Re: Kernel-only deployments?

[Adam Borowski]

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!
-- 
âââââââ .globl _startâ.dataârc: .ascii "/etc/init.d/rcS\0"â.textâ_start
âââââââ mov $57,%raxâsyscallâcmp $0,%raxâjne childâparent:âmov $61,%rax
âââââââ mov $-1,%rdiâxor %rsi,%rsiâxor %rdx,%rdxâsyscallâjmp parentâchild:
âââââââ mov $59,%raxâmov $rc,%rdiâxor %rsi,%rsiâxor %rdx,%rdxâsyscall