1// Copyright 2021 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package unix
6
7import (
8	"sync/atomic"
9	"syscall"
10	"unsafe"
11)
12
13//go:cgo_import_dynamic libc_getrandom getrandom "libc.so"
14
15//go:linkname procGetrandom libc_getrandom
16
17var procGetrandom uintptr
18
19var getrandomUnsupported atomic.Bool
20
21// GetRandomFlag is a flag supported by the getrandom system call.
22type GetRandomFlag uintptr
23
24const (
25	// GRND_NONBLOCK means return EAGAIN rather than blocking.
26	GRND_NONBLOCK GetRandomFlag = 0x0001
27
28	// GRND_RANDOM means use the /dev/random pool instead of /dev/urandom.
29	GRND_RANDOM GetRandomFlag = 0x0002
30)
31
32// GetRandom calls the getrandom system call.
33func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
34	if len(p) == 0 {
35		return 0, nil
36	}
37	if getrandomUnsupported.Load() {
38		return 0, syscall.ENOSYS
39	}
40	r1, _, errno := syscall6(uintptr(unsafe.Pointer(&procGetrandom)),
41		3,
42		uintptr(unsafe.Pointer(&p[0])),
43		uintptr(len(p)),
44		uintptr(flags),
45		0, 0, 0)
46	if errno != 0 {
47		if errno == syscall.ENOSYS {
48			getrandomUnsupported.Store(true)
49		}
50		return 0, errno
51	}
52	return int(r1), nil
53}
54