commit 5dd0c6e3c048dc2ff6e8204440765219094ee362
parent b1a89ae8c3927d94be33cfb93ed646a5a21a8232
Author: Daniel Barlow <dan@telent.net>
Date: Sat, 15 Apr 2023 17:35:02 +0100
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 $gp at
function entry of main(), so we do it by hand - but I'd rather find
out why gcc doesn't.
Diffstat:
4 files changed, 126 insertions(+), 68 deletions(-)
diff --git a/modules/initramfs.nix b/modules/initramfs.nix
@@ -16,85 +16,25 @@ in
};
};
config = mkIf config.boot.initramfs.enable {
- kernel.config.BLK_DEV_INITRD = "y";
- kernel.config.INITRAMFS_SOURCE = builtins.toJSON "${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
'';
};
diff --git a/pkgs/default.nix b/pkgs/default.nix
@@ -40,6 +40,7 @@
# https://sourceforge.net/p/squashfs/mailman/message/26599379/
lzma = callPackage ./lzma {};
+ preinit = callPackage ./preinit {};
swconfig = callPackage ./swconfig {};
openwrt = callPackage ./openwrt {};
diff --git a/pkgs/preinit/default.nix b/pkgs/preinit/default.nix
@@ -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
+ '';
+}
diff --git a/pkgs/preinit/preinit.c b/pkgs/preinit/preinit.c
@@ -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));
+ }
+}
