cmd/earlyinit: early /dev/ and io setup

This establishes an environment where devtmpfs is mounted, and if the kernel fails to set up console, 1 and 2 is pointed at /dev/kmsg.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/earlyinit/main.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"log"
+	"os"
+	"runtime"
+	. "syscall"
+)
+
+func main() {
+	runtime.LockOSThread()
+	log.SetFlags(0)
+	log.SetPrefix("earlyinit: ")
+
+	if err := Mount(
+		"devtmpfs",
+		"/dev/",
+		"devtmpfs",
+		MS_NOSUID|MS_NOEXEC,
+		"",
+	); err != nil {
+		log.Fatalf("cannot mount devtmpfs: %v", err)
+	}
+
+	// The kernel might be unable to set up the console. When that happens,
+	// printk is called with "Warning: unable to open an initial console."
+	// and the init runs with no files. The checkfds runtime function
+	// populates 0-2 by opening /dev/null for them.
+	//
+	// This check replaces 1 and 2 with /dev/kmsg to improve the chance
+	// of output being visible to the user.
+	if fi, err := os.Stdout.Stat(); err == nil {
+		if stat, ok := fi.Sys().(*Stat_t); ok {
+			if stat.Rdev == 0x103 {
+				var fd int
+				if fd, err = Open(
+					"/dev/kmsg",
+					O_WRONLY|O_CLOEXEC,
+					0,
+				); err != nil {
+					log.Fatalf("cannot open kmsg: %v", err)
+				}
+
+				if err = Dup3(fd, Stdout, 0); err != nil {
+					log.Fatalf("cannot open stdout: %v", err)
+				}
+				if err = Dup3(fd, Stderr, 0); err != nil {
+					log.Fatalf("cannot open stderr: %v", err)
+				}
+
+				if err = Close(fd); err != nil {
+					log.Printf("cannot close kmsg: %v", err)
+				}
+			}
+		}
+	}
+
+}