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Zircon Kernel objects

Zircon is an object-based kernel. User mode code almost exclusively interacts with OS resources via object handles. A handle can be thought of as an active session with a specific OS subsystem scoped to a particular resource.

Zircon actively manages the following resources:

  • processor time
  • memory and address spaces
  • device-io memory
  • interrupts
  • signaling and waiting
  • inter-process communication

Kernel objects for applications

IPC

Tasks

Scheduling

Signaling

Memory and address space

Waiting

Kernel objects for drivers

Kernel object lifetime

Kernel objects are reference-counted. Most kernel objects are created during a 'create' syscall and are held alive by the first handle, given as the output of the create syscall. The caller gets the numeric id of the handle and the handle itself is placed in the handle table of the process.

A handle is held alive as long it exists in the handle table. Handles are removed from the handle table by:

  • Closing them via zx_handle_close which decrements the reference count of the corresponding kernel object. Usually, when the last handle is closed the kernel object reference count will reach 0 which causes the kernel object to be destroyed.

  • When the process that owns the handle table is destroyed. The kernel effectively iterates over the entire handle table closing each handle in turn.

The reference count increases when new handles (referring to the same object) are created via zx_handle_duplicate, but also when a direct pointer reference (by some kernel code) is acquired; therefore a kernel object lifetime might be longer than the lifetime of the code that created it.

There are three important cases in which kernel objects are kept alive when there are no outstanding handles to them:

  • The object is referenced by a message which has not been consumed. This can happen via the channel APIs. While such message is in the channel the kernel keeps the object alive.

  • The object is the parent of another object which is alive. This is the case of VMOs attached to live VMARs, of processes with live threads and jobs with live processes or child jobs.

  • Threads are kept alive by the scheduler. A thread that is alive will continue to live until it voluntarily exits by calling zx_thread_exit or the process is terminated via zx_task_kill.

The outcome of the last case is that a single thread can keep its process and the entire lineage of jobs up to the root job alive.

Kernel Object security

Kernel objects do not have an intrinsic notion of security and do not do authorization checks; security rights are held by each handle. A single process can have two different handles to the same object with different rights.

See Also

Handles