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 
5 package unix
6 
7 import (
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 
17 var procGetrandom uintptr
18 
19 var getrandomUnsupported atomic.Bool
20 
21 // GetRandomFlag is a flag supported by the getrandom system call.
22 type GetRandomFlag uintptr
23 
24 const (
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.
33 func 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