crypto/rand: batch large calls to linux getrandom

The linux getrandom system call returns at most 33554431 = 2^25-1 bytes per
call. The existing behavior for larger reads is to report a failure, because
there appears to have been an unexpected short read. In this case the system
falls back to reading from "/dev/urandom".

This change performs reads of 2^25 bytes or more with multiple calls to
getrandom.

Fixes #20877

Change-Id: I618855bdedafd86cd11219fe453af1d6fa2c88a7
Reviewed-on: https://go-review.googlesource.com/49170
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/rand/rand_linux.go

@@ -9,7 +9,30 @@ import (
 )
 
 func init() {
-	altGetRandom = getRandomLinux
+	altGetRandom = batched(getRandomLinux, maxGetRandomRead)
+}
+
+// maxGetRandomRead is the maximum number of bytes to ask for in one call to the
+// getrandom() syscall. In linux at most 2^25-1 bytes will be returned per call.
+// From the manpage
+//
+//	*  When reading from the urandom source, a maximum of 33554431 bytes
+//	   is returned by a single call to getrandom() on systems where int
+//	   has a size of 32 bits.
+const maxGetRandomRead = (1 << 25) - 1
+
+// batched returns a function that calls f to populate a []byte by chunking it
+// into subslices of, at most, readMax bytes.
+func batched(f func([]byte) bool, readMax int) func([]byte) bool {
+	return func(buf []byte) bool {
+		for len(buf) > readMax {
+			if !f(buf[:readMax]) {
+				return false
+			}
+			buf = buf[readMax:]
+		}
+		return len(buf) == 0 || f(buf)
+	}
 }
 
 // If the kernel is too old (before 3.17) to support the getrandom syscall(),

src/crypto/rand/rand_linux_test.go

@@ -0,0 +1,42 @@
+// 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 (
+	"bytes"
+	"testing"
+)
+
+func TestBatched(t *testing.T) {
+	fillBatched := batched(func(p []byte) bool {
+		for i := range p {
+			p[i] = byte(i)
+		}
+		return true
+	}, 5)
+
+	p := make([]byte, 13)
+	if !fillBatched(p) {
+		t.Fatal("batched function returned false")
+	}
+	expected := []byte{0, 1, 2, 3, 4, 0, 1, 2, 3, 4, 0, 1, 2}
+	if !bytes.Equal(expected, p) {
+		t.Errorf("incorrect batch result: got %x, want %x", p, expected)
+	}
+}
+
+func TestBatchedError(t *testing.T) {
+	b := batched(func(p []byte) bool { return false }, 5)
+	if b(make([]byte, 13)) {
+		t.Fatal("batched function should have returned false")
+	}
+}
+
+func TestBatchedEmpty(t *testing.T) {
+	b := batched(func(p []byte) bool { return false }, 5)
+	if !b(make([]byte, 0)) {
+		t.Fatal("empty slice should always return true")
+	}
+}