github.com/metacubex/gvisor@v0.0.0-20240320004321-933faba989ec/pkg/rand/rand_linux.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package rand
    16  
    17  import (
    18  	"bufio"
    19  	"crypto/rand"
    20  	"io"
    21  
    22  	"golang.org/x/sys/unix"
    23  	"github.com/metacubex/gvisor/pkg/sync"
    24  )
    25  
    26  // reader implements an io.Reader that returns pseudorandom bytes.
    27  type reader struct {
    28  	once         sync.Once
    29  	useGetrandom bool
    30  }
    31  
    32  // Read implements io.Reader.Read.
    33  func (r *reader) Read(p []byte) (int, error) {
    34  	r.once.Do(func() {
    35  		_, err := unix.Getrandom(p, 0)
    36  		if err != unix.ENOSYS {
    37  			r.useGetrandom = true
    38  		}
    39  	})
    40  
    41  	if r.useGetrandom {
    42  		return unix.Getrandom(p, 0)
    43  	}
    44  	return rand.Read(p)
    45  }
    46  
    47  // bufferedReader implements a threadsafe buffered io.Reader.
    48  type bufferedReader struct {
    49  	mu sync.Mutex
    50  	r  *bufio.Reader
    51  }
    52  
    53  // Read implements io.Reader.Read.
    54  func (b *bufferedReader) Read(p []byte) (int, error) {
    55  	// In Linux, reads of up to page size bytes will always complete fully.
    56  	// See drivers/char/random.c:get_random_bytes_user().
    57  	// NOTE(gvisor.dev/issue/9445): Some applications rely on this behavior.
    58  	const pageSize = 4096
    59  	min := len(p)
    60  	if min > pageSize {
    61  		min = pageSize
    62  	}
    63  	b.mu.Lock()
    64  	defer b.mu.Unlock()
    65  	return io.ReadAtLeast(b.r, p, min)
    66  }
    67  
    68  // Reader is the default reader.
    69  var Reader io.Reader = &bufferedReader{r: bufio.NewReader(&reader{})}
    70  
    71  // Read reads from the default reader.
    72  func Read(b []byte) (int, error) {
    73  	return io.ReadFull(Reader, b)
    74  }
    75  
    76  // Init can be called to make sure /dev/urandom is pre-opened on kernels that
    77  // do not support getrandom(2).
    78  func Init() error {
    79  	p := make([]byte, 1)
    80  	_, err := Read(p)
    81  	return err
    82  }