genext2fs included in buildroot, misses this patch: https://github.com/bestouff/genext2fs/pull/14/commits/8e4b9ae6f051454953a80b8e64443bcea6c9cc46 Without this patch you can't set the proper permissions for the root node, which makes genext2fs quite useless in scenarios that need a non-default root node perms.
Fixed by commit 51d55fc132.
Fixed for 2021.02.x in: https://git.buildroot.net/buildroot/commit/?h=2021.02.x&id=a4822f31ca985192595cf04246e4d6c1f88f168f