IPC Namespace

The IPC namespace isolates two old but still-used interprocess communication families:

• System V IPC — shmget, msgget, semget (key-based, identifier-returning)
• POSIX message queues mounted under /dev/mqueue

Things not in the IPC namespace: pipes, UNIX domain sockets, signals, eventfd — those are isolated by other namespaces (mount, PID).

Why it matters

A shared-memory segment in one container should be invisible to another. Without IPC namespacing, a container that knows the host's SysV key could shmat() a segment created by a sibling — a cross-container data leak.

          (no IPC ns)
Container A: shmget(0xDEAD, 4096, IPC_CREAT)  -> id 32768
Container B: shmget(0xDEAD, 0, 0)             -> id 32768 (!!)
Container B: shmat(32768)                     -> reads A's memory

With CLONE_NEWIPC, the second shmget returns ENOENT or creates a brand-new segment in B's own namespace.

The simulator

We model two operations: shared memory (key -> id) and message queues (key -> id with a FIFO of messages). Each namespace has its own key tables and id allocators — but ids are globally unique (matches the kernel: SysV ids come from a single global allocator, but they're only resolvable within the namespace they were created in).

Reference: man 7 ipc_namespaces; Kerrisk, TLPI, ch. 45-48.