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rewrite preinit as very small C program

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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 $gp at
function entry of main(), so we do it by hand - but I'd rather find
out why gcc doesn't.
This commit is contained in:
Daniel Barlow
2023-04-15 17:35:02 +01:00
parent b1a89ae8c3
commit 5dd0c6e3c0
4 changed files with 126 additions and 68 deletions
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76
modules/initramfs.nix
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@@ -16,85 +16,25 @@ in
};
};
config = mkIf config.boot.initramfs.enable {
kernel.config.BLK_DEV_INITRD = "y";
kernel.config.INITRAMFS_SOURCE = builtins.toJSON "${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
'';
};