rewrite preinit as very small C program

By using the kernel "nolibc" header to avoid requiring a C library, we
can bring the initramfs size to around 4k

This does involve a tiny bit of inline mips assembly which I'm not
sure about. gcc seems unwilling to generate the code to load $gp at
function entry of main(), so we do it by hand - but I'd rather find
out why gcc doesn't.
This commit is contained in:
Daniel Barlow 2023-04-15 17:35:02 +01:00
parent b1a89ae8c3
commit 5dd0c6e3c0
4 changed files with 126 additions and 68 deletions

View file

@ -16,85 +16,25 @@ in
};
};
config = mkIf config.boot.initramfs.enable {
kernel.config.BLK_DEV_INITRD = "y";
kernel.config.INITRAMFS_SOURCE = builtins.toJSON "${config.outputs.initramfs}";
kernel.config = {
BLK_DEV_INITRD = "y";
INITRAMFS_SOURCE = builtins.toJSON "${config.outputs.initramfs}";
# INITRAMFS_COMPRESSION_LZO = "y";
};
outputs = {
initramfs =
let
bb1 = pkgs.busybox.override {
enableStatic = true;
enableMinimal = true;
enableAppletSymlinks = false;
extraConfig = ''
CONFIG_DESKTOP n
CONFIG_ASH n
CONFIG_HUSH y
CONFIG_HUSH_TICK y
CONFIG_HUSH_LOOPS y
CONFIG_HUSH_CASE y
CONFIG_HUSH_ECHO y
CONFIG_HUSH_SET y
CONFIG_LN y
CONFIG_CAT y
CONFIG_MOUNT y
CONFIG_PRINTF y
CONFIG_FEATURE_MOUNT_FLAGS y
CONFIG_FEATURE_MOUNT_VERBOSE y
CONFIG_ECHO y
CONFIG_CHROOT y
CONFIG_CHMOD y
CONFIG_MKDIR y
CONFIG_MKNOD y
CONFIG_BASH_IS_NONE y
CONFIG_SH_IS_NONE y
CONFIG_SH_IS_ASH n
CONFIG_FEATURE_SH_STANDALONE y
CONFIG_FEATURE_PREFER_APPLETS y
CONFIG_BUSYBOX_EXEC_PATH "/bin/busybox"
'';
};
bb = bb1.overrideAttrs(o: {
makeFlags = [];
});
slashinit = pkgs.writeScript "init" ''
#!/bin/hush
exec >/dev/console
echo Running in initramfs
mount -t proc none /proc
set -- `cat /proc/cmdline`
for i in "$@" ; do
case "''${i}" in
root=*)
rootdevice="''${i#root=}"
;;
esac
done
echo mount -t jffs2 ''${rootdevice} /target/persist
mount -t jffs2 ''${rootdevice} /target/persist
mount -o bind /target/persist/nix /target/nix
hush /target/persist/activate /target
cd /target
mount -o bind /target /
exec chroot . /bin/init "$@"
'';
refs = pkgs.writeReferencesToFile bb;
gen_init_cpio = pkgs.pkgsBuildBuild.gen_init_cpio;
let inherit (pkgs.pkgsBuildBuild) gen_init_cpio;
in runCommand "initramfs.cpio" {} ''
cat << SPECIALS | ${gen_init_cpio}/bin/gen_init_cpio /dev/stdin > $out
dir /proc 0755 0 0
dir /dev 0755 0 0
nod /dev/console 0600 0 0 c 5 1
nod /dev/mtdblock0 0600 0 0 b 31 0
dir /target 0755 0 0
dir /target/persist 0755 0 0
dir /target/nix 0755 0 0
nod /dev/console 0600 0 0 c 5 1
dir /bin 0755 0 0
file /bin/busybox ${bb}/bin/busybox 0755 0 0
slink /bin/hush /bin/busybox 0755 0 0
slink /bin/chroot /bin/busybox 0755 0 0
file /init ${slashinit} 0755 0 0
file /init ${pkgs.preinit}/bin/preinit 0755 0 0
SPECIALS
'';
};

View file

@ -40,6 +40,7 @@
# https://sourceforge.net/p/squashfs/mailman/message/26599379/
lzma = callPackage ./lzma {};
preinit = callPackage ./preinit {};
swconfig = callPackage ./swconfig {};
openwrt = callPackage ./openwrt {};

30
pkgs/preinit/default.nix Normal file
View file

@ -0,0 +1,30 @@
{
stdenv
, fetchzip
, gdb
}:
let kernel = fetchzip {
name = "linux";
url = "https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.71.tar.gz";
hash = "sha256-pq6QNa0PJVeheaZkuvAPD0rLuEeKrViKk65dz+y4kqo=";
};
in
stdenv.mkDerivation {
name = "preinit";
src = ./.;
# NIX_DEBUG=2;
hardeningDisable = [ "all" ];
CFLAGS = "-Os -nostartfiles -nostdlib -fno-stack-protector -fpic -fPIC -I ./ -I ${kernel}/tools/include/nolibc";
postBuild = ''
$STRIP --remove-section=.note --remove-section=.comment preinit
'';
makeFlags = ["preinit"];
stripAllList = ["bin"];
installPhase = ''
mkdir -p $out/bin
cp preinit $out/bin
'';
}

87
pkgs/preinit/preinit.c Normal file
View file

@ -0,0 +1,87 @@
#ifdef USE_LIBC
#include <string.h>
#include <unistd.h>
#include <sys/mount.h>
#include <sys/wait.h>
#include <string.h>
#else
#include <nolibc.h>
#endif
#include <asm/setup.h>
#define ERR(x) write(2, x, strlen(x))
#define AVER(c) do { if(c < 0) ERR("failed: " #c); } while(0)
static int begins_with(char * str, char * prefix)
{
while(*prefix) {
if(*str == '\0') return 0;
if(*str != *prefix) return 0;
str++;
prefix++;
}
return 1;
}
static void fork_exec(char * command, char *args[])
{
int fork_pid = fork();
AVER(fork_pid);
if(fork_pid > 0)
wait(NULL);
else
AVER(execve(command, args, NULL));
}
char banner[] = "Running pre-init...\n";
char buf[COMMAND_LINE_SIZE];
int main(int argc, char *argv[], char *envp[])
{
asm("la $gp, _gp\nsw $gp,16($sp)");
char *rootdevice = 0;
char *p = buf;
write(1, banner, strlen(banner));
mount("none", "/proc", "proc", 0, NULL);
int cmdline = open("/proc/cmdline", O_RDONLY, 0);
if(cmdline>=0) {
int len = read(cmdline, buf, sizeof buf - 1);
buf[len]='\0';
write(1, "cmdline ", 8);
write(1, buf, len);
};
while(*p) {
if(begins_with(p, "root=")) {
rootdevice = p + 5;
while(*p && (*p != ' ')) p++;
*p= '\0';
}
while(*p && (*p != ' ')) p++;
p++;
}
if(rootdevice) {
write(1, "rootdevice ", 11);
write(1, rootdevice, strlen(rootdevice));
write(1, "\n", 1);
AVER(mount(rootdevice, "/target/persist", "jffs2", 0, NULL));
AVER(mount("/target/persist/nix", "/target/nix",
"bind", MS_BIND, NULL));
char *exec_args[] = { "activate", "/target" };
fork_exec("/target/persist/activate", exec_args);
AVER(chdir("/target"));
AVER(mount("/target", "/", "bind", MS_BIND | MS_REC, NULL));
AVER(chroot("."));
argv[0] = "init";
argv[1] = NULL;
AVER(execve("/persist/init", argv, envp));
}
}