10.5. Process Groups, Sessions, and Terminal Control The process-control (job-control) facilities, described in Section 4.8, depend on the terminal I/O system to control access to the terminal. Each job (a process group that is manipulated as a single entity) is known by a process-group ID. Each terminal structure contains a pointer to an associated session. When a process creates a new session, that session has no associated terminal. To acquire an associated terminal, the session leader must make an ioctl system call using a file descriptor associated with the terminal and specifying the TIOCSCTTY flag. When the ioctl succeeds, the session leader is known as the controlling process. In addition, each terminal structure contains the process group ID of the foreground process group. When a session leader acquires an associated terminal, the terminal process group is set to the process group of the session leader. The terminal process group may be changed by making an ioctl system call using a file descriptor associated with the terminal and specifying the TIOCSPGRP flag. Any process group in the session is permitted to become the foreground process group for the terminal. Signals that are generated by characters typed at the terminal are sent to all the processes in the terminal's foreground process group. By default, some of those signals cause the process group to stop. The shell creates jobs as process groups, setting the process group ID to be the PID of the first process in the process group. Each time it places a new job in the foreground, the shell sets the terminal process group to the new process group. Thus, the terminal process group is the identifier for the process group that is currently in control of the terminal that is, for the process group running in the foreground. Other process groups may run in the background. If a background process attempts to read from the terminal, its process group is sent another signal, which stops the process group. Optionally, background processes that attempt terminal output may be stopped as well. These rules for control of input and output operations apply to only those operations on the controlling terminal. When carrier is lost for the terminal for example, at modem disconnect or when a network connection is lost the session leader of the session associated with the terminal is sent a SIGHUP signal. If the session leader exits, the controlling terminal is revoked, and that invalidates any open file descriptors in the system for the terminal. This revocation ensures that processes holding file descriptors for a terminal cannot access the terminal after the terminal is acquired by another user. The revocation operates at the vnode layer. It is possible for a process to have a read or write sleeping for some reason for example, it was in a background process group. Since such a process would have already resolved the file descriptor through the vnode layer, a single read or write by the sleeping process could complete after the revoke system call. To avoid this security problem, the system checks a tty generation number when a process wakes up from sleeping on a terminal and, if the number has changed, restarts the read or write system call. |
