client: implement map/unmap commands for .img backups

Allows mapping fixed-index .img files (usually from VM backups) to be
mapped to a local loopback device.

The architecture uses a FUSE-backed temp file mapped to a loopdev:

  /dev/loopX -> FUSE /run/pbs-loopdev/xxx -> backup client -> PBS

Since unmapping requires some cleanup (unmap the loopdev, stop FUSE,
remove the temp files) a special 'unmap' command is added, which uses a
PID file to send SIGINT to the backup-client instance started with
'map', which will handle the cleanup itself.

The polling with select! in mount.rs needs to be split in two, since we
have a chicken and egg problem between running FUSE and setting up the
loop device - so we need to do them concurrently, until the loopdev is
assigned, at which point we can report success and daemonize, and then
continue polling the FUSE loop future.

A loopdev module is added to tools containing all required functions for
mapping a loop device to the FUSE file, with the ioctls moved into an
inline module to avoid exposing them directly.

The client code is placed in the 'mount' module, which, while
admittedly a loose fit, allows reuse of the daemonizing code.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/bin/proxmox-backup-client.rs

@@ -1970,6 +1970,8 @@ fn main() {
         .insert("status", status_cmd_def)
         .insert("key", key::cli())
         .insert("mount", mount_cmd_def())
+        .insert("map", map_cmd_def())
+        .insert("unmap", unmap_cmd_def())
         .insert("catalog", catalog_mgmt_cli())
         .insert("task", task_mgmt_cli())
         .insert("version", version_cmd_def)

src/bin/proxmox_backup_client/mount.rs

@@ -3,6 +3,7 @@ use std::sync::Arc;
 use std::os::unix::io::RawFd;
 use std::path::Path;
 use std::ffi::OsStr;
+use std::collections::HashMap;
 
 use anyhow::{bail, format_err, Error};
 use serde_json::Value;
@@ -10,6 +11,7 @@ use tokio::signal::unix::{signal, SignalKind};
 use nix::unistd::{fork, ForkResult, pipe};
 use futures::select;
 use futures::future::FutureExt;
+use futures::stream::{StreamExt, TryStreamExt};
 
 use proxmox::{sortable, identity};
 use proxmox::api::{ApiHandler, ApiMethod, RpcEnvironment, schema::*, cli::*};
@@ -23,6 +25,7 @@ use proxmox_backup::backup::{
     BackupDir,
     BackupGroup,
     BufferedDynamicReader,
+    AsyncIndexReader,
 };
 
 use proxmox_backup::client::*;
@@ -50,7 +53,34 @@ const API_METHOD_MOUNT: ApiMethod = ApiMethod::new(
             ("target", false, &StringSchema::new("Target directory path.").schema()),
             ("repository", true, &REPO_URL_SCHEMA),
             ("keyfile", true, &StringSchema::new("Path to encryption key.").schema()),
-            ("verbose", true, &BooleanSchema::new("Verbose output.").default(false).schema()),
+            ("verbose", true, &BooleanSchema::new("Verbose output and stay in foreground.").default(false).schema()),
+        ]),
+    )
+);
+
+#[sortable]
+const API_METHOD_MAP: ApiMethod = ApiMethod::new(
+    &ApiHandler::Sync(&mount),
+    &ObjectSchema::new(
+        "Map a drive image from a VM backup to a local loopback device. Use 'unmap' to undo.
+WARNING: Only do this with *trusted* backups!",
+        &sorted!([
+            ("snapshot", false, &StringSchema::new("Group/Snapshot path.").schema()),
+            ("archive-name", false, &StringSchema::new("Backup archive name.").schema()),
+            ("repository", true, &REPO_URL_SCHEMA),
+            ("keyfile", true, &StringSchema::new("Path to encryption key.").schema()),
+            ("verbose", true, &BooleanSchema::new("Verbose output and stay in foreground.").default(false).schema()),
+        ]),
+    )
+);
+
+#[sortable]
+const API_METHOD_UNMAP: ApiMethod = ApiMethod::new(
+    &ApiHandler::Sync(&unmap),
+    &ObjectSchema::new(
+        "Unmap a loop device mapped with 'map' and release all resources.",
+        &sorted!([
+            ("loopdev", false, &StringSchema::new("Path to loopdev (/dev/loopX) or loop device number.").schema()),
         ]),
     )
 );
@@ -65,6 +95,22 @@ pub fn mount_cmd_def() -> CliCommand {
         .completion_cb("target", tools::complete_file_name)
 }
 
+pub fn map_cmd_def() -> CliCommand {
+
+    CliCommand::new(&API_METHOD_MAP)
+        .arg_param(&["snapshot", "archive-name"])
+        .completion_cb("repository", complete_repository)
+        .completion_cb("snapshot", complete_group_or_snapshot)
+        .completion_cb("archive-name", complete_pxar_archive_name)
+}
+
+pub fn unmap_cmd_def() -> CliCommand {
+
+    CliCommand::new(&API_METHOD_UNMAP)
+        .arg_param(&["loopdev"])
+        .completion_cb("loopdev", tools::complete_file_name)
+}
+
 fn mount(
     param: Value,
     _info: &ApiMethod,
@@ -100,9 +146,10 @@ fn mount(
 async fn mount_do(param: Value, pipe: Option<RawFd>) -> Result<Value, Error> {
     let repo = extract_repository_from_value(&param)?;
     let archive_name = tools::required_string_param(&param, "archive-name")?;
-    let target = tools::required_string_param(&param, "target")?;
     let client = connect(repo.host(), repo.port(), repo.user())?;
 
+    let target = param["target"].as_str();
+
     record_repository(&repo);
 
     let path = tools::required_string_param(&param, "snapshot")?;
@@ -124,9 +171,17 @@ async fn mount_do(param: Value, pipe: Option<RawFd>) -> Result<Value, Error> {
     };
 
     let server_archive_name = if archive_name.ends_with(".pxar") {
+        if let None = target {
+            bail!("use the 'mount' command to mount pxar archives");
+        }
         format!("{}.didx", archive_name)
+    } else if archive_name.ends_with(".img") {
+        if let Some(_) = target {
+            bail!("use the 'map' command to map drive images");
+        }
+        format!("{}.fidx", archive_name)
     } else {
-        bail!("Can only mount pxar archives.");
+        bail!("Can only mount/map pxar archives and drive images.");
     };
 
     let client = BackupReader::start(
@@ -143,25 +198,7 @@ async fn mount_do(param: Value, pipe: Option<RawFd>) -> Result<Value, Error> {
 
     let file_info = manifest.lookup_file_info(&server_archive_name)?;
 
-    if server_archive_name.ends_with(".didx") {
-        let index = client.download_dynamic_index(&manifest, &server_archive_name).await?;
-        let most_used = index.find_most_used_chunks(8);
-        let chunk_reader = RemoteChunkReader::new(client.clone(), crypt_config, file_info.chunk_crypt_mode(), most_used);
-        let reader = BufferedDynamicReader::new(index, chunk_reader);
-        let archive_size = reader.archive_size();
-        let reader: proxmox_backup::pxar::fuse::Reader =
-            Arc::new(BufferedDynamicReadAt::new(reader));
-        let decoder = proxmox_backup::pxar::fuse::Accessor::new(reader, archive_size).await?;
-        let options = OsStr::new("ro,default_permissions");
-
-        let session = proxmox_backup::pxar::fuse::Session::mount(
-            decoder,
-            &options,
-            false,
-            Path::new(target),
-        )
-        .map_err(|err| format_err!("pxar mount failed: {}", err))?;
-
+    let daemonize = || -> Result<(), Error> {
         if let Some(pipe) = pipe {
             nix::unistd::chdir(Path::new("/")).unwrap();
             // Finish creation of daemon by redirecting filedescriptors.
@@ -182,10 +219,35 @@ async fn mount_do(param: Value, pipe: Option<RawFd>) -> Result<Value, Error> {
             nix::unistd::close(pipe).unwrap();
         }
 
-        // handle SIGINT and SIGTERM
-        let mut interrupt_int = signal(SignalKind::interrupt())?;
-        let mut interrupt_term = signal(SignalKind::terminate())?;
-        let mut interrupt = futures::future::select(interrupt_int.next(), interrupt_term.next());
+        Ok(())
+    };
+
+    let options = OsStr::new("ro,default_permissions");
+
+    // handle SIGINT and SIGTERM
+    let mut interrupt_int = signal(SignalKind::interrupt())?;
+    let mut interrupt_term = signal(SignalKind::terminate())?;
+    let mut interrupt = futures::future::select(interrupt_int.next(), interrupt_term.next());
+
+    if server_archive_name.ends_with(".didx") {
+        let index = client.download_dynamic_index(&manifest, &server_archive_name).await?;
+        let most_used = index.find_most_used_chunks(8);
+        let chunk_reader = RemoteChunkReader::new(client.clone(), crypt_config, file_info.chunk_crypt_mode(), most_used);
+        let reader = BufferedDynamicReader::new(index, chunk_reader);
+        let archive_size = reader.archive_size();
+        let reader: proxmox_backup::pxar::fuse::Reader =
+            Arc::new(BufferedDynamicReadAt::new(reader));
+        let decoder = proxmox_backup::pxar::fuse::Accessor::new(reader, archive_size).await?;
+
+        let session = proxmox_backup::pxar::fuse::Session::mount(
+            decoder,
+            &options,
+            false,
+            Path::new(target.unwrap()),
+        )
+        .map_err(|err| format_err!("pxar mount failed: {}", err))?;
+
+        daemonize()?;
 
         select! {
             res = session.fuse() => res?,
@@ -193,9 +255,73 @@ async fn mount_do(param: Value, pipe: Option<RawFd>) -> Result<Value, Error> {
                 // exit on interrupted
             }
         }
+    } else if server_archive_name.ends_with(".fidx") {
+        let index = client.download_fixed_index(&manifest, &server_archive_name).await?;
+        let size = index.index_bytes();
+        let chunk_reader = RemoteChunkReader::new(client.clone(), crypt_config, file_info.chunk_crypt_mode(), HashMap::new());
+        let reader = AsyncIndexReader::new(index, chunk_reader);
+
+        let mut session = tools::fuse_loop::FuseLoopSession::map_loop(size, reader, options).await?;
+        let loopdev = session.loopdev_path.clone();
+
+        let (st_send, st_recv) = futures::channel::mpsc::channel(1);
+        let (mut abort_send, abort_recv) = futures::channel::mpsc::channel(1);
+        let mut st_recv = st_recv.fuse();
+        let mut session_fut = session.main(st_send, abort_recv).boxed().fuse();
+
+        // poll until loop file is mapped (or errors)
+        select! {
+            res = session_fut => {
+                bail!("FUSE session unexpectedly ended before loop file mapping");
+            },
+            res = st_recv.try_next() => {
+                if let Err(err) = res {
+                    // init went wrong, abort now
+                    abort_send.try_send(()).map_err(|err|
+                        format_err!("error while sending abort signal - {}", err))?;
+                    // ignore and keep original error cause
+                    let _ = session_fut.await;
+                    return Err(err);
+                }
+            }
+        }
+
+        // daemonize only now to be able to print mapped loopdev or startup errors
+        println!("Image mapped as {}", loopdev);
+        daemonize()?;
+
+        // continue polling until complete or interrupted (which also happens on unmap)
+        select! {
+            res = session_fut => res?,
+            _ = interrupt => {
+                // exit on interrupted
+                abort_send.try_send(()).map_err(|err|
+                    format_err!("error while sending abort signal - {}", err))?;
+                session_fut.await?;
+            }
+        }
+
+        println!("Image unmapped");
     } else {
-        bail!("unknown archive file extension (expected .pxar)");
+        bail!("unknown archive file extension (expected .pxar or .img)");
     }
 
     Ok(Value::Null)
 }
+
+fn unmap(
+    param: Value,
+    _info: &ApiMethod,
+    _rpcenv: &mut dyn RpcEnvironment,
+) -> Result<Value, Error> {
+
+    let mut path = tools::required_string_param(&param, "loopdev")?.to_owned();
+
+    if let Ok(num) = path.parse::<u8>() {
+        path = format!("/dev/loop{}", num);
+    }
+
+    tools::fuse_loop::unmap(path)?;
+
+    Ok(Value::Null)
+}

src/tools.rs

@@ -33,6 +33,8 @@ pub mod statistics;
 pub mod systemd;
 pub mod nom;
 pub mod logrotate;
+pub mod loopdev;
+pub mod fuse_loop;
 
 mod parallel_handler;
 pub use parallel_handler::*;

src/tools/fuse_loop.rs

@@ -0,0 +1,258 @@
+use anyhow::{Error, format_err, bail};
+use std::ffi::OsStr;
+use std::path::{Path, PathBuf};
+use std::fs::{File, remove_file, read_to_string};
+use std::io::SeekFrom;
+use std::io::prelude::*;
+
+use nix::unistd::{Pid, mkstemp};
+use nix::sys::signal::{self, Signal};
+
+use tokio::io::{AsyncRead, AsyncSeek, AsyncReadExt, AsyncSeekExt};
+use futures::stream::{StreamExt, TryStreamExt};
+use futures::channel::mpsc::{Sender, Receiver};
+
+use proxmox::try_block;
+use proxmox_fuse::{*, requests::FuseRequest};
+use super::loopdev;
+
+const RUN_DIR: &'static str = "/run/pbs-loopdev";
+
+pub struct FuseLoopSession<R: AsyncRead + AsyncSeek + Unpin> {
+    session: Option<Fuse>,
+    stat: libc::stat,
+    reader: R,
+    fuse_path: String,
+    pid_path: String,
+    pub loopdev_path: String,
+}
+
+impl<R: AsyncRead + AsyncSeek + Unpin> FuseLoopSession<R> {
+
+    /// Prepare for mapping the given reader as a block device node at
+    /// /dev/loopN. Creates a temporary file for FUSE and a PID file for unmap.
+    pub async fn map_loop(size: u64, mut reader: R, options: &OsStr)
+        -> Result<Self, Error>
+    {
+        // attempt a single read to check if the reader is configured correctly
+        let _ = reader.read_u8().await?;
+
+        std::fs::create_dir_all(RUN_DIR)?;
+        let mut base_path = PathBuf::from(RUN_DIR);
+        base_path.push("XXXXXX"); // template for mkstemp
+        let (_, path) = mkstemp(&base_path)?;
+        let mut pid_path = path.clone();
+        pid_path.set_extension("pid");
+
+        let res: Result<(Fuse, String), Error> = try_block!{
+            let session = Fuse::builder("pbs-block-dev")?
+                .options_os(options)?
+                .enable_read()
+                .build()?
+                .mount(&path)?;
+
+            let loopdev_path = loopdev::get_or_create_free_dev().map_err(|err| {
+                format_err!("loop-control GET_FREE failed - {}", err)
+            })?;
+
+            // write pidfile so unmap can later send us a signal to exit
+            Self::write_pidfile(&pid_path)?;
+
+            Ok((session, loopdev_path))
+        };
+
+        match res {
+            Ok((session, loopdev_path)) =>
+                Ok(Self {
+                    session: Some(session),
+                    reader,
+                    stat: minimal_stat(size as i64),
+                    fuse_path: path.to_string_lossy().into_owned(),
+                    pid_path: pid_path.to_string_lossy().into_owned(),
+                    loopdev_path,
+                }),
+            Err(e) => {
+                // best-effort temp file cleanup in case of error
+                let _ = remove_file(&path);
+                let _ = remove_file(&pid_path);
+                Err(e)
+            }
+        }
+    }
+
+    fn write_pidfile(path: &Path) -> Result<(), Error> {
+        let pid = unsafe { libc::getpid() };
+        let mut file = File::create(path)?;
+        write!(file, "{}", pid)?;
+        Ok(())
+    }
+
+    /// Runs the FUSE request loop and assigns the loop device. Will send a
+    /// message on startup_chan once the loop device is assigned (or assignment
+    /// fails). Send a message on abort_chan to trigger cleanup and exit FUSE.
+    /// An error on loopdev assignment does *not* automatically close the FUSE
+    /// handle or do cleanup, trigger abort_chan manually in case startup fails.
+    pub async fn main(
+        &mut self,
+        mut startup_chan: Sender<Result<(), Error>>,
+        abort_chan: Receiver<()>,
+    ) -> Result<(), Error> {
+
+        if let None = self.session {
+            panic!("internal error: fuse_loop::main called before ::map_loop");
+        }
+        let mut session = self.session.take().unwrap().fuse();
+        let mut abort_chan = abort_chan.fuse();
+
+        let (loopdev_path, fuse_path) = (self.loopdev_path.clone(), self.fuse_path.clone());
+        tokio::task::spawn_blocking(move || {
+            if let Err(err) = loopdev::assign(loopdev_path, fuse_path) {
+                let _ = startup_chan.try_send(Err(format_err!("error while assigning loop device - {}", err)));
+            } else {
+                // device is assigned successfully, which means not only is the
+                // loopdev ready, but FUSE is also okay, since the assignment
+                // would have failed otherwise
+                let _ = startup_chan.try_send(Ok(()));
+            }
+        });
+
+        let (loopdev_path, fuse_path, pid_path) =
+            (self.loopdev_path.clone(), self.fuse_path.clone(), self.pid_path.clone());
+        let cleanup = |session: futures::stream::Fuse<Fuse>| {
+            // only warn for errors on cleanup, if these fail nothing is lost
+            if let Err(err) = loopdev::unassign(&loopdev_path) {
+                eprintln!(
+                    "cleanup: warning: could not unassign file {} from loop device {} - {}",
+                    &fuse_path,
+                    &loopdev_path,
+                    err,
+                );
+            }
+
+            // force close FUSE handle before attempting to remove backing file
+            std::mem::drop(session);
+
+            if let Err(err) = remove_file(&fuse_path) {
+                eprintln!(
+                    "cleanup: warning: could not remove temporary file {} - {}",
+                    &fuse_path,
+                    err,
+                );
+            }
+            if let Err(err) = remove_file(&pid_path) {
+                eprintln!(
+                    "cleanup: warning: could not remove PID file {} - {}",
+                    &pid_path,
+                    err,
+                );
+            }
+        };
+
+        loop {
+            tokio::select!{
+                _ = abort_chan.next() => {
+                    // aborted, do cleanup and exit
+                    break;
+                },
+                req = session.try_next() => {
+                    let res = match req? {
+                        Some(Request::Lookup(req)) => {
+                            let stat = self.stat;
+                            let entry = EntryParam::simple(stat.st_ino, stat);
+                            req.reply(&entry)
+                        },
+                        Some(Request::Getattr(req)) => {
+                            req.reply(&self.stat, std::f64::MAX)
+                        },
+                        Some(Request::Read(req)) => {
+                            match self.reader.seek(SeekFrom::Start(req.offset)).await {
+                                Ok(_) => {
+                                    let mut buf = vec![0u8; req.size];
+                                    match self.reader.read_exact(&mut buf).await {
+                                        Ok(_) => {
+                                            req.reply(&buf)
+                                        },
+                                        Err(e) => {
+                                            req.io_fail(e)
+                                        }
+                                    }
+                                },
+                                Err(e) => {
+                                    req.io_fail(e)
+                                }
+                            }
+                        },
+                        Some(_) => {
+                            // only FUSE requests necessary for loop-mapping are implemented
+                            eprintln!("Unimplemented FUSE request type encountered");
+                            Ok(())
+                        },
+                        None => {
+                            // FUSE connection closed
+                            break;
+                        }
+                    };
+                    if let Err(err) = res {
+                        // error during FUSE reply, cleanup and exit
+                        cleanup(session);
+                        bail!(err);
+                    }
+                }
+            }
+        }
+
+        // non-error FUSE exit
+        cleanup(session);
+        Ok(())
+    }
+}
+
+/// Try and unmap a running proxmox-backup-client instance from the given
+/// /dev/loopN device
+pub fn unmap(loopdev: String) -> Result<(), Error> {
+    if loopdev.len() < 10 || !loopdev.starts_with("/dev/loop") {
+        bail!("malformed loopdev path, must be in format '/dev/loopX'");
+    }
+    let num = loopdev.split_at(9).1.parse::<u8>().map_err(|err|
+        format_err!("malformed loopdev path, does not end with valid number - {}", err))?;
+
+    let block_path = PathBuf::from(format!("/sys/devices/virtual/block/loop{}/loop/backing_file", num));
+    let backing_file = read_to_string(block_path).map_err(|err| {
+        if err.kind() == std::io::ErrorKind::NotFound {
+            format_err!("nothing mapped to {}", loopdev)
+        } else {
+            format_err!("error reading backing file - {}", err)
+        }
+    })?;
+
+    let backing_file = backing_file.trim();
+    if !backing_file.starts_with(RUN_DIR) {
+        bail!(
+            "loopdev {} is in use, but not by proxmox-backup-client (mapped to '{}')",
+            loopdev,
+            backing_file,
+        );
+    }
+
+    let mut pid_path = PathBuf::from(backing_file);
+    pid_path.set_extension("pid");
+
+    let pid_str = read_to_string(&pid_path).map_err(|err|
+        format_err!("error reading pidfile {:?}: {}", &pid_path, err))?;
+    let pid = pid_str.parse::<i32>().map_err(|err|
+        format_err!("malformed PID ({}) in pidfile - {}", pid_str, err))?;
+
+    // send SIGINT to trigger cleanup and exit in target process
+    signal::kill(Pid::from_raw(pid), Signal::SIGINT)?;
+
+    Ok(())
+}
+
+fn minimal_stat(size: i64) -> libc::stat {
+    let mut stat: libc::stat = unsafe { std::mem::zeroed() };
+    stat.st_mode = libc::S_IFREG;
+    stat.st_ino = 1;
+    stat.st_nlink = 1;
+    stat.st_size = size;
+    stat
+}

src/tools/loopdev.rs

@@ -0,0 +1,93 @@
+use anyhow::Error;
+use std::fs::{File, OpenOptions};
+use std::path::Path;
+use std::os::unix::io::{RawFd, AsRawFd};
+
+const LOOP_CONTROL: &str = "/dev/loop-control";
+const LOOP_NAME: &str = "/dev/loop";
+
+/// Implements a subset of loop device ioctls necessary to assign and release
+/// a single file from a free loopdev.
+mod loop_ioctl {
+    use nix::{ioctl_none, ioctl_write_int_bad, ioctl_write_ptr_bad};
+
+    const LOOP_IOCTL: u16 = 0x4C; // 'L'
+    const LOOP_SET_FD: u16 = 0x00;
+    const LOOP_CLR_FD: u16 = 0x01;
+    const LOOP_SET_STATUS64: u16 = 0x04;
+
+    const LOOP_CTRL_GET_FREE: u16 = 0x82;
+
+    ioctl_write_int_bad!(ioctl_set_fd, (LOOP_IOCTL << 8) | LOOP_SET_FD);
+    ioctl_none!(ioctl_clr_fd, LOOP_IOCTL, LOOP_CLR_FD);
+    ioctl_none!(ioctl_ctrl_get_free, LOOP_IOCTL, LOOP_CTRL_GET_FREE);
+    ioctl_write_ptr_bad!(ioctl_set_status64, (LOOP_IOCTL << 8) | LOOP_SET_STATUS64, LoopInfo64);
+
+    pub const LO_FLAGS_READ_ONLY: u32 = 1;
+    pub const LO_FLAGS_PARTSCAN: u32 = 8;
+
+    const LO_NAME_SIZE: usize = 64;
+    const LO_KEY_SIZE: usize = 32;
+
+    #[repr(C)]
+    pub struct LoopInfo64 {
+        pub lo_device: u64,
+        pub lo_inode: u64,
+        pub lo_rdevice: u64,
+        pub lo_offset: u64,
+        pub lo_sizelimit: u64,
+        pub lo_number: u32,
+        pub lo_encrypt_type: u32,
+        pub lo_encrypt_key_size: u32,
+        pub lo_flags: u32,
+        pub lo_file_name: [u8; LO_NAME_SIZE],
+        pub lo_crypt_name: [u8; LO_NAME_SIZE],
+        pub lo_encrypt_key: [u8; LO_KEY_SIZE],
+        pub lo_init: [u64; 2],
+    }
+}
+
+// ioctl helpers create public fns, do not export them outside the module
+// users should use the wrapper functions below
+use loop_ioctl::*;
+
+/// Use the GET_FREE ioctl to get or add a free loop device, of which the
+/// /dev/loopN path will be returned. This is inherently racy because of the
+/// delay between this and calling assign, but since assigning is atomic it
+/// does not matter much and will simply cause assign to fail.
+pub fn get_or_create_free_dev() -> Result<String, Error> {
+    let ctrl_file = File::open(LOOP_CONTROL)?;
+    let free_num = unsafe { ioctl_ctrl_get_free(ctrl_file.as_raw_fd())? };
+    let loop_file_path = format!("{}{}", LOOP_NAME, free_num);
+    Ok(loop_file_path)
+}
+
+fn assign_dev(fd: RawFd, backing_fd: RawFd) -> Result<(), Error> {
+    unsafe { ioctl_set_fd(fd, backing_fd)?; }
+
+    // set required read-only flag and partscan for convenience
+    let mut info: LoopInfo64 = unsafe { std::mem::zeroed() };
+    info.lo_flags = LO_FLAGS_READ_ONLY | LO_FLAGS_PARTSCAN;
+    unsafe { ioctl_set_status64(fd, &info)?; }
+
+    Ok(())
+}
+
+/// Open the next available /dev/loopN file and assign the given path to
+/// it as it's backing file in read-only mode.
+pub fn assign<P: AsRef<Path>>(loop_dev: P, backing: P) -> Result<(), Error> {
+    let loop_file = File::open(loop_dev)?;
+    let backing_file = OpenOptions::new()
+        .read(true)
+        .open(backing)?;
+    assign_dev(loop_file.as_raw_fd(), backing_file.as_raw_fd())?;
+    Ok(())
+}
+
+/// Unassign any file descriptors currently attached to the given
+/// /dev/loopN device.
+pub fn unassign<P: AsRef<Path>>(path: P) -> Result<(), Error> {
+    let loop_file = File::open(path)?;
+    unsafe { ioctl_clr_fd(loop_file.as_raw_fd())?; }
+    Ok(())
+}