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// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package rand
import (
"bufio"
"crypto/rand"
"io"
"golang.org/x/sys/unix"
"gvisor.dev/gvisor/pkg/sync"
)
// reader implements an io.Reader that returns pseudorandom bytes.
type reader struct {
once sync.Once
useGetrandom bool
}
// Read implements io.Reader.Read.
func (r *reader) Read(p []byte) (int, error) {
r.once.Do(func() {
_, err := unix.Getrandom(p, 0)
if err != unix.ENOSYS {
r.useGetrandom = true
}
})
if r.useGetrandom {
return unix.Getrandom(p, 0)
}
return rand.Read(p)
}
// bufferedReader implements a threadsafe buffered io.Reader.
type bufferedReader struct {
mu sync.Mutex
r *bufio.Reader
}
// Read implements io.Reader.Read.
func (b *bufferedReader) Read(p []byte) (int, error) {
// In Linux, reads of up to page size bytes will always complete fully.
// See drivers/char/random.c:get_random_bytes_user().
// NOTE(gvisor.dev/issue/9445): Some applications rely on this behavior.
const pageSize = 4096
min := len(p)
if min > pageSize {
min = pageSize
}
b.mu.Lock()
defer b.mu.Unlock()
return io.ReadAtLeast(b.r, p, min)
}
// Reader is the default reader.
var Reader io.Reader = &bufferedReader{r: bufio.NewReader(&reader{})}
// Read reads from the default reader.
func Read(b []byte) (int, error) {
return io.ReadFull(Reader, b)
}
// Init can be called to make sure /dev/urandom is pre-opened on kernels that
// do not support getrandom(2).
func Init() error {
p := make([]byte, 1)
_, err := Read(p)
return err
}
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