Support nsfs

kernel/src/fs/path/mount_namespace.rs

@@ -5,6 +5,7 @@ use spin::Once;
 use crate::{
     fs::{
         path::{Mount, Path, PathResolver},
+        pseudofs::{NsCommonOps, NsFs, NsType},
         ramfs::RamFs,
     },
     prelude::*,
@@ -22,6 +23,8 @@ pub struct MountNamespace {
     root: Arc<Mount>,
     /// The user namespace that owns this mount namespace.
     owner: Arc<UserNamespace>,
+    /// The path in nsfs.
+    path: Path,
 }
 
 impl MountNamespace {
@@ -36,7 +39,8 @@ impl MountNamespace {
 
             Arc::new_cyclic(|weak_self| {
                 let root = Mount::new_root(rootfs, weak_self.clone());
-                MountNamespace { root, owner }
+                let path = NsFs::new_path(weak_self.clone());
+                MountNamespace { root, owner, path }
             })
         })
     }
@@ -73,10 +77,11 @@ impl MountNamespace {
         let new_mnt_ns = Arc::new_cyclic(|weak_self| {
             let new_root =
                 root_mount.clone_mount_tree(root_mount.root_dentry(), Some(weak_self), true);
-
+            let path = NsFs::new_path(weak_self.clone());
             MountNamespace {
                 root: new_root,
                 owner,
+                path,
             }
         });
 
@@ -134,3 +139,26 @@ impl Drop for MountNamespace {
         }
     }
 }
+
+impl NsCommonOps for MountNamespace {
+    const TYPE: NsType = NsType::Mnt;
+
+    fn get_owner_user_ns(&self) -> Result<&Arc<UserNamespace>> {
+        Ok(&self.owner)
+    }
+
+    fn get_parent(&self) -> Result<Arc<Self>> {
+        return_errno_with_message!(
+            Errno::EINVAL,
+            "mnt namespace does not have parent namespace"
+        );
+    }
+
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}

kernel/src/fs/procfs/pid/task/mod.rs

@@ -12,8 +12,8 @@ use crate::{
                 cgroup::CgroupFileOps, cmdline::CmdlineFileOps, comm::CommFileOps,
                 environ::EnvironFileOps, exe::ExeSymOps, fd::FdDirOps, gid_map::GidMapFileOps,
                 maps::MapsFileOps, mem::MemFileOps, mountinfo::MountInfoFileOps,
-                mounts::MountsFileOps, oom_score_adj::OomScoreAdjFileOps, stat::StatFileOps,
-                status::StatusFileOps, uid_map::UidMapFileOps,
+                mounts::MountsFileOps, ns::NsDirOps, oom_score_adj::OomScoreAdjFileOps,
+                stat::StatFileOps, status::StatusFileOps, uid_map::UidMapFileOps,
             },
             template::{
                 DirOps, ProcDir, ProcDirBuilder, lookup_child_from_table,
@@ -38,6 +38,7 @@ mod maps;
 mod mem;
 mod mountinfo;
 mod mounts;
+mod ns;
 mod oom_score_adj;
 mod stat;
 mod status;
@@ -111,6 +112,7 @@ impl TidDirOps {
         ("gid_map", GidMapFileOps::new_inode),
         ("mem", MemFileOps::new_inode),
         ("mountinfo", MountInfoFileOps::new_inode),
+        ("ns", NsDirOps::new_inode),
         ("oom_score_adj", OomScoreAdjFileOps::new_inode),
         ("stat", StatFileOps::new_inode),
         ("status", StatusFileOps::new_inode),

kernel/src/fs/procfs/pid/task/ns.rs

@@ -0,0 +1,180 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use core::marker::PhantomData;
+
+use aster_util::slot_vec::SlotVec;
+use ostd::sync::RwMutexUpgradeableGuard;
+
+use crate::{
+    fs::{
+        path::{MountNamespace, Path},
+        procfs::{
+            DirOps, ProcDir, ProcDirBuilder, ProcSymBuilder, SymOps, pid::TidDirOps,
+            template::ProcSym,
+        },
+        pseudofs::NsCommonOps,
+        utils::{DirEntryVecExt, Inode, SymbolicLink, mkmod},
+    },
+    net::uts_ns::UtsNamespace,
+    prelude::*,
+    process::{NsProxy, UserNamespace, posix_thread::AsPosixThread},
+};
+
+/// Represents the inode at `/proc/[pid]/task/[tid]/ns` (and also `/proc/[pid]/ns`).
+pub(super) struct NsDirOps {
+    dir: TidDirOps,
+}
+
+impl NsDirOps {
+    /// Creates a new directory inode for the `ns` directory.
+    pub fn new_inode(dir: &TidDirOps, parent: Weak<dyn Inode>) -> Arc<dyn Inode> {
+        ProcDirBuilder::new(
+            Self { dir: dir.clone() },
+            // Reference: <https://elixir.bootlin.com/linux/v6.18/source/fs/proc/base.c#L3321>
+            mkmod!(u + r, a + x),
+        )
+        .parent(parent)
+        .build()
+        .unwrap()
+    }
+}
+
+impl NsDirOps {
+    /// The set of namespace entries that depend on the thread's [`NsProxy`].
+    #[expect(clippy::type_complexity)]
+    const NS_PROXY_ENTRIES: &[(&str, fn(&NsProxy, Weak<dyn Inode>) -> Arc<dyn Inode>)] = &[
+        ("uts", |proxy, parent| {
+            NsSymOps::new_inode(proxy.uts_ns(), parent)
+        }),
+        ("mnt", |proxy, parent| {
+            NsSymOps::new_inode(proxy.mnt_ns(), parent)
+        }),
+    ];
+
+    /// Looks up a namespace symlink backed by the thread's [`NsProxy`].
+    fn lookup_ns_proxy_child(
+        &self,
+        name: &str,
+        parent: Weak<dyn Inode>,
+    ) -> Option<Result<Arc<dyn Inode>>> {
+        let constructor = Self::NS_PROXY_ENTRIES
+            .iter()
+            .find(|(entry_name, _)| *entry_name == name)
+            .map(|(_, ctor)| ctor)?;
+
+        let thread = self.dir.thread();
+        let ns_proxy = thread.as_posix_thread().unwrap().ns_proxy().lock();
+        let Some(ns_proxy) = ns_proxy.as_ref() else {
+            return Some(Err(Error::with_message(
+                Errno::ENOENT,
+                "the thread's namespace proxy no longer exists",
+            )));
+        };
+
+        Some(Ok(constructor(ns_proxy, parent)))
+    }
+}
+
+impl DirOps for NsDirOps {
+    fn lookup_child(&self, dir: &ProcDir<Self>, name: &str) -> Result<Arc<dyn Inode>> {
+        let inode = if let Some(result) = self.lookup_ns_proxy_child(name, dir.this_weak().clone())
+        {
+            result?
+        } else if name == "user" {
+            let user_ns = self.dir.process_ref.user_ns().lock();
+            NsSymOps::new_inode(&*user_ns, dir.this_weak().clone())
+        } else {
+            return_errno_with_message!(Errno::ENOENT, "the file does not exist");
+        };
+
+        let mut cached_children = dir.cached_children().write();
+        cached_children.remove_entry_by_name(name);
+        cached_children.put((name.to_string(), inode.clone()));
+        Ok(inode)
+    }
+
+    fn populate_children<'a>(
+        &self,
+        dir: &'a ProcDir<Self>,
+    ) -> RwMutexUpgradeableGuard<'a, SlotVec<(String, Arc<dyn Inode>)>> {
+        let mut cached_children = dir.cached_children().write();
+
+        let thread = self.dir.thread();
+        let ns_proxy = thread.as_posix_thread().unwrap().ns_proxy().lock();
+
+        // Refresh NsProxy-backed entries: remove stale ones and re-add if the proxy is alive.
+        for &(name, constructor) in Self::NS_PROXY_ENTRIES {
+            cached_children.remove_entry_by_name(name);
+            if let Some(ns_proxy) = ns_proxy.as_ref() {
+                let inode = constructor(ns_proxy, dir.this_weak().clone());
+                cached_children.put((name.to_string(), inode));
+            }
+        }
+
+        // The user namespace never changes, so only insert if absent.
+        cached_children.put_entry_if_not_found("user", || {
+            let user_ns = self.dir.process_ref.user_ns().lock();
+            NsSymOps::new_inode(&*user_ns, dir.this_weak().clone())
+        });
+
+        cached_children.downgrade()
+    }
+
+    fn validate_child(&self, child: &dyn Inode) -> bool {
+        // The user namespace of a thread/process never changes,
+        // so a user-ns symlink is always valid.
+        if child.downcast_ref::<NsSymlink<UserNamespace>>().is_some() {
+            return true;
+        }
+
+        // Checks whether `child` still matches the corresponding namespace
+        // in the thread's current `NsProxy`.
+
+        let thread = self.dir.thread();
+        let ns_proxy = thread.as_posix_thread().unwrap().ns_proxy().lock();
+        let Some(ns_proxy) = ns_proxy.as_ref() else {
+            return false;
+        };
+
+        if let Some(sym) = child.downcast_ref::<NsSymlink<UtsNamespace>>() {
+            return &sym.inner().ns_path == ns_proxy.uts_ns().path();
+        }
+
+        if let Some(sym) = child.downcast_ref::<NsSymlink<MountNamespace>>() {
+            return &sym.inner().ns_path == ns_proxy.mnt_ns().path();
+        }
+
+        // TODO: Support additional namespace types.
+        false
+    }
+}
+
+type NsSymlink<T> = ProcSym<NsSymOps<T>>;
+
+/// Represents the inode at `/proc/[pid]/task/[tid]/ns/<type>` (and also `/proc/[pid]/ns/<type>`).
+pub struct NsSymOps<T: NsCommonOps> {
+    ns_path: Path,
+    phantom: PhantomData<T>,
+}
+
+impl<T: NsCommonOps> NsSymOps<T> {
+    /// Creates a new symlink inode pointing to the given namespace.
+    fn new_inode(ns: &Arc<T>, parent: Weak<dyn Inode>) -> Arc<dyn Inode> {
+        ProcSymBuilder::new(
+            Self {
+                ns_path: ns.path().clone(),
+                phantom: PhantomData,
+            },
+            mkmod!(a + rwx),
+        )
+        .parent(parent)
+        .build()
+        .unwrap()
+    }
+}
+
+impl<T: NsCommonOps> SymOps for NsSymOps<T> {
+    fn read_link(&self) -> Result<SymbolicLink> {
+        Ok(SymbolicLink::Path(self.ns_path.clone()))
+    }
+}

kernel/src/fs/pseudofs/mod.rs

@@ -6,6 +6,7 @@ use core::{
 };
 
 pub use anon_inode_fs::AnonInodeFs;
+pub use nsfs::{NsCommonOps, NsFile, NsFs, NsType};
 pub use pidfdfs::PidfdFs;
 pub(super) use pipefs::PipeFs;
 use pipefs::PipeFsType;
@@ -28,6 +29,7 @@ use crate::{
 };
 
 mod anon_inode_fs;
+mod nsfs;
 mod pidfdfs;
 mod pipefs;
 mod sockfs;
@@ -122,6 +124,7 @@ pub enum PseudoInodeType {
     Socket,
     AnonInode,
     Pidfd,
+    Ns,
 }
 
 impl From<PseudoInodeType> for InodeType {
@@ -132,6 +135,7 @@ impl From<PseudoInodeType> for InodeType {
             PseudoInodeType::Socket => InodeType::Socket,
             PseudoInodeType::AnonInode => InodeType::Unknown,
             PseudoInodeType::Pidfd => InodeType::Unknown,
+            PseudoInodeType::Ns => InodeType::File,
         }
     }
 }

kernel/src/fs/pseudofs/nsfs.rs

@@ -0,0 +1,365 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use alloc::format;
+use core::time::Duration;
+
+use inherit_methods_macro::inherit_methods;
+use ostd::task::Task;
+use spin::Once;
+
+use crate::{
+    events::IoEvents,
+    fs::{
+        file_table::{FdFlags, FileDesc},
+        inode_handle::{FileIo, InodeHandle},
+        path::{Mount, Path},
+        pseudofs::{PseudoFs, PseudoInode, PseudoInodeType},
+        utils::{
+            AccessMode, Extension, FileSystem, Inode, InodeIo, InodeMode, InodeType, Metadata,
+            StatusFlags, mkmod,
+        },
+    },
+    prelude::*,
+    process::{
+        CloneFlags, Gid, Uid, UserNamespace,
+        signal::{PollHandle, Pollable},
+    },
+    util::ioctl::{RawIoctl, dispatch_ioctl},
+};
+
+/// A pseudo filesystem for namespace files.
+pub struct NsFs {
+    _private: (),
+}
+
+impl NsFs {
+    /// Returns the singleton instance of the ns filesystem.
+    pub fn singleton() -> &'static Arc<PseudoFs> {
+        static NSFS: Once<Arc<PseudoFs>> = Once::new();
+        PseudoFs::singleton(&NSFS, "nsfs", NSFS_MAGIC)
+    }
+
+    /// Creates a pseudo [`Path`] for a namespace file.
+    pub fn new_path<T: NsCommonOps>(ns: Weak<T>) -> Path {
+        let ns_inode = {
+            let ino = Self::singleton().alloc_id();
+            let fs = Arc::downgrade(Self::singleton());
+            Arc::new(NsInode::new(ino, Uid::new_root(), Gid::new_root(), ns, fs))
+        };
+
+        Path::new_pseudo(Self::mount_node().clone(), ns_inode, |inode| {
+            let inode = inode.downcast_ref::<NsInode<T>>().unwrap();
+            inode.name().to_string()
+        })
+    }
+
+    /// Returns the pseudo mount node of the ns filesystem.
+    pub fn mount_node() -> &'static Arc<Mount> {
+        static NSFS_MOUNT: Once<Arc<Mount>> = Once::new();
+        NSFS_MOUNT.call_once(|| Mount::new_pseudo(Self::singleton().clone()))
+    }
+}
+
+/// An inode representing a namespace entry in [`NsFs`].
+struct NsInode<T: NsCommonOps> {
+    common: PseudoInode,
+    ns: Weak<T>,
+    name: String,
+}
+
+impl<T: NsCommonOps> NsInode<T> {
+    fn new(ino: u64, uid: Uid, gid: Gid, ns: Weak<T>, fs: Weak<PseudoFs>) -> Self {
+        let mode = mkmod!(a + r);
+        let common = PseudoInode::new(ino, PseudoInodeType::Ns, mode, uid, gid, fs);
+        let name = format!("{}:[{}]", T::NAME, ino);
+
+        Self { common, ns, name }
+    }
+
+    fn name(&self) -> &str {
+        &self.name
+    }
+}
+
+#[inherit_methods(from = "self.common")]
+impl<T: NsCommonOps> Inode for NsInode<T> {
+    fn size(&self) -> usize;
+    fn resize(&self, _new_size: usize) -> Result<()>;
+    fn metadata(&self) -> Metadata;
+    fn extension(&self) -> &Extension;
+    fn ino(&self) -> u64;
+    fn type_(&self) -> InodeType;
+    fn mode(&self) -> Result<InodeMode>;
+    fn set_mode(&self, mode: InodeMode) -> Result<()>;
+    fn owner(&self) -> Result<Uid>;
+    fn set_owner(&self, uid: Uid) -> Result<()>;
+    fn group(&self) -> Result<Gid>;
+    fn set_group(&self, gid: Gid) -> Result<()>;
+    fn atime(&self) -> Duration;
+    fn set_atime(&self, time: Duration);
+    fn mtime(&self) -> Duration;
+    fn set_mtime(&self, time: Duration);
+    fn ctime(&self) -> Duration;
+    fn set_ctime(&self, time: Duration);
+    fn fs(&self) -> Arc<dyn FileSystem>;
+
+    fn open(
+        &self,
+        access_mode: AccessMode,
+        _status_flags: StatusFlags,
+    ) -> Option<Result<Box<dyn FileIo>>> {
+        // FIXME: This may not be the most appropriate place to check the access mode,
+        // but the check must not be bypassed even if the current process has the
+        // CAP_DAC_OVERRIDE capability. It is hard to find a better place for it,
+        // and an extra check here does no harm.
+        if access_mode.is_writable() {
+            return Some(Err(Error::with_message(
+                Errno::EPERM,
+                "ns files cannot be opened as writable",
+            )));
+        }
+
+        let ns = self
+            .ns
+            .upgrade()
+            .ok_or_else(|| Error::with_message(Errno::EPERM, "the namespace no longer exists"));
+
+        Some(ns.map(|ns| Box::new(NsFile { ns }) as Box<dyn FileIo>))
+    }
+}
+
+#[inherit_methods(from = "self.common")]
+impl<T: NsCommonOps> InodeIo for NsInode<T> {
+    fn read_at(
+        &self,
+        _offset: usize,
+        _writer: &mut VmWriter,
+        _status: StatusFlags,
+    ) -> Result<usize>;
+    fn write_at(
+        &self,
+        _offset: usize,
+        _reader: &mut VmReader,
+        _status: StatusFlags,
+    ) -> Result<usize>;
+}
+
+/// A file handle referencing a live namespace.
+pub struct NsFile<T: NsCommonOps> {
+    ns: Arc<T>,
+}
+
+impl<T: NsCommonOps> NsFile<T> {
+    /// Returns a reference to the underlying namespace.
+    pub fn ns(&self) -> &Arc<T> {
+        &self.ns
+    }
+}
+
+impl<T: NsCommonOps> FileIo for NsFile<T> {
+    fn check_seekable(&self) -> Result<()> {
+        return_errno_with_message!(Errno::ESPIPE, "ns files is not seekable");
+    }
+
+    fn is_offset_aware(&self) -> bool {
+        false
+    }
+
+    fn ioctl(&self, raw_ioctl: RawIoctl) -> Result<i32> {
+        use ioctl_defs::*;
+        dispatch_ioctl!(match raw_ioctl {
+            _cmd @ GetUserNs => {
+                let user_ns = self.ns.get_owner_user_ns()?;
+
+                let current = current!();
+                let current_user_ns = current.user_ns().lock();
+                if !current_user_ns.is_same_or_ancestor_of(user_ns) {
+                    return_errno_with_message!(
+                        Errno::EPERM,
+                        "the owner user namespace is not an ancestor of the current namespace"
+                    );
+                }
+
+                open_ns_as_file(user_ns.as_ref())
+            }
+            _cmd @ GetParent => {
+                let parent = self.ns.get_parent()?;
+                open_ns_as_file(parent.as_ref())
+            }
+            _cmd @ GetType => {
+                let clone_flags = CloneFlags::from(T::TYPE);
+                Ok(clone_flags.bits().cast_signed())
+            }
+            cmd @ GetOwnerUid => {
+                let user_ns = self
+                    .ns
+                    .as_any()
+                    .downcast_ref::<UserNamespace>()
+                    .ok_or_else(|| {
+                        Error::with_message(
+                            Errno::EINVAL,
+                            "the ns file does not correspond to a user namespace",
+                        )
+                    })?;
+                let uid = user_ns.get_owner_uid()?;
+                cmd.write(&uid.into())?;
+                Ok(0)
+            }
+            // TODO: Support additional iotcl commands
+            _ => return_errno_with_message!(Errno::ENOTTY, "unsupported ioctl command"),
+        })
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}
+
+impl<T: NsCommonOps> Pollable for NsFile<T> {
+    fn poll(&self, mask: IoEvents, _poller: Option<&mut PollHandle>) -> IoEvents {
+        (IoEvents::IN | IoEvents::OUT | IoEvents::RDNORM) & mask
+    }
+}
+
+impl<T: NsCommonOps> InodeIo for NsFile<T> {
+    fn read_at(
+        &self,
+        _offset: usize,
+        _writer: &mut VmWriter,
+        _status_flags: StatusFlags,
+    ) -> Result<usize> {
+        return_errno_with_message!(Errno::EINVAL, "ns files do not support read_at");
+    }
+
+    fn write_at(
+        &self,
+        _offset: usize,
+        _reader: &mut VmReader,
+        _status_flags: StatusFlags,
+    ) -> Result<usize> {
+        return_errno_with_message!(Errno::EINVAL, "ns files do not support write_at");
+    }
+}
+
+/// Opens a namespace as a file and returns the file descriptor.
+fn open_ns_as_file<T: NsCommonOps>(ns: &T) -> Result<FileDesc> {
+    let path = ns.path();
+    let inode_handle = InodeHandle::new(path.clone(), AccessMode::O_RDONLY, StatusFlags::empty())?;
+
+    let current_task = Task::current().unwrap();
+    let thread_local = current_task.as_thread_local().unwrap();
+    let mut file_table_ref = thread_local.borrow_file_table_mut();
+    let mut file_table = file_table_ref.unwrap().write();
+    let fd = file_table.insert(Arc::new(inode_handle), FdFlags::CLOEXEC);
+
+    Ok(fd)
+}
+
+/// Common operations shared by all namespace types.
+///
+/// Implementors represent a specific kind of namespace (e.g., UTS, mount, user)
+/// and must provide the associated metadata and traversal methods required by
+/// [`NsFs`] and [`NsFile`].
+pub trait NsCommonOps: Any + Send + Sync + 'static {
+    /// The human-readable name of this namespace kind (derived from [`Self::TYPE`]).
+    const NAME: &str = Self::TYPE.as_str();
+
+    /// The [`NsType`] discriminant for this namespace kind.
+    const TYPE: NsType;
+
+    /// Returns the owner user namespace.
+    fn get_owner_user_ns(&self) -> Result<&Arc<UserNamespace>>;
+
+    /// Returns the parent namespace, if one exists.
+    fn get_parent(&self) -> Result<Arc<Self>>;
+
+    /// Returns the pseudo filesystem [`Path`] associated with this namespace.
+    fn path(&self) -> &Path;
+
+    fn as_any(&self) -> &dyn Any;
+}
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum NsType {
+    Uts,
+    User,
+    Mnt,
+    #[expect(unused)]
+    Pid,
+    #[expect(unused)]
+    Time,
+    #[expect(unused)]
+    Cgroup,
+    #[expect(unused)]
+    Ipc,
+    #[expect(unused)]
+    Net,
+}
+
+impl NsType {
+    const fn as_str(&self) -> &'static str {
+        match self {
+            NsType::Uts => "uts",
+            NsType::User => "user",
+            NsType::Mnt => "mnt",
+            NsType::Pid => "pid",
+            NsType::Time => "time",
+            NsType::Cgroup => "cgroup",
+            NsType::Ipc => "ipc",
+            NsType::Net => "net",
+        }
+    }
+}
+
+impl From<NsType> for CloneFlags {
+    fn from(value: NsType) -> Self {
+        match value {
+            NsType::Uts => CloneFlags::CLONE_NEWUTS,
+            NsType::User => CloneFlags::CLONE_NEWUSER,
+            NsType::Mnt => CloneFlags::CLONE_NEWNS,
+            NsType::Pid => CloneFlags::CLONE_NEWPID,
+            NsType::Time => CloneFlags::CLONE_NEWTIME,
+            NsType::Cgroup => CloneFlags::CLONE_NEWCGROUP,
+            NsType::Ipc => CloneFlags::CLONE_NEWIPC,
+            NsType::Net => CloneFlags::CLONE_NEWNET,
+        }
+    }
+}
+
+// Reference: <https://elixir.bootlin.com/linux/v6.16.5/source/include/uapi/linux/magic.h#L95>
+const NSFS_MAGIC: u64 = 0x6e736673;
+
+mod ioctl_defs {
+    use crate::util::ioctl::{InData, NoData, OutData, ioc};
+
+    // Legacy encoding ioctl commands
+
+    /// Returns a file descriptor of the owner user namespace.
+    pub type GetUserNs       = ioc!(NS_GET_USERNS, 0xb701, NoData);
+    /// Returns a file descriptor of the parent namespace.
+    pub type GetParent     = ioc!(NS_GET_PARENT, 0xb702, NoData);
+    /// Gets the type of the namespace (e.g., user, pid, mnt, etc.).
+    pub type GetType       = ioc!(NS_GET_NSTYPE, 0xb703, NoData);
+    /// Gets the user ID of the namespace owner.
+    ///
+    /// Only user namespace supports this operation.
+    pub type GetOwnerUid = ioc!(NS_GET_OWNER_ID, 0xb704, OutData<u32>);
+
+    // Modern encoding ioctl commands
+
+    #[expect(unused)]
+    /// Gets the ID of the mount namespace.
+    pub type GetMntNsId    = ioc!(NS_GET_MNTNS_ID, 0xb7, 0x5, OutData<u64>);
+    /// Translates thread ID from the target PID namespace into the caller's PID namespace.
+    #[expect(unused)]
+    pub type GetTidFromPidNs = ioc!(NS_GET_PID_FROM_PIDNS, 0xb7, 0x6, InData<i32>);
+    /// Translates process ID from the target PID namespace into the caller's PID namespace.
+    #[expect(unused)]
+    pub type GetPidFromPidNs = ioc!(NS_GET_TGID_FROM_PIDNS, 0xb7, 0x7, InData<i32>);
+    /// Translates thread ID from the caller's PID namespace into the target PID namespace.
+    #[expect(unused)]
+    pub type GetTidInPidNs = ioc!(NS_GET_PID_IN_PIDNS, 0xb7, 0x8, InData<i32>);
+    /// Translates process ID from the caller's PID namespace into the target PID namespace.
+    #[expect(unused)]
+    pub type GetPidInPidNs = ioc!(NS_GET_TGID_IN_PIDNS, 0xb7, 0x9, InData<i32>);
+}

kernel/src/net/uts_ns.rs

@@ -4,6 +4,10 @@ use ostd::{const_assert, sync::RwMutexReadGuard};
 use spin::Once;
 
 use crate::{
+    fs::{
+        path::Path,
+        pseudofs::{NsCommonOps, NsFs, NsType},
+    },
     prelude::*,
     process::{UserNamespace, credentials::capabilities::CapSet, posix_thread::PosixThread},
     util::padded,
@@ -13,6 +17,7 @@ use crate::{
 pub struct UtsNamespace {
     uts_name: RwMutex<UtsName>,
     owner: Arc<UserNamespace>,
+    path: Path,
 }
 
 impl UtsNamespace {
@@ -33,11 +38,18 @@ impl UtsNamespace {
             };
 
             let owner = UserNamespace::get_init_singleton().clone();
+            Self::new(uts_name, owner)
+        })
+    }
 
-            Arc::new(Self {
+    fn new(uts_name: UtsName, owner: Arc<UserNamespace>) -> Arc<Self> {
+        Arc::new_cyclic(|weak_self| {
+            let path = NsFs::new_path(weak_self.clone());
+            Self {
                 uts_name: RwMutex::new(uts_name),
                 owner,
-            })
+                path,
+            }
         })
     }
 
@@ -48,10 +60,7 @@ impl UtsNamespace {
         posix_thread: &PosixThread,
     ) -> Result<Arc<Self>> {
         owner.check_cap(CapSet::SYS_ADMIN, posix_thread)?;
-        Ok(Arc::new(Self {
-            uts_name: RwMutex::new(*self.uts_name.read()),
-            owner,
-        }))
+        Ok(Self::new(*self.uts_name.read(), owner))
     }
 
     /// Returns the owner user namespace of the namespace.
@@ -195,3 +204,26 @@ impl UtsName {
         }
     };
 }
+
+impl NsCommonOps for UtsNamespace {
+    const TYPE: NsType = NsType::Uts;
+
+    fn get_owner_user_ns(&self) -> Result<&Arc<UserNamespace>> {
+        Ok(&self.owner)
+    }
+
+    fn get_parent(&self) -> Result<Arc<Self>> {
+        return_errno_with_message!(
+            Errno::EINVAL,
+            "UTS namespace does not have parent namespace"
+        );
+    }
+
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+}

kernel/src/process/namespace/user_ns.rs

@@ -3,13 +3,18 @@
 use spin::Once;
 
 use crate::{
+    fs::{
+        path::Path,
+        pseudofs::{NsCommonOps, NsFs, NsType},
+    },
     prelude::*,
-    process::{credentials::capabilities::CapSet, posix_thread::PosixThread},
+    process::{Uid, credentials::capabilities::CapSet, posix_thread::PosixThread},
 };
 
 /// The user namespace.
 pub struct UserNamespace {
     _private: (),
+    path: Path,
 }
 
 impl UserNamespace {
@@ -17,7 +22,14 @@ impl UserNamespace {
     pub fn get_init_singleton() -> &'static Arc<UserNamespace> {
         static INIT: Once<Arc<UserNamespace>> = Once::new();
 
-        INIT.call_once(|| Arc::new(UserNamespace { _private: () }))
+        INIT.call_once(Self::new)
+    }
+
+    fn new() -> Arc<Self> {
+        Arc::new_cyclic(|weak_self| {
+            let path = NsFs::new_path(weak_self.clone());
+            Self { _private: (), path }
+        })
     }
 
     /// Checks whether the thread has the required capability in this user namespace.
@@ -37,4 +49,47 @@ impl UserNamespace {
             "the thread does not have the required capability"
         )
     }
+
+    /// Returns the owner UID of the user namespace.
+    pub fn get_owner_uid(&self) -> Result<Uid> {
+        // FIXME: The owner of the user namespace is not yet tracked.
+        // Return the correct user ID once ownership tracking is implemented.
+        Ok(Uid::new_root())
+    }
+
+    /// Returns whether this namespace is the same as, or an ancestor of, the other namespace.
+    pub fn is_same_or_ancestor_of(self: &Arc<Self>, other: &Arc<Self>) -> bool {
+        // FIXME: Creating new user namespaces is not yet supported,
+        // so we simply check pointer equality.
+        // Once user namespace creation is implemented,
+        // this should walk up the ancestor chain to verify
+        // whether `self` is an ancestor of `other`.
+        Arc::ptr_eq(self, other)
+    }
+}
+
+impl NsCommonOps for UserNamespace {
+    const TYPE: NsType = NsType::User;
+
+    fn get_owner_user_ns(&self) -> Result<&Arc<UserNamespace>> {
+        return_errno_with_message!(
+            Errno::EPERM,
+            "a user namespace does not have an owner user namespace"
+        );
+    }
+
+    fn get_parent(&self) -> Result<Arc<Self>> {
+        return_errno_with_message!(
+            Errno::EPERM,
+            "getting the parent of a user namespace is not supported"
+        );
+    }
+
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
 }