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 $gp at function entry of main(), so we do it by hand - but I'd rather find out why gcc doesn't.
This commit is contained in:
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b1a89ae8c3
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5dd0c6e3c0
4 changed files with 126 additions and 68 deletions
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@ -16,85 +16,25 @@ in
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};
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};
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config = mkIf config.boot.initramfs.enable {
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kernel.config.BLK_DEV_INITRD = "y";
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kernel.config.INITRAMFS_SOURCE = builtins.toJSON "${config.outputs.initramfs}";
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kernel.config = {
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BLK_DEV_INITRD = "y";
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INITRAMFS_SOURCE = builtins.toJSON "${config.outputs.initramfs}";
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# INITRAMFS_COMPRESSION_LZO = "y";
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};
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outputs = {
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initramfs =
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let
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bb1 = pkgs.busybox.override {
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enableStatic = true;
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enableMinimal = true;
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enableAppletSymlinks = false;
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extraConfig = ''
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CONFIG_DESKTOP n
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CONFIG_ASH n
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CONFIG_HUSH y
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CONFIG_HUSH_TICK y
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CONFIG_HUSH_LOOPS y
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CONFIG_HUSH_CASE y
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CONFIG_HUSH_ECHO y
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CONFIG_HUSH_SET y
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CONFIG_LN y
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CONFIG_CAT y
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CONFIG_MOUNT y
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CONFIG_PRINTF y
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CONFIG_FEATURE_MOUNT_FLAGS y
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CONFIG_FEATURE_MOUNT_VERBOSE y
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CONFIG_ECHO y
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CONFIG_CHROOT y
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CONFIG_CHMOD y
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CONFIG_MKDIR y
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CONFIG_MKNOD y
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CONFIG_BASH_IS_NONE y
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CONFIG_SH_IS_NONE y
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CONFIG_SH_IS_ASH n
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CONFIG_FEATURE_SH_STANDALONE y
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CONFIG_FEATURE_PREFER_APPLETS y
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CONFIG_BUSYBOX_EXEC_PATH "/bin/busybox"
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'';
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};
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bb = bb1.overrideAttrs(o: {
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makeFlags = [];
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});
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slashinit = pkgs.writeScript "init" ''
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#!/bin/hush
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exec >/dev/console
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echo Running in initramfs
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mount -t proc none /proc
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set -- `cat /proc/cmdline`
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for i in "$@" ; do
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case "''${i}" in
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root=*)
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rootdevice="''${i#root=}"
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;;
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esac
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done
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echo mount -t jffs2 ''${rootdevice} /target/persist
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mount -t jffs2 ''${rootdevice} /target/persist
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mount -o bind /target/persist/nix /target/nix
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hush /target/persist/activate /target
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cd /target
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mount -o bind /target /
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exec chroot . /bin/init "$@"
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'';
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refs = pkgs.writeReferencesToFile bb;
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gen_init_cpio = pkgs.pkgsBuildBuild.gen_init_cpio;
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let inherit (pkgs.pkgsBuildBuild) gen_init_cpio;
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in runCommand "initramfs.cpio" {} ''
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cat << SPECIALS | ${gen_init_cpio}/bin/gen_init_cpio /dev/stdin > $out
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dir /proc 0755 0 0
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dir /dev 0755 0 0
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nod /dev/console 0600 0 0 c 5 1
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nod /dev/mtdblock0 0600 0 0 b 31 0
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dir /target 0755 0 0
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dir /target/persist 0755 0 0
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dir /target/nix 0755 0 0
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nod /dev/console 0600 0 0 c 5 1
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dir /bin 0755 0 0
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file /bin/busybox ${bb}/bin/busybox 0755 0 0
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slink /bin/hush /bin/busybox 0755 0 0
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slink /bin/chroot /bin/busybox 0755 0 0
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file /init ${slashinit} 0755 0 0
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file /init ${pkgs.preinit}/bin/preinit 0755 0 0
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SPECIALS
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'';
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};
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@ -40,6 +40,7 @@
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# https://sourceforge.net/p/squashfs/mailman/message/26599379/
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lzma = callPackage ./lzma {};
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preinit = callPackage ./preinit {};
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swconfig = callPackage ./swconfig {};
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openwrt = callPackage ./openwrt {};
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30
pkgs/preinit/default.nix
Normal file
30
pkgs/preinit/default.nix
Normal file
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@ -0,0 +1,30 @@
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{
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stdenv
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, fetchzip
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, gdb
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}:
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let kernel = fetchzip {
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name = "linux";
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url = "https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.15.71.tar.gz";
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hash = "sha256-pq6QNa0PJVeheaZkuvAPD0rLuEeKrViKk65dz+y4kqo=";
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};
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in
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stdenv.mkDerivation {
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name = "preinit";
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src = ./.;
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# NIX_DEBUG=2;
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hardeningDisable = [ "all" ];
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CFLAGS = "-Os -nostartfiles -nostdlib -fno-stack-protector -fpic -fPIC -I ./ -I ${kernel}/tools/include/nolibc";
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postBuild = ''
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$STRIP --remove-section=.note --remove-section=.comment preinit
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'';
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makeFlags = ["preinit"];
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stripAllList = ["bin"];
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installPhase = ''
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mkdir -p $out/bin
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cp preinit $out/bin
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'';
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}
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87
pkgs/preinit/preinit.c
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87
pkgs/preinit/preinit.c
Normal file
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@ -0,0 +1,87 @@
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#ifdef USE_LIBC
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#include <string.h>
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#include <unistd.h>
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#include <sys/mount.h>
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#include <sys/wait.h>
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#include <string.h>
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#else
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#include <nolibc.h>
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#endif
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#include <asm/setup.h>
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#define ERR(x) write(2, x, strlen(x))
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#define AVER(c) do { if(c < 0) ERR("failed: " #c); } while(0)
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static int begins_with(char * str, char * prefix)
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{
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while(*prefix) {
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if(*str == '\0') return 0;
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if(*str != *prefix) return 0;
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str++;
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prefix++;
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}
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return 1;
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}
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static void fork_exec(char * command, char *args[])
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{
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int fork_pid = fork();
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AVER(fork_pid);
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if(fork_pid > 0)
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wait(NULL);
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else
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AVER(execve(command, args, NULL));
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}
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char banner[] = "Running pre-init...\n";
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char buf[COMMAND_LINE_SIZE];
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int main(int argc, char *argv[], char *envp[])
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{
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asm("la $gp, _gp\nsw $gp,16($sp)");
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char *rootdevice = 0;
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char *p = buf;
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write(1, banner, strlen(banner));
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mount("none", "/proc", "proc", 0, NULL);
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int cmdline = open("/proc/cmdline", O_RDONLY, 0);
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if(cmdline>=0) {
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int len = read(cmdline, buf, sizeof buf - 1);
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buf[len]='\0';
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write(1, "cmdline ", 8);
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write(1, buf, len);
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};
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while(*p) {
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if(begins_with(p, "root=")) {
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rootdevice = p + 5;
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while(*p && (*p != ' ')) p++;
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*p= '\0';
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}
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while(*p && (*p != ' ')) p++;
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p++;
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}
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if(rootdevice) {
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write(1, "rootdevice ", 11);
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write(1, rootdevice, strlen(rootdevice));
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write(1, "\n", 1);
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AVER(mount(rootdevice, "/target/persist", "jffs2", 0, NULL));
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AVER(mount("/target/persist/nix", "/target/nix",
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"bind", MS_BIND, NULL));
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char *exec_args[] = { "activate", "/target" };
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fork_exec("/target/persist/activate", exec_args);
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AVER(chdir("/target"));
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AVER(mount("/target", "/", "bind", MS_BIND | MS_REC, NULL));
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AVER(chroot("."));
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argv[0] = "init";
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argv[1] = NULL;
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AVER(execve("/persist/init", argv, envp));
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}
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}
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