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support loop-dev mount in mount_rootfs #305

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235 changes: 234 additions & 1 deletion crates/shim/src/mount.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,6 +22,8 @@ use std::{
ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, Not},
path::Path,
};
#[cfg(target_os = "linux")]
use std::{fs::File, os::fd::AsRawFd};

use lazy_static::lazy_static;
use log::error;
Expand All @@ -42,7 +44,62 @@ struct Flag {
flags: MsFlags,
}

#[cfg(target_os = "linux")]
#[derive(Debug, Default)]
pub struct LoopParams {
readonly: bool,
auto_clear: bool,
direct: bool,
}

#[cfg(target_os = "linux")]
#[repr(C)]
#[derive(Debug)]
pub struct LoopInfo {
device: u64,
inode: u64,
rdevice: u64,
offset: u64,
size_limit: u64,
number: u32,
encrypt_type: u32,
encrypt_key_size: u32,
flags: u32,
file_name: [u8; 64],
crypt_name: [u8; 64],
encrypt_key: [u8; 32],
init: [u64; 2],
}

#[cfg(target_os = "linux")]
impl Default for LoopInfo {
fn default() -> Self {
LoopInfo {
device: 0,
inode: 0,
rdevice: 0,
offset: 0,
size_limit: 0,
number: 0,
encrypt_type: 0,
encrypt_key_size: 0,
flags: 0,
file_name: [0; 64],
crypt_name: [0; 64],
encrypt_key: [0; 32],
init: [0; 2],
}
}
}

#[cfg(target_os = "linux")]
const OVERLAY_LOWERDIR_PREFIX: &str = "lowerdir=";
#[cfg(target_os = "linux")]
const LOOP_CONTROL_PATH: &str = "/dev/loop-control";
#[cfg(target_os = "linux")]
const LOOP_DEV_FORMAT: &str = "/dev/loop";
#[cfg(target_os = "linux")]
const EBUSY_STRING: &str = "device or resource busy";

#[cfg(target_os = "linux")]
lazy_static! {
Expand Down Expand Up @@ -543,13 +600,17 @@ pub fn mount_rootfs(

let mut flags: MsFlags = MsFlags::from_bits(0).unwrap();
let mut data = Vec::new();
let mut lo_setup = false;
let mut loop_params = LoopParams::default();
options.iter().for_each(|x| {
if let Some(f) = MOUNT_FLAGS.get(x.as_str()) {
if f.clear {
flags.bitand_assign(f.flags.not());
} else {
flags.bitor_assign(f.flags)
}
} else if x.as_str() == "loop" {
lo_setup = true;
} else {
data.push(x.as_str())
}
Expand All @@ -570,9 +631,23 @@ pub fn mount_rootfs(
}
// mount with non-propagation first, or remount with changed data
let oflags = flags.bitand(PROPAGATION_TYPES.not());
if lo_setup {
loop_params = LoopParams {
readonly: oflags.bitand(MsFlags::MS_RDONLY) == MsFlags::MS_RDONLY,
auto_clear: true,
direct: false,
};
}
let zero: MsFlags = MsFlags::from_bits(0).unwrap();
if flags.bitand(MsFlags::MS_REMOUNT).eq(&zero) || data.is_some() {
mount(source, target.as_ref(), fs_type, oflags, data).map_err(mount_error!(
let mut lo_file = String::new();
let s = if lo_setup {
lo_file = setup_loop(source, loop_params)?;
Some(lo_file.as_str())
} else {
source
};
mount(s, target.as_ref(), fs_type, oflags, data).map_err(mount_error!(
e,
"Mount {:?} to {}",
source,
Expand Down Expand Up @@ -615,6 +690,164 @@ pub fn mount_rootfs(
Err(Error::Unimplemented("start".to_string()))
}

#[cfg(target_os = "linux")]
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I wonder if we should move linux specific code to mount_linux.rs ?
So we don't have to specify target for each function.

pub fn setup_loop(source: Option<&str>, params: LoopParams) -> Result<String> {
if source.is_none() {
return Err(Error::Other("not exist".to_string()));
}
for _ in 0..100 {
let num = get_free_loop_dev()?;
let loop_dev = format!("{}{}", LOOP_DEV_FORMAT, num);
match setup_loop_dev(source.unwrap(), loop_dev.as_str(), &params) {
Ok(_) => return Ok(loop_dev),
Err(e) => {
if e.to_string().contains(EBUSY_STRING) {
continue;
} else {
return Err(e);
}
}
}
}
Err(Error::Other(
"creating new loopback device after 100 times".to_string(),
))
}

#[cfg(target_os = "linux")]
pub fn get_free_loop_dev() -> Result<i32> {
const LOOP_CTL_GET_FREE: i32 = 0x4c82;
let loop_control = File::options()
.read(true)
.write(true)
.open(LOOP_CONTROL_PATH)
.map_err(|e| Error::IoError {
context: format!("open {} error: ", LOOP_CONTROL_PATH),
err: e,
})?;
unsafe {
#[cfg(target_env = "gnu")]
let ret = libc::ioctl(
loop_control.as_raw_fd() as libc::c_int,
LOOP_CTL_GET_FREE as libc::c_ulong,
) as i32;
#[cfg(target_env = "musl")]
let ret = libc::ioctl(
loop_control.as_raw_fd() as libc::c_int,
LOOP_CTL_GET_FREE as libc::c_int,
) as i32;
match nix::errno::Errno::result(ret) {
Ok(ret) => Ok(ret),
Err(e) => Err(Error::Nix(e)),
}
}
}

#[cfg(target_os = "linux")]
pub fn setup_loop_dev(backing_file: &str, loop_dev: &str, params: &LoopParams) -> Result<File> {
const LOOP_SET_FD: u32 = 0x4c00;
const LOOP_CLR_FD: u32 = 0x4c01;
const LOOP_SET_STATUS64: u32 = 0x4c04;
const LOOP_SET_DIRECT_IO: u32 = 0x4c08;
const LO_FLAGS_READ_ONLY: u32 = 0x1;
const LO_FLAGS_AUTOCLEAR: u32 = 0x4;
let mut open_options = File::options();
open_options.read(true);
if !params.readonly {
open_options.write(true);
}
// 1. open backing file
let back = open_options
.open(backing_file)
.map_err(|e| Error::IoError {
context: format!("open {} error: ", backing_file),
err: e,
})?;
let loop_dev = open_options.open(loop_dev).map_err(|e| Error::IoError {
context: format!("open {} error: ", loop_dev),
err: e,
})?;
// 2. set FD
unsafe {
#[cfg(target_env = "gnu")]
let ret = libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_SET_FD as libc::c_ulong,
back.as_raw_fd() as libc::c_int,
);
#[cfg(target_env = "musl")]
let ret = libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_SET_FD as libc::c_int,
back.as_raw_fd() as libc::c_int,
);
if let Err(e) = nix::errno::Errno::result(ret) {
return Err(Error::Nix(e));
}
}
// 3. set info
let mut info = LoopInfo::default();
info.file_name[..backing_file.as_bytes().len()].copy_from_slice(backing_file.as_bytes());
if params.readonly {
info.flags |= LO_FLAGS_READ_ONLY;
}

if params.auto_clear {
info.flags |= LO_FLAGS_AUTOCLEAR;
}
unsafe {
#[cfg(target_env = "gnu")]
let ret = libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_SET_STATUS64 as libc::c_ulong,
info,
);
#[cfg(target_env = "musl")]
if let Err(e) = nix::errno::Errno::result(ret) {
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@Tim-Zhang Tim-Zhang Jul 17, 2025 •
edited
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The variable ret does not defined in musl env

libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_CLR_FD as libc::c_int,
0,
);
return Err(Error::Nix(e));
}
}

// 4. Set Direct IO
if params.direct {
unsafe {
#[cfg(target_env = "gnu")]
let ret = libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_SET_DIRECT_IO as libc::c_ulong,
1,
);
#[cfg(target_env = "musl")]
let ret = libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_SET_DIRECT_IO as libc::c_int,
1,
);
if let Err(e) = nix::errno::Errno::result(ret) {
#[cfg(target_env = "gnu")]
libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_CLR_FD as libc::c_ulong,
0,
);
#[cfg(target_env = "musl")]
libc::ioctl(
loop_dev.as_raw_fd() as libc::c_int,
LOOP_CLR_FD as libc::c_int,
0,
);
return Err(Error::Nix(e));
}
}
}
Ok(loop_dev)
}

#[cfg(test)]
#[cfg(target_os = "linux")]
mod tests {
Expand Down