Handles are kernel constructs that allow user-mode programs to reference a kernel object. A handle can be thought of as a session or connection to a particular kernel object.
It is often the case that multiple processes concurrently access the same object via different handles. However, a single handle can only be either bound to a single process or be bound to the kernel.
When it is bound to the kernel we say it's 'in-transit'.
In user mode a handle is simply a specific number returned by some syscall. Only handles that are not in-transit are visible to user-mode.
The integer that represents a handle is only meaningful for that process. The same number in another process might not map to any handle or it might map to a handle pointing to a completely different kernel object.
The integer value for a handle is any 32-bit number except the value corresponding to ZX_HANDLE_INVALID which will always have the value of 0. In addition to this, the integer value of a valid handle will always have two least significant bits of the handle set. The mask representing these bits may be accessed using ZX_HANDLE_FIXED_BITS_MASK
For kernel mode, a handle is a C++ object that contains three logical fields:
- A reference to a kernel object
- The rights to the kernel object
- The process it is bound to (or if it's bound to the kernel)
The 'rights' specify what operations on the kernel object are allowed. It is possible for a single process to have two different handles to the same kernel object with different rights.
Figure 1. A user process starts the creation of a handle.
Figure 2. The user process creates the kernel object (for example, an event) with a system call and holds an integer reference to the object.
Figure 3. Handles are created with a set of basic rights and any additional rights applicable to the kernel object type.
Figure 4. Handles can be duplicated. Rights can be dropped during this process.
There are many syscalls that create a new kernel object and that return a handle to it. To name a few:
These calls create both the kernel object and the first handle pointing to it. The handle is bound to the process that issued the syscall and the rights are the default rights for that type of kernel object.
There is only one syscall that can make a copy of a handle, which points to the same kernel object and is bound to the same process that issued the syscall:
There is one syscall that creates an equivalent handle (possibly with fewer rights), invalidating the original handle:
There is one syscall that just destroys a handle:
There is one syscall that takes a handle bound to the calling process and binds it into the kernel (puts the handle in-transit):
There are two syscalls that take an in-transit handle and bind it to the calling process:
The channel and socket syscalls above are used to transfer a handle from
one process to another. For example it is possible to connect
two processes with a channel. To transfer a handle the source process
zx_channel_call and then the destination
zx_channel_read on the same channel.
Finally, there is a single syscall that gives a new process its bootstrapping handle, that is, the handle that it can use to request other handles:
The bootstrapping handle can be of any transferable kernel object but the most reasonable case is that it points to one end of a channel so this initial channel can be used to send further handles into the new process.
If a handle is valid, the kernel object it points to is guaranteed to be valid. This is ensured because kernel objects are ref-counted and each handle holds a reference to its kernel object.
The opposite does not hold. When a handle is destroyed it does not mean its object is destroyed. There could be other handles pointing to the object or the kernel itself could be holding a reference to the kernel object. An example of this is a handle to a thread; the fact that the last handle to a thread is closed it does not mean that the thread has been terminated.
When the last reference to a kernel object is released, the kernel object is either destroyed or the kernel marks the object for garbage collection; the object will be destroyed at a later time when the current set of pending operations on it is completed.
When a handle is in-transit and the channel or socket it was written to is destroyed, the handle is closed.
Debugging sessions (and debuggers) might have special syscalls to get access to handles.
Invalid Handles and handle reuse
It is an error to pass to any syscall except for
the following values:
- A handle value that corresponds to a closed handle
- The ZX_HANDLE_INVALID value, except for
The kernel is free to re-use the integer values of closed handles for newly created objects. Therefore, it is important to make sure that proper handle hygiene is observed:
Don't have one thread close a given handle and another thread use the same handle in a racy way. Even if the second thread is also closing it.
Don't ignore ZX_ERR_BAD_HANDLE return codes. They usually mean the code has a logic error.
Detecting invalid handle usage can be automated by using the ZX_POL_BAD_HANDLE Job policy with ZX_POL_ACTION_EXCEPTION to generate an exception when a process under such job object attempts any of the mentioned invalid cases.