crypto/rand: supports for getrandom syscall in DragonFlyBSD

Since the 5.7 release, DragonFlyBSD supports as well
the getrandom function, the actual stable is 5.8.

Change-Id: I2b8fc468771b10ac12b38ea7e8e5314342de6375
GitHub-Last-Rev: c5c496f418
GitHub-Pull-Request: golang/go#42617
Reviewed-on: https://go-review.googlesource.com/c/go/+/269999
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>
Trust: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/crypto/rand/rand_batched.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux || freebsd
+//go:build linux || freebsd || dragonfly
-// +build linux freebsd
+// +build linux freebsd dragonfly
 
 package rand
 

src/crypto/rand/rand_batched_test.go

@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux || freebsd
+//go:build linux || freebsd || dragonfly
-// +build linux freebsd
+// +build linux freebsd dragonfly
 
 package rand
 

src/crypto/rand/rand_dragonfly.go

@@ -0,0 +1,9 @@
+// Copyright 2021 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 rand
+
+// maxGetRandomRead is the maximum number of bytes to ask for in one call to the
+// getrandom() syscall. In DragonFlyBSD at most 256 bytes will be returned per call.
+const maxGetRandomRead = 1 << 8

src/internal/syscall/unix/getrandom_dragonfly.go

@@ -0,0 +1,51 @@
+// Copyright 2021 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 unix
+
+import (
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+var randomUnsupported int32 // atomic
+
+// DragonFlyBSD getrandom system call number.
+const randomTrap uintptr = 550
+
+// GetRandomFlag is a flag supported by the getrandom system call.
+type GetRandomFlag uintptr
+
+const (
+	// GRND_RANDOM is only set for portability purpose, no-op on DragonFlyBSD.
+	GRND_RANDOM GetRandomFlag = 0x0001
+
+	// GRND_NONBLOCK means return EAGAIN rather than blocking.
+	GRND_NONBLOCK GetRandomFlag = 0x0002
+
+	// GRND_INSECURE is an GRND_NONBLOCK alias
+	GRND_INSECURE GetRandomFlag = 0x0004
+)
+
+// GetRandom calls the DragonFlyBSD getrandom system call.
+func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	if atomic.LoadInt32(&randomUnsupported) != 0 {
+		return 0, syscall.ENOSYS
+	}
+	r1, _, errno := syscall.Syscall(randomTrap,
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		uintptr(flags))
+	if errno != 0 {
+		if errno == syscall.ENOSYS {
+			atomic.StoreInt32(&randomUnsupported, 1)
+		}
+		return 0, errno
+	}
+	return int(r1), nil
+}