Created attachment 7476 [details] defconfig When selecting BR2_TARGET_ROOTFS_ISO9660 and BR2_TARGET_SYSLINUX_ISOLINUX, Linux fails to build until "make linux-menuconfig" is run. It would seem that the kernel is expecting output/images/rootfs.cpio to exist, but it does not unless linux-menuconfig is run. Once linux-menuconfig has been run once, the problem disappears until I rm -rf everything in output/images. Then running linux-menuconfig again solves the issue. Obviously the workaround is to run linux-menuconfig, but trying to build in a freshly checked out copy of our repo always fails. A branch of our repo that is identical except it does not have BR2_TARGET_ROOTFS_ISO9660 and BR2_TARGET_SYSLINUX_ISOLINUX set does not see this issue.
Created attachment 7481 [details] linux_defconfig linux_defconfig used when bug was discovered
I'm indeed able to reproduce, I'll try to have a look. Thanks for the bug report.
You have CONFIG_INITRAMFS_SOURCE defined in your linux_defconfig, but you have not enabled initramfs support in Buildroot. So obviously this cannot work. If you enable initramfs support in Buildroot, then Buildroot will make sure to have a dummy rootfs.cpio when the kernel is first built, will make sure to set CONFIG_INITRAMFS_SOURCE to the correct value, and will rebuild the kernel image at the end of the build with the final rootfs.cpio. Therefore, there is no bug per se: there is just a mis-use of the initramfs support.