runtime: get randomness from AT_RANDOM AUXV on linux/arm64

Fixes #15147.

Change-Id: Ibfe46c747dea987787a51eb0c95ccd8c5f24f366
Reviewed-on: https://go-review.googlesource.com/21580
Run-TryBot: Minux Ma <minux@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/os_linux_arm64.go

@@ -4,6 +4,11 @@
 
 package runtime
 
+import (
+	"runtime/internal/sys"
+	"unsafe"
+)
+
 const (
 	_AT_NULL   = 0
 	_AT_RANDOM = 25 // introduced in 2.6.29
@@ -11,6 +16,27 @@ const (
 
 var randomNumber uint32
 
+func sysargs(argc int32, argv **byte) {
+	// skip over argv, envv to get to auxv
+	n := argc + 1
+	for argv_index(argv, n) != nil {
+		n++
+	}
+	n++
+	auxv := (*[1 << 29]uint64)(add(unsafe.Pointer(argv), uintptr(n)*sys.PtrSize))
+
+	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+		switch auxv[i] {
+		case _AT_RANDOM: // kernel provides a pointer to 16-bytes worth of random data
+			startupRandomData = (*[16]byte)(unsafe.Pointer(uintptr(auxv[i+1])))[:]
+			// the pointer provided may not be word aligned, so we must treat it
+			// as a byte array.
+			randomNumber = uint32(startupRandomData[4]) | uint32(startupRandomData[5])<<8 |
+				uint32(startupRandomData[6])<<16 | uint32(startupRandomData[7])<<24
+		}
+	}
+}
+
 //go:nosplit
 func cputicks() int64 {
 	// Currently cputicks() is used in blocking profiler and to seed fastrand1().

src/runtime/vdso_none.go

@@ -5,6 +5,7 @@
 // +build !linux !amd64
 // +build !linux !386
 // +build !linux !arm
+// +build !linux !arm64
 
 package runtime