crypto/rand, internal/syscall/unix: add support for getrandom on NetBSD ≥ 10.0

The getrandom syscall was added to NetBSD in version 10.0, see
https://man.netbsd.org/NetBSD-10.0-STABLE/getrandom.2

Change-Id: I2714c1040791f7f4728be8d869058a38cbd93d4d
Reviewed-on: https://go-review.googlesource.com/c/go/+/463123
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>

src/crypto/rand/rand.go

@@ -11,7 +11,7 @@ import "io"
 // Reader is a global, shared instance of a cryptographically
 // secure random number generator.
 //
-// On Linux, FreeBSD, Dragonfly and Solaris, Reader uses getrandom(2) if
+// On Linux, FreeBSD, Dragonfly, NetBSD and Solaris, Reader uses getrandom(2) if
 // available, /dev/urandom otherwise.
 // On OpenBSD and macOS, Reader uses getentropy(2).
 // On other Unix-like systems, Reader reads from /dev/urandom.

src/crypto/rand/rand_getrandom.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux || freebsd || dragonfly || solaris
+//go:build dragonfly || freebsd || linux || netbsd || solaris
 
 package rand
 
@@ -21,7 +21,7 @@ func init() {
 		//     is returned by a single call to getrandom() on systems where int
 		//     has a size of 32 bits.
 		maxGetRandomRead = (1 << 25) - 1
-	case "freebsd", "dragonfly", "solaris", "illumos":
+	case "dragonfly", "freebsd", "illumos", "netbsd", "solaris":
 		maxGetRandomRead = 1 << 8
 	default:
 		panic("no maximum specified for GetRandom")

src/internal/syscall/unix/getrandom_netbsd.go

@@ -0,0 +1,56 @@
+// Copyright 2023 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"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+)
+
+// NetBSD getrandom system call number.
+const getrandomTrap uintptr = 91
+
+var getrandomUnsupported int32 // atomic
+
+// GetRandomFlag is a flag supported by the getrandom system call.
+type GetRandomFlag uintptr
+
+// GetRandom calls the getrandom system call.
+func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	if atomic.LoadInt32(&getrandomUnsupported) != 0 {
+		return 0, syscall.ENOSYS
+	}
+	// getrandom(2) was added in NetBSD 10.0
+	if getOSRevision() < 1000000000 {
+		atomic.StoreInt32(&getrandomUnsupported, 1)
+		return 0, syscall.ENOSYS
+	}
+	r1, _, errno := syscall.Syscall(getrandomTrap,
+		uintptr(unsafe.Pointer(&p[0])),
+		uintptr(len(p)),
+		uintptr(flags))
+	if errno != 0 {
+		if errno == syscall.ENOSYS {
+			atomic.StoreInt32(&getrandomUnsupported, 1)
+		}
+		return 0, errno
+	}
+	return int(r1), nil
+}
+
+var (
+	osrevisionOnce sync.Once
+	osrevision     uint32
+)
+
+func getOSRevision() uint32 {
+	osrevisionOnce.Do(func() { osrevision, _ = syscall.SysctlUint32("kern.osrevision") })
+	return osrevision
+}