github.com/u-root/u-root@v7.0.1-0.20200915234505-ad7babab0a8e+incompatible/pkg/rand/random_linux.go (about)

     1  // Copyright 2019 the u-root Authors. All rights reserved
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package rand
     6  
     7  import (
     8  	"context"
     9  	"log"
    10  	"sync"
    11  	"syscall"
    12  	"time"
    13  
    14  	"github.com/u-root/u-root/pkg/cmdline"
    15  	"golang.org/x/sys/unix"
    16  )
    17  
    18  var defaultContextReader = &getrandomReader{}
    19  
    20  var backupReader = &urandomReader{}
    21  
    22  type getrandomReader struct {
    23  	once   sync.Once
    24  	backup bool
    25  }
    26  
    27  // ReadContext implements a cancelable read from /dev/urandom.
    28  func (r *getrandomReader) ReadContext(ctx context.Context, b []byte) (int, error) {
    29  	r.once.Do(func() {
    30  		if cmdline.ContainsFlag("uroot.nohwrng") {
    31  			r.backup = true
    32  			return
    33  		}
    34  		if _, err := unix.Getrandom(b, unix.GRND_NONBLOCK); err == syscall.ENOSYS {
    35  			r.backup = true
    36  		}
    37  	})
    38  	if r.backup {
    39  		return backupReader.ReadContext(ctx, b)
    40  	}
    41  
    42  	for {
    43  		// getrandom(2) with GRND_NONBLOCK uses the urandom number
    44  		// source, but only returns numbers if the crng has been
    45  		// initialized.
    46  		//
    47  		// This is preferrable to /dev/urandom, as /dev/urandom will
    48  		// make up fake random numbers until the crng has been
    49  		// initialized.
    50  		n, err := unix.Getrandom(b, unix.GRND_NONBLOCK)
    51  		if err == nil {
    52  			return n, err
    53  		}
    54  		select {
    55  		case <-ctx.Done():
    56  			return 0, ctx.Err()
    57  
    58  		default:
    59  			if err != nil && err != syscall.EAGAIN && err != syscall.EINTR {
    60  				return n, err
    61  			}
    62  		}
    63  	}
    64  }
    65  
    66  // ReadContextWithSlowLogs logs a helpful message if it takes a significant
    67  // amount of time (>2s) to produce random data.
    68  func (r *getrandomReader) ReadContextWithSlowLogs(ctx context.Context, b []byte) (int, error) {
    69  	d := 2 * time.Second
    70  	t := time.AfterFunc(d, func() {
    71  		log.Printf("getrandom is taking a long time (>%v). "+
    72  			"If running on hardware, consider enabling Linux's CONFIG_RANDOM_TRUST_CPU=y. "+
    73  			"If running in a VM/emulator, try setting up virtio-rng.", d)
    74  	})
    75  	defer t.Stop()
    76  	return r.ReadContext(ctx, b)
    77  }