Re: Kernel-only deployments?

Adam Borowski
2018-08-23T18:54:28Z

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.

If you want to do anything more complex, though -- you really want musl
or another lightweight libc instead. Glibc is utterly unfit for static
linking.

Meow!
--
⢀⣴⠾⠻⢶⣦⠀ .globl _start↵.data↵rc: .ascii "/etc/init.d/rcS\0"↵.text↵_start
⣾⠁⢰⠒⠀⣿⡁ mov $57,%rax↵syscall↵cmp $0,%rax↵jne child↵parent:↵mov $61,%rax
⢿⡄⠘⠷⠚⠋⠀ mov $-1,%rdi↵xor %rsi,%rsi↵xor %rdx,%rdx↵syscall↵jmp parent↵child:
⠈⠳⣄⠀⠀⠀⠀ mov $59,%rax↵mov $rc,%rdi↵xor %rsi,%rsi↵xor %rdx,%rdx↵syscall