crypto/rand: remove all buffering

The kernel's RNG is fast enough, and buffering means taking locks, which
we don't want to do. So just remove all buffering. This also means the
randomness we get is "fresher". That also means we don't need any
locking, making this potentially faster if multiple cores are hitting
GetRandom() at the same time on newer Linuxes.

Also, change the build tag of the tests to be 'unix' instead of
enumerating them.

Change-Id: Ia773fab768270d2aa20c0649f4171c5326b71d02
Reviewed-on: https://go-review.googlesource.com/c/go/+/390038
Reviewed-by: Filippo Valsorda <valsorda@google.com>
Run-TryBot: Jason Donenfeld <Jason@zx2c4.com>
Auto-Submit: Jason Donenfeld <Jason@zx2c4.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 19 insertions, 45 deletions

diff --git a/src/crypto/rand/rand_batched_test.go b/src/crypto/rand/rand_batched_test.go
index 7a981e7892f..dfb9517d5e0 100644
--- a/src/crypto/rand/rand_batched_test.go
+++ b/src/crypto/rand/rand_batched_test.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 aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris
+//go:build unix
 
 package rand
 
diff --git a/src/crypto/rand/rand_unix.go b/src/crypto/rand/rand_unix.go
index 2dd4158888f..07715bd5ee0 100644
--- a/src/crypto/rand/rand_unix.go
+++ b/src/crypto/rand/rand_unix.go
@@ -10,11 +10,11 @@
 package rand
 
 import (
-	"bufio"
 	"errors"
 	"io"
 	"os"
 	"sync"
+	"sync/atomic"
 	"syscall"
 	"time"
 )
@@ -29,7 +29,7 @@ func init() {
 type reader struct {
 	f    io.Reader
 	mu   sync.Mutex
-	used bool // whether this reader has been used
+	used uint32 // Atomic: 0 - never used, 1 - used, but f == nil, 2 - used, and f != nil
 }
 
 // altGetRandom if non-nil specifies an OS-specific function to get
@@ -37,24 +37,11 @@ type reader struct {
 var altGetRandom func([]byte) (ok bool)
 
 // batched returns a function that calls f to populate a []byte by chunking it
-// into subslices of, at most, readMax bytes, buffering min(readMax, 4096)
-// bytes at a time.
+// into subslices of, at most, readMax bytes.
 func batched(f func([]byte) error, readMax int) func([]byte) bool {
-	bufferSize := 4096
-	if bufferSize > readMax {
-		bufferSize = readMax
-	}
-	fullBuffer := make([]byte, bufferSize)
-	var buf []byte
 	return func(out []byte) bool {
-		// First we copy any amount remaining in the buffer.
-		n := copy(out, buf)
-		out, buf = out[n:], buf[n:]
-
-		// Then, if we're requesting more than the buffer size,
-		// generate directly into the output, chunked by readMax.
-		for len(out) >= len(fullBuffer) {
-			read := len(out) - (len(out) % len(fullBuffer))
+		for len(out) > 0 {
+			read := len(out)
 			if read > readMax {
 				read = readMax
 			}
@@ -63,22 +50,6 @@ func batched(f func([]byte) error, readMax int) func([]byte) bool {
 			}
 			out = out[read:]
 		}
-
-		// If there's a partial block left over, fill the buffer,
-		// and copy in the remainder.
-		if len(out) > 0 {
-			if f(fullBuffer[:]) != nil {
-				return false
-			}
-			buf = fullBuffer[:]
-			n = copy(out, buf)
-			out, buf = out[n:], buf[n:]
-		}
-
-		if len(out) > 0 {
-			panic("crypto/rand batching failed to fill buffer")
-		}
-
 		return true
 	}
 }
@@ -88,10 +59,7 @@ func warnBlocked() {
 }
 
 func (r *reader) Read(b []byte) (n int, err error) {
-	r.mu.Lock()
-	defer r.mu.Unlock()
-	if !r.used {
-		r.used = true
+	if atomic.CompareAndSwapUint32(&r.used, 0, 1) {
 		// First use of randomness. Start timer to warn about
 		// being blocked on entropy not being available.
 		t := time.AfterFunc(time.Minute, warnBlocked)
@@ -100,14 +68,20 @@ func (r *reader) Read(b []byte) (n int, err error) {
 	if altGetRandom != nil && altGetRandom(b) {
 		return len(b), nil
 	}
-	if r.f == nil {
-		f, err := os.Open(urandomDevice)
-		if err != nil {
-			return 0, err
+	if atomic.LoadUint32(&r.used) != 2 {
+		r.mu.Lock()
+		if r.used != 2 {
+			f, err := os.Open(urandomDevice)
+			if err != nil {
+				r.mu.Unlock()
+				return 0, err
+			}
+			r.f = hideAgainReader{f}
+			atomic.StoreUint32(&r.used, 2)
 		}
-		r.f = bufio.NewReader(hideAgainReader{f})
+		r.mu.Unlock()
 	}
-	return r.f.Read(b)
+	return io.ReadFull(r.f, b)
 }
 
 // hideAgainReader masks EAGAIN reads from /dev/urandom.