: root: Permission denied
> $ cd root/subdir
> $ pwd
> /tmp/mnt/root/subdir
> 
> Also tar is ok as well:
> $ cd /tmp/mnt
> $ tar cf /tmp/tar.tar .
> $ tar tf /tmp/tar.tar | grep -e "^./root/subdir/$"
> ./root/subdir/
> 
> This is for sure some fuse magic to allow the regular user to list files
> just as if he is root (after all, guestmount did not require to be root
> in the first place).

This isn't exactly what's happening.  libguestfs runs an appliance,
and inside the appliance everything runs as root [not the same as host
root], so has access to every file in the disk image.  guestmount runs
as your [host] user, translating everything into libguestfs API calls.

However FUSE is a bit strange in that it allows you to create files
which appear to come from other users.  There are various options
which let you change this behaviour, have a look at the `-o' options
in the `guestmount --fuse-help' output and the guestmount and FUSE
manuals.

In this case you've created a root-owned directory which you (as
non-root) cannot access.

The usual solution to all these problems is just to use the libguestfs
API directly, which gives you a great deal more control, supports
xattrs properly, and is also a bit faster.

Rich.

> I'm very ok with that since this is very useful,
> but then I would expect `cd root' to be OK as well ?
> 
> I'm using
> $ guestmount -V
> guestmount 1.32.7
> 
> Best regards,
> 
> PS: @moderators: sorry for the noise with cancelled messages, I fixed
> typos... :/
> -- 
> Pierre
> 
> 
> 
> 



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Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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