feat(vfs): Implement mount propagation mechanism in VFS

[fslongjin]

• Added support for mount propagation types: Shared, Private, Slave, and Unbindable.
• Introduced a new module for managing mount propagation semantics, including peer group registration and event propagation.
• Updated existing mount functions to handle propagation logic during mount and unmount operations.
• Enhanced documentation to include details on the new mount propagation features and their usage.
• Added unit tests to verify the correctness of mount propagation behavior across different scenarios.

This implementation aligns with Linux semantics for mount propagation, ensuring compatibility and expected behavior in containerized environments.;

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Note

Adds full mount propagation semantics with peer groups, bind/recursive bind, propagation-type syscalls, namespace propagation, plus docs and tests.

• Kernel/VFS:
  • Propagation Core: Introduces namespace::propagation with PropagationType, MountPropagation, PropagationGroupId, and global peer registry; implements propagate_mount/propagate_umount.
  • MountFS Integration: Clones propagation on deepcopy; registers/unregisters peers on mount/umount; exposes mount_id.
  • sys_mount Enhancements:
    • Supports MS_BIND and MS_REC (recursive bind) via do_bind_mount/do_recursive_bind_mount.
    • Supports --make-{shared,private,slave,unbindable} (and recursive) via change_mnt_propagation_recursive.
    • Validates unbindable sources for bind.
  • Mount Namespace: Copies mounts with propagation state; registers peers across namespaces during CLONE_NEWNS.
  • Init: Uses new MountPropagation::new_private() for initramfs/root mounts.
• Docs:
  • Adds docs/kernel/filesystem/vfs/mount_propagation.md and links from index.rst detailing propagation types, peer groups, and flows.
• Tests:
  • Adds user/apps/c_unitest/test_mount_propagation.c covering API flags, shared/private/slave/unbindable behavior, recursive bind, and namespace propagation/umount.

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[cursor]

Bug: Child mount unregistered before propagation can fail

The child mount is unregistered from its peer group at line 551 before propagate_umount is called at line 558. If propagation fails (which only logs a warning), the source child is unregistered while peer children remain registered in the same peer group, creating an inconsistent state. The unregistration should happen after successful propagation, or the unregistration at line 551 should be removed since propagate_umount already handles unregistering children at all peers via umount_at_peer.

 

[cursor]

Bug: Missing slash in recursive bind mount paths

Path construction for recursive bind mounts can produce invalid paths. When target_base_path doesn't end with '/' and relative_path doesn't start with '/' (which occurs when source_base_path ends with '/'), line 499 concatenates them without a separator, producing paths like /targetchild instead of /target/child. The code should ensure a single '/' separator between the base path and relative path regardless of trailing/leading slashes.