The C IPC gate interface, see L4::Ipc_gate for the C++ interface. More...

Collaboration diagram for IPC-Gate API:

Functions
l4_msgtag_t	l4_ipc_gate_get_infos (l4_cap_idx_t gate, l4_umword_t *label)
	Get information about the IPC-gate.

l4_msgtag_t	l4_rcv_ep_bind_thread (l4_cap_idx_t ep, l4_cap_idx_t thread, l4_umword_t label)
	Bind the IPC receive endpoint to a thread.

Detailed Description

The C IPC gate interface, see L4::Ipc_gate for the C++ interface.

IPC gates are used to create secure communication channels between protection domains. An IPC gate can be created using the Factory interface.

Depending on the permissions of the capability used, an IPC gate forwards IPC to the Thread that is bound to the IPC gate (cf. l4_rcv_ep_bind_thread()). If the capability has the L4_FPAGE_C_IPCGATE_SVR permission, only IPC using a protocol different from the L4_PROTO_KOBJECT protocol is forwarded. Without the L4_FPAGE_C_IPCGATE_SVR permission, all IPC is forwarded. The latter is the usual case for a client in a client/server scenario. When no thread is bound yet, the forwarded IPC blocks until a thread is bound or the IPC times out.

Forwarded IPC is always forwarded to the userland of the bound thread. That means, the Thread interface of the bound thread is not accessible via an IPC gate. The IPC-Gate API of an IPC gate is only accessible if the capability used has the L4_FPAGE_C_IPCGATE_SVR permission (cf. previous paragraph). Conversely that means, if the capability used lacks the L4_FPAGE_C_IPCGATE_SVR permission, IPC-Gate API calls are forwarded to the bound thread instead of being processed by the IPC gate itself. In a client/server scenario, a client should only get IPC gate capabilities without L4_FPAGE_C_IPCGATE_SVR permission so the client cannot tamper with the IPC gate.

When binding a thread to an IPC gate, a user-defined, kernel protected, machine-word sized payload called the IPC gate’s label is assigned to the IPC gate (cf. l4_rcv_ep_bind_thread()). When a send-only IPC or call IPC is forwarded via an IPC gate, the label provided by the sender is ignored and replaced by the IPC gate’s label where the two least significant bits are the result of bitwise disjunction of the corresponding label bits with the L4_CAP_FPAGE_S and L4_CAP_FPAGE_W permissions of the capability used. Hence, the label provided via l4_rcv_ep_bind_thread() should usually have its two least significant bits set to zero. The replaced label is only visible to the bound thread upon receive. However, the configured label of an IPC gate can also be queried via l4_ipc_gate_get_infos() if the capability used has the L4_FPAGE_C_IPCGATE_SVR permission.

When deleting an IPC gate or when unbinding it from a thread, the label of IPC already in flight won't be changed. To ensure that no IPC from this IPC gate is received by a thread with an unexpected label, l4_thread_modify_sender_start() shall be used to change the labels of every pending IPC to that gate. This is also required if the label of an already bound IPC gate is changed. It is not necessary after binding the IPC gate to a thread for the first time.

When binding a new thread to an IPC gate that is currently bound, the same label should be used that was used with the old thread. Otherwise the old and the new thread need to synchronize to avoid IPC messages with unexpected labels.

Include File: #include <l4/sys/ipc_gate.h>

ipc_gate.h
The C IPC gate interface, see L4::Ipc_gate for the C++ interface.

For the C++ interface refer to the L4::Ipc_gate documentation.

See also: Object Invocation

Function Documentation

◆ l4_ipc_gate_get_infos()

l4_msgtag_t l4_ipc_gate_get_infos	(	l4_cap_idx_t	gate,
		l4_umword_t *	label
	)

inline

Get information about the IPC-gate.

Parameters

	gate	The IPC gate object to get information about.
[out]	label	The label of the IPC gate is returned here.

Returns: System call return tag.

Precondition: If gate does not possess the L4_FPAGE_C_IPCGATE_SVR right, the kernel will not perform this operation. Instead, the underlying IPC message will be forwarded to the thread bound to the IPC gate, blocking the caller if no thread is bound yet.

Definition at line 159 of file ipc_gate.h.

References l4_utcb().

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◆ l4_rcv_ep_bind_thread()

l4_msgtag_t l4_rcv_ep_bind_thread	(	l4_cap_idx_t	ep,
		l4_cap_idx_t	thread,
		l4_umword_t	label
	)

inline

Bind the IPC receive endpoint to a thread.

Parameters

ep	The IPC receive endpoint object.
thread	The thread object that shall be bound to `ep`.
label	Label to assign to `ep`. The two least significant bits should usually be set to zero.

Returns: Syscall return tag containing one of the following return codes.

Return values

L4_EOK	Operation successful.
-L4_EINVAL	`thread` is not a thread object or other arguments were malformed.
-L4_EPERM	No L4_CAP_FPAGE_S right on `ep` or `thread`.

Precondition: If ep is an IPC gate capability without the L4_FPAGE_C_IPCGATE_SVR right, the kernel will not perform this operation. Instead, the underlying IPC message will be forwarded to the thread bound to the IPC gate, blocking the caller if no thread is bound yet.

The specified label is passed to the receiver of the incoming IPC. It is possible to re-bind a receive endpoint to the same or a different thread. In this case, IPC already in flight will be delivered with the old label to the previously bound thread unless l4_thread_modify_sender_start() is used to change these labels.

Examples: examples/sys/isr/main.c.