[Libguestfs] libldm crashes in a linux-sandbox context

[Richard W.M. Jones]

On Thu, Jun 15, 2023 at 09:18:38PM +0900, Vincent Mailhol wrote:
> Hello,
> 
> I am using libguestfs in a Bazel's linux-sandbox environment[1].
> 
> When executing in that sandbox environment, I got frequent crashes.
> 
> Please find attached below the results of libguestfs-test-tool when
> run into that linux-sandbox environment. The most relevant part seems
> to be:
> 
>   [    0.797233] ldmtool[164]: segfault at 0 ip 0000564a892506a5 sp 00007fff8ee5b900 error 4 in ldmtool[564a8924e000+3000]
>   [    0.798117] Code: 18 64 48 33 1c 25 28 00 00 00 75 5e 48 83 c4 28 5b 5d 41 5c 41 5d 41 5e 41 5f c3 66 2e 0f 1f 84 00 00 00 00 00 e8 db fd ff ff <4c> 8b 20 48 89 44 24 08 4c 89 e7 e8 0b e1 ff ff 45 31 c0 4c 89 e1
>   /init: line 154:   164 Segmentation fault      ldmtool create all
> 
> So the root cause seems to be around libldm. This mailing list seems
> to cover both libguestfs and libldm, so hopefully, I am at the right
> place to ask :)
> 
> Needless to say, when run outside of the sandbox environment, no crash
> were observed.
> 
> [1] linux-sandbox.cc
> Link: https://github.com/bazelbuild/bazel/blob/master/src/main/tools/linux-sandbox.cc
> 
> ---
...
> supermin: picked /sys/block/sdb/dev (8:16) as root device
> supermin: creating /dev/root as block special 8:16
> supermin: mounting new root on /root
> [    0.678248] EXT4-fs (sdb): mounting ext2 file system using the ext4 subsystem
> [    0.679832] EXT4-fs (sdb): mounted filesystem without journal. Opts: . Quota mode: none.
> supermin: deleting initramfs files
> supermin: chroot
> Starting /init script ...
> mount: only root can use "--types" option (effective UID is 65534)
> /init: line 38: /proc/cmdline: No such file or directory
> mount: only root can use "--types" option (effective UID is 65534)
> mount: only root can use "--options" option (effective UID is 65534)
> mount: only root can use "--types" option (effective UID is 65534)
> mount: only root can use "--types" option (effective UID is 65534)
> mount: only root can use "--options" option (effective UID is 65534)

It really goes wrong from here, where apparently it's not running as
root (instead UID 65534), even though we're supposed to be running
inside a Linux appliance virtual machine.

Any idea why that would be?

I looked at the sandbox and that would run the qemu process as UID
"nobody" (which might be 65534).  However I don't understand why that
would affect anything running on the new kernel inside the appliance.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org