[Bug] virtio-mmio kernel cmdline append breaks init arguments separated by `--`

[leonjza]

Describe the bug

Currently, it is not possible to pass arguments to init using -- (ref) as extra arguments are appended after the user provided command line here. When a user uses --, the behaviour causes essential arguments to be ignored as they will be passed to init.

In a bit of a contrived example, but for illustrations sake, let's imagine my init was to /bin/cat a file.
When specifying kernel boot_args such as console=ttyS0 nomodules random.trust_cpu=on reboot=k panic=1 pci=off ip=:::::eth0:dhcp init=/bin/cat -- /etc/password, the aforementioned code block will append extra arguments which technically get discarded because of the the --. The resultant command line would therefore be:

[    0.000000] Command line: console=ttyS0 nomodules random.trust_cpu=on reboot=k panic=1 pci=off ip=:::::eth0:dhcp init=/bin/cat -- /etc/password root=/dev/vda rw virtio_mmio.device=4K@0xd0000000:5 virtio_mmio.device=4K@0xd0001000:6 virtio_mmio.device=4K@0xd0002000:7

To Reproduce

1. Start firecracker with firecracker --api-sock /tmp/firecracker.socket.
2. Configure the kernel with the following JSON body:

{
   "kernel_image_path":"/path/to/kern",
   "boot_args":"console=ttyS0 nomodules random.trust_cpu=on reboot=k panic=1 pci=off ip=:::::eth0:dhcp init=/bin/cat -- /etc/password"
}

3. Configure other parts to get a VM to boot.
4. Start the instance and observe the Command line: output.

Expected behaviour

Kernel arguments such as root= and virtio_mmio.device= should appear before user supplied arguments.

Environment

1. Arch : x86_64
2. OS: Ubuntu 21.04
3. Firecracker 0.24.6

Checks

• Have you searched the Firecracker Issues database for similar problems?
• Have you read the existing relevant Firecracker documentation?
• Are you certain the bug being reported is a Firecracker issue?

[dianpopa]

Hi @leonjza ,

I posted a fix for this issue #2716. Take a look and let me know if this fixes your problem.

[georgepisaltu]

Pending on this PR in rust-vmm/linux-loader, we will have a fix for this.