Hi all! On Thu, 2018-08-23 at 10:43 -0700, Paul E. McKenney wrote: > [...] > Does anyone do kernel-only deployments, for example, setting up an > embedded device having a Linux kernel and absolutely no userspace > whatsoever? [...] > You see, rcutorture runs entirely out of initrd, never mounting a real > root partition. The user has been required to supply the initrd, but
IMHO running programs from the initrd is in user-space, but anyways: Ages ago at some former employer, we built an embedded Linux device on an MPC-860 board (but that shouldn't make a significant difference to other architectures) based on the (at that time) brand new 2.4 kernel which ran completely out of the initrd (which obviously contained the whole root filesystem). [...] > by throwing out everything not absolutely needed by the dash and sleep > binaries, which got me down to about 2.5MB, 1.8MB of which was libc. We had a working glibc binary (which as the largest binary on the filesystem) and just used it (and never got time and/or necessity to use something else like ulibc, newlibc or build glibc ourselves to leave all unneeded stuff out). We basically built the filesystem - the distribution as such;-) - from scratch (only self-crafted `configure` calls around[0]) and - thus - used busybox and ash (IIRC) - so throw dash, core-utils etc. away and just use busybox (or something similar) for further space savings. The whole startup and daemon management was done with busybox' "init" via a simple /etc/inittab (that were the good old times;-) and it was enough as one can start one-time programs at boot time (e.g. to load kernel modules (and remove the file in the filesystem from the filesystem[0]) or configure stuff via sysctl) and restart daemons. We didn't need run-levels ... > This situation of course prompted me to create an initrd containing > a statically linked binary named "init" and absolutely nothing else > (not even /dev or /tmp directories), which weighs in at not quite 800KB. That is probably the smallest solution - if it's enough. If it's all GPL, just link it statically against dietlibc .... We had all of the usual directories and a somewhat filled /dev (completely static in the initrd IIRC, no udev or similar dynamic stuff was needed) as we had dropbear as ssh-server, a small webserver+CGI- script for a web interface and a SNMP agent (hacked net-smtp as we had our own configuration daemon and needed SNMP only as a transport protocol). [...] MfG, Bernd [0]: Every byte counts and size does matter;-) -- Bernd Petrovitsch Email : be...@petrovitsch.priv.at LUGA : http://www.luga.at