crypto/rand: use getrandom system call on Linux

Adds internal/syscall package.

Fixes Issue 8520

LGTM=r, agl
R=agl, rsc, r
CC=golang-codereviews, iant
https://codereview.appspot.com/123260044

2 files changed, 49 insertions, 1 deletions

diff --git a/src/pkg/crypto/rand/rand_linux.go b/src/pkg/crypto/rand/rand_linux.go
new file mode 100644
index 000000000..8cb59c75d
--- /dev/null
+++ b/src/pkg/crypto/rand/rand_linux.go
@@ -0,0 +1,39 @@
+// Copyright 2014 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
+
+import (
+	"internal/syscall"
+	"sync"
+)
+
+func init() {
+	altGetRandom = getRandomLinux
+}
+
+var (
+	once       sync.Once
+	useSyscall bool
+)
+
+func pickStrategy() {
+	// Test whether we should use the system call or /dev/urandom.
+	// We'll fall back to urandom if:
+	// - the kernel is too old (before 3.17)
+	// - the machine has no entropy available (early boot + no hardware
+	//   entropy source?) and we want to avoid blocking later.
+	var buf [1]byte
+	n, err := syscall.GetRandom(buf[:], syscall.GRND_NONBLOCK)
+	useSyscall = n == 1 && err == nil
+}
+
+func getRandomLinux(p []byte) (ok bool) {
+	once.Do(pickStrategy)
+	if !useSyscall {
+		return false
+	}
+	n, err := syscall.GetRandom(p, 0)
+	return n == len(p) && err == nil
+}
diff --git a/src/pkg/crypto/rand/rand_unix.go b/src/pkg/crypto/rand/rand_unix.go
index 1e741fda1..62d0fbdb3 100644
--- a/src/pkg/crypto/rand/rand_unix.go
+++ b/src/pkg/crypto/rand/rand_unix.go
@@ -20,6 +20,8 @@ import (
 	"time"
 )
 
+const urandomDevice = "/dev/urandom"
+
 // Easy implementation: read from /dev/urandom.
 // This is sufficient on Linux, OS X, and FreeBSD.
 
@@ -27,7 +29,7 @@ func init() {
 	if runtime.GOOS == "plan9" {
 		Reader = newReader(nil)
 	} else {
-		Reader = &devReader{name: "/dev/urandom"}
+		Reader = &devReader{name: urandomDevice}
 	}
 }
 
@@ -38,7 +40,14 @@ type devReader struct {
 	mu   sync.Mutex
 }
 
+// altGetRandom if non-nil specifies an OS-specific function to get
+// urandom-style randomness.
+var altGetRandom func([]byte) (ok bool)
+
 func (r *devReader) Read(b []byte) (n int, err error) {
+	if altGetRandom != nil && r.name == urandomDevice && altGetRandom(b) {
+		return len(b), nil
+	}
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	if r.f == nil {