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.

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
+  '';
+}

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));
+	}
+}