src/runtime/os_linux_arm64.go

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
	"runtime/internal/sys"
	"unsafe"
)

const (
	_AT_NULL   = 0
	_AT_RANDOM = 25 // introduced in 2.6.29
)

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().
	// nanotime() is a poor approximation of CPU ticks that is enough for the profiler.
	// randomNumber provides better seeding of fastrand1.
	return nanotime() + int64(randomNumber)
}