From fe633972f3c9c0f9fa034a2219f5b3bccd5363c8 Mon Sep 17 00:00:00 2001
From: Wang Siyuan <wsy@stu.pku.edu.cn>
Date: Wed, 24 Dec 2025 03:40:52 +0000
Subject: [PATCH] Add pseudo `Mount`

---
 kernel/src/fs/path/mount.rs  | 15 +++++++++++++--
 kernel/src/fs/pseudofs.rs    | 22 ++++++++++++++++++++++
 kernel/src/fs/ramfs/memfd.rs | 23 ++++++++++++++++++++---
 3 files changed, 55 insertions(+), 5 deletions(-)

diff --git a/kernel/src/fs/path/mount.rs b/kernel/src/fs/path/mount.rs
index c07ff8edb..10341ef02 100644
--- a/kernel/src/fs/path/mount.rs
+++ b/kernel/src/fs/path/mount.rs
@@ -64,6 +64,8 @@ bitflags! {
         const NODIRATIME     = 1 << 11;
         /// Update atime relative to mtime/ctime.
         const RELATIME       = 1 << 21;
+        /// Kernel (pseudo) mount.
+        const KERNMOUNT      = 1 << 22;
         /// Always perform atime updates.
         const STRICTATIME    = 1 << 24;
     }
@@ -188,11 +190,19 @@ impl Mount {
         Self::new(fs, PerMountFlags::default(), None, mnt_ns)
     }
 
+    /// Creates a pseudo mount node with an associated FS.
+    ///
+    /// This pseudo mount is not mounted on other mount nodes, has no parent, and does not
+    /// belong to any mount namespace.
+    pub(in crate::fs) fn new_pseudo(fs: Arc<dyn FileSystem>) -> Arc<Self> {
+        Self::new(fs, PerMountFlags::KERNMOUNT, None, Weak::new())
+    }
+
     /// The internal constructor.
     ///
-    /// Root mount node has no mountpoint which other mount nodes must have mountpoint.
+    /// A root mount node has no mountpoint, while other mount nodes must have one.
     ///
-    /// Here, a Mount is instantiated without an initial mountpoint,
+    /// Here, a `Mount` is instantiated without an initial mountpoint,
     /// avoiding fixed mountpoint limitations. This allows the root mount node to
     /// exist without a mountpoint, ensuring uniformity and security, while all other
     /// mount nodes must be explicitly assigned a mountpoint to maintain structural integrity.
@@ -203,6 +213,7 @@ impl Mount {
         mnt_ns: Weak<MountNamespace>,
     ) -> Arc<Self> {
         let id = ID_ALLOCATOR.get().unwrap().lock().alloc().unwrap();
+
         Arc::new_cyclic(|weak_self| Self {
             id,
             root_dentry: Dentry::new_root(fs.root_inode()),
diff --git a/kernel/src/fs/pseudofs.rs b/kernel/src/fs/pseudofs.rs
index ac2220a7a..af2e4e2cd 100644
--- a/kernel/src/fs/pseudofs.rs
+++ b/kernel/src/fs/pseudofs.rs
@@ -10,6 +10,7 @@ use spin::Once;
 use super::utils::{Extension, InodeIo, StatusFlags};
 use crate::{
     fs::{
+        path::Mount,
         registry::{FsProperties, FsType},
         utils::{
             FileSystem, FsEventSubscriberStats, FsFlags, Inode, InodeMode, InodeType, Metadata,
@@ -105,6 +106,13 @@ impl PipeFs {
 
         PseudoFs::singleton(&PIPEFS, "pipefs", PIPEFS_MAGIC)
     }
+
+    /// Returns the pseudo mount node of the pipe file system.
+    fn mount_node() -> &'static Arc<Mount> {
+        static PIPEFS_MOUNT: Once<Arc<Mount>> = Once::new();
+
+        PIPEFS_MOUNT.call_once(|| Mount::new_pseudo(Self::singleton().clone()))
+    }
 }
 
 pub struct SockFs {
@@ -118,6 +126,13 @@ impl SockFs {
 
         PseudoFs::singleton(&SOCKFS, "sockfs", SOCKFS_MAGIC)
     }
+
+    /// Returns the pseudo mount node of the socket file system.
+    pub fn mount_node() -> &'static Arc<Mount> {
+        static SOCKFS_MOUNT: Once<Arc<Mount>> = Once::new();
+
+        SOCKFS_MOUNT.call_once(|| Mount::new_pseudo(Self::singleton().clone()))
+    }
 }
 
 pub struct AnonInodeFs {
@@ -132,6 +147,13 @@ impl AnonInodeFs {
         PseudoFs::singleton(&ANON_INODEFS, "anon_inodefs", ANON_INODEFS_MAGIC)
     }
 
+    /// Returns the pseudo mount node of the anonymous inode file system.
+    pub fn mount_node() -> &'static Arc<Mount> {
+        static ANON_INODEFS_MOUNT: Once<Arc<Mount>> = Once::new();
+
+        ANON_INODEFS_MOUNT.call_once(|| Mount::new_pseudo(Self::singleton().clone()))
+    }
+
     /// Returns the shared inode of the anonymous inode file system singleton.
     //
     // Some members of anon_inodefs (such as epollfd, eventfd, timerfd, etc.) share
diff --git a/kernel/src/fs/ramfs/memfd.rs b/kernel/src/fs/ramfs/memfd.rs
index f0aaf7acd..947180936 100644
--- a/kernel/src/fs/ramfs/memfd.rs
+++ b/kernel/src/fs/ramfs/memfd.rs
@@ -22,7 +22,7 @@ use crate::{
         file_handle::{FileLike, Mappable},
         file_table::FdFlags,
         inode_handle::{do_fallocate_util, do_resize_util, do_seek_util},
-        path::{RESERVED_MOUNT_ID, check_open_util},
+        path::{Mount, RESERVED_MOUNT_ID, check_open_util},
         tmpfs::TmpFs,
         utils::{
             AccessMode, CachePage, CreationFlags, Extension, FallocMode, FileSystem, Inode,
@@ -231,9 +231,26 @@ impl Inode for MemfdInode {
     }
 
     fn fs(&self) -> Arc<dyn FileSystem> {
-        // Reference: <https://elixir.bootlin.com/linux/v6.16.5/source/mm/shmem.c#L3828-L3850>
+        MemfdTmpFs::singleton().clone()
+    }
+}
+
+struct MemfdTmpFs {
+    _private: (),
+}
+
+impl MemfdTmpFs {
+    // Reference: <https://elixir.bootlin.com/linux/v6.16.5/source/mm/shmem.c#L3828-L3850>
+    fn singleton() -> &'static Arc<TmpFs> {
         static MEMFD_TMPFS: Once<Arc<TmpFs>> = Once::new();
-        MEMFD_TMPFS.call_once(TmpFs::new).clone()
+
+        MEMFD_TMPFS.call_once(TmpFs::new)
+    }
+
+    fn mount_node() -> &'static Arc<Mount> {
+        static MEMFD_TMPFS_MOUNT: Once<Arc<Mount>> = Once::new();
+
+        MEMFD_TMPFS_MOUNT.call_once(|| Mount::new_pseudo(Self::singleton().clone()))
     }
 }