defconfig

BR2_aarch64=y
BR2_cortex_a57=y
BR2_OPTIMIZE_3=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TARGET_GENERIC_ROOT_PASSWD="root"
BR2_TARGET_GENERIC_GETTY_PORT="ttyAMA0"
BR2_PACKAGE_BUSYBOX_SHOW_OTHERS=y
BR2_PACKAGE_BASH=y
BR2_PACKAGE_HTOP=y
BR2_PACKAGE_VIM=y
BR2_TARGET_ROOTFS_CPIO=y
# BR2_TARGET_ROOTFS_TAR is not set

This is the document I was following for buildroot, as per our dynamic tools team:
https://github.com/google/syzkaller/blob/master/docs/linux/setup_linux-host_qemu-vm_arm64-kernel.md

Thanks for the link @nathanchance, that worked.

$ cd buildroot
$ mkdir -p overlays/etc/
$ cp output/target/etc/inittab overlays/etc/.
$ vim overlays/etc/inittab

-console::respawn:/sbin/getty -L console 0 vt100
+::respawn:-/bin/sh

$ make menuconfig

System configuration > Root filesystem overlay directories set to overlays

$ make
$ cd ~/linux
$ qemu-system-aarch64 -kernel arch/arm64/boot/Image.gz -machine virt -cpu cortex-a57 -m 2048 -append "console=ttyAMA0 root=/dev/vda" -nographic -hda ~/buildroot/output/images/rootfs.ext2 -no-reboot

$ make savedefconfig

doesn't create a defconfig file for me, though. Am I holding it wrong? 🐈

Am I holding it wrong?

[Yes] 🌵

$ make savedefconfig BR2_DEFCONFIG=path/to/save/defconfig