Virtio-vga with blobs on fails, when qemu compiled with enabled modules

Host environment

• Operating system: Manjaro Linux
• OS/kernel version: Linux max 5.13.11-1-MANJARO #1 SMP PREEMPT Sun Aug 15 13:15:18 UTC 2021 x86_64 GNU/Linux
• Architecture: x86 (others may be affected too)
• QEMU flavor: qemu-system-x86_64
• QEMU version: QEMU emulator version 6.0.94 (v6.1.0-rc4-dirty)
• QEMU command line:

   sudo ./qemu-system-x86_64  \
     -object memory-backend-memfd,id=mem1,size=512M \
     -machine memory-backend=mem1 \
     -display sdl \
     -device virtio-vga,blob=on

Emulated/Virtualized environment

• Bug happens before launching system

Description of problem

When using qemu configured with --enabled-modules and starting qemu with command line above, qemu crashes with following output:

qemu-system-x86_64: -device virtio-vga,blob=on: cannot enable blob resources without udmabuf

While qemu configured without --enabled-modules runs this command successfully.

Steps to reproduce

1. Get latest qemu source code
2. Build qemu mkdir build && cd build && ../configure && ninja
3. Check if following command runs without errors and show sdl qemu window

sudo ./qemu-system-x86_64  \
     -object memory-backend-memfd,id=mem1,size=512M \
     -machine memory-backend=mem1 \
     -display sdl \
     -device virtio-vga,blob=on

4. Then try to build with modules enabled mkdir build && cd build && ../configure --enable-modules && ninja
5. Try to do step 3 again

Additional information

I tried to debug this bug, and found that problem is with function virtio_gpu_have_udmabuf: when qemu is build without modules this function is from hw/display/virtio-gpu-udmabuf.c (which is correct), but when qemu compiled with modules this function comes from stubs/virtio-gpu-udmabuf.c and when hw-display-virtio-gpu.so is loaded, virtio_gpu_have_udmabuf is not replaced, and remains function from stub (which always return 0) and command fails.

I think I will submit patch that fix it tomorrow