Re: Kernel-only deployments?

Josh Triplett
2018-08-23T19:16:18Z

On Thu, Aug 23, 2018 at 08:54:17PM +0200, Adam Borowski wrote:
> On Thu, Aug 23, 2018 at 10:43:59AM -0700, Paul E. McKenney wrote:
> > The mkinitramfs approach results in about 40MB of initrd, and dracut
> > about 10MB. Most of this is completely useless for rcutorture, which
> > isn't interested in mounting filesystems, opening devices, and almost
> > all of the other interesting things that mkinitramfs and dracut enable.
> >
> > Those who know me will not be at all surprised to learn that I went
> > overboard making the resulting initrd as small as possible. I started
> > 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.
> > 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.
> > This is a great improvement over 10MB, to say nothing of 40MB, but 800KB
> > for a C-language "for" loop containing nothing more than a single call to
> > sleep()?
>
> .globl _start
> .data
> req: .8byte 999999999, 999999999
> .text
> _start:
> mov $35, %rax # syscall: nanosleep
> mov $req, %rdi
> xor %rsi, %rsi
> syscall
> jmp _start
>
>
> as sl.s -o sl.o
> ld sl.o -o init
>
> 'Ere you go, no libc needed. If your arch is not amd64, just say so.

"pause" ($34) would also suffice, and would not require an argument or a
.data section.

- Josh Triplett