linux-kernel - Re: Kernel-only deployments?

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Date:   Thu, 23 Aug 2018 13:39:11 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Adam Borowski <kilobyte@...band.pl>
Cc:     nicolas.pitre@...aro.org, josh@...htriplett.org,
        linux-kernel@...r.kernel.org
Subject: Re: Kernel-only deployments?

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.

I need to be arch-independent, but I will save off your solution,
thank you!

> 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.

Got it, thank you!

							Thanx, Paul

> 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
>