Failed to reuse ram.qcow2 in parallel fuzzing mode.

[Bl1nnnk]

The snapshot file "ram.qcow2" is used concurrently by the instances in parallel fuzzing mode. But maybe modern QEMU introduced a new feature "file lock"?

The error log looks like:

qemu-system-x86_64: -hdb /home/jungu/kafl/snapshots/win8_x64/ram.qcow2: Failed to get "write" lock

Does this error take place in your environment?

[il-steffen]

Hi! I have not seen this but I also barely use the VM-style launching.
Have you tried with qemu-4.2? Just enable it in install.sh and point your kafl.ini to it.

[moehw]

Hello! I have the same problem and this error appears on both qemu-5.0 and qemu-4.2.
I solved it just to take advantage of parallel fuzzing by creating one more ram.qcow2. I rewrote qemu.py to use vm_ram exactly the same as vm_dir (ram_<id>.qcow2 and overlay_<id>.qcow2). Is there a prettier way?

[il-steffen]

It seems this is a feature and there are half-documented options to get around it.
https://bugs.launchpad.net/juju/+bug/1816170/comments/8

I'm not sure from the description if you just need to supply -U/--force-share to qemu-img when creating the backing image, or supply shared-rw (renamed to force-share=on?) on qemu cmdline, or both. Also it looks like force-share=on may only work for -blockdev, which is the modern version of -drive..

[Bl1nnnk]

I also use the method that @nola-milkin mentioned, it's useful.

[il-steffen]

The separate vm_ram option was removed some time ago when testing on Windows. Do we know if the above approach also allows using multiple cores for Windows VM fuzzing? Cc @vient

[vient]

Do you mean if we can make several RAM copies and use them for Windows fuzzing instead of removing vm_ram option? For that answer is yes, this was the first thing I tried. The reason why I went to remove vm_ram instead is because I don't understand it's meaning: RAM still needs to be copied as well as overlay, we might just combine them in one file then.

[il-steffen]

Thanks. I just also confirmed that the Linux VM use case works with just the overlay_N file. Will update the guide accordingly.