github.com/craftyguy/u-root@v1.0.0/pkg/loop/losetup_linux.go (about)

     1  // Copyright 2018 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 loop
     6  
     7  import (
     8  	"fmt"
     9  	"os"
    10  	"syscall"
    11  )
    12  
    13  const (
    14  	/*
    15  	 * IOCTL commands --- we will commandeer 0x4C ('L')
    16  	 */
    17  	LOOP_SET_CAPACITY = 0x4C07
    18  	LOOP_CHANGE_FD    = 0x4C06
    19  	LOOP_GET_STATUS64 = 0x4C05
    20  	LOOP_SET_STATUS64 = 0x4C04
    21  	LOOP_GET_STATUS   = 0x4C03
    22  	LOOP_SET_STATUS   = 0x4C02
    23  	LOOP_CLR_FD       = 0x4C01
    24  	LOOP_SET_FD       = 0x4C00
    25  	LO_NAME_SIZE      = 64
    26  	LO_KEY_SIZE       = 32
    27  	/* /dev/loop-control interface */
    28  	LOOP_CTL_ADD      = 0x4C80
    29  	LOOP_CTL_REMOVE   = 0x4C81
    30  	LOOP_CTL_GET_FREE = 0x4C82
    31  )
    32  
    33  // FindDevice finds an unused loop device.
    34  func FindDevice() (name string, err error) {
    35  	cfd, err := os.OpenFile("/dev/loop-control", os.O_RDWR, 0644)
    36  	if err != nil {
    37  		return "", err
    38  	}
    39  	defer cfd.Close()
    40  
    41  	if number, err := CtlGetFree(cfd.Fd()); err != nil {
    42  		return "", err
    43  	} else {
    44  		return fmt.Sprintf("/dev/loop%d", number), nil
    45  	}
    46  
    47  }
    48  
    49  // ClearFd clears the loop device associated with filedescriptor fd.
    50  func ClearFd(fd uintptr) error {
    51  	if _, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd, LOOP_CLR_FD, 0); err != 0 {
    52  		return err
    53  	}
    54  
    55  	return nil
    56  }
    57  
    58  // CtlGetFree finds a free loop device querying the loop control device pointed
    59  // by fd. It returns the number of the free loop device /dev/loopX
    60  func CtlGetFree(fd uintptr) (uintptr, error) {
    61  	number, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd, LOOP_CTL_GET_FREE, 0)
    62  	if err != 0 {
    63  		return 0, err
    64  	}
    65  	return number, nil
    66  }
    67  
    68  // SetFd associates a loop device pointed by lfd with a regular file pointed by ffd.
    69  func SetFd(lfd, ffd uintptr) error {
    70  	_, _, err := syscall.Syscall(syscall.SYS_IOCTL, lfd, LOOP_SET_FD, ffd)
    71  	if err != 0 {
    72  		return err
    73  	}
    74  
    75  	return nil
    76  }
    77  
    78  // SetFdFiles associates loop device "devicename" with regular file "filename"
    79  func SetFdFiles(devicename, filename string) error {
    80  	mode := os.O_RDWR
    81  	file, err := os.OpenFile(filename, mode, 0644)
    82  	if err != nil {
    83  		mode = os.O_RDONLY
    84  		file, err = os.OpenFile(filename, mode, 0644)
    85  		if err != nil {
    86  			return err
    87  		}
    88  	}
    89  	defer file.Close()
    90  
    91  	device, err := os.OpenFile(devicename, mode, 0644)
    92  	if err != nil {
    93  		return err
    94  	}
    95  	defer device.Close()
    96  
    97  	return SetFd(device.Fd(), file.Fd())
    98  }
    99  
   100  // ClearFdFile clears the loop device "devicename"
   101  func ClearFdFile(devicename string) error {
   102  	device, err := os.Open(devicename)
   103  	if err != nil {
   104  		return err
   105  	}
   106  	defer device.Close()
   107  
   108  	return ClearFd(device.Fd())
   109  }