Signals and Session Management

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Signals and Session Management

• Chapter 4

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Contents

• Signal Generation and Handling
• Unreliable Signals
• Reliable Signals
• Signals in SVR4
• Signals Implementation
• Exceptions
• Process Groups and Terminal Management
• The SVR4 Sessions Architecture

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4.2 Signal Generation Handling

• Signal A way to call a procedure when some
  events occur.
• Generation when an event occurs.
• Delivery when the process recognizes the
  signals arrival (handling)
• Pending between generated and delivered.
• System V 15 signals
• 4BSD/SVR4 31 signals
• Signal numbers different in different system or
  versions

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Signal Handling

• Default actions each signal has a .
• Abort Terminate the process after generating a
  core dump.
• Exit Terminate the process without generating a
  core dump.
• Ignore Ignores the signal.
• Stop Suspend the process.
• Continue Resume the process, if suspended
• Default actions may be overridden by signal
  handlers. (p86)

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Signal Handling

• Any action can only be done by the process.
• issig() (Kernel call)
• Before returning to user mode from a system call
  or interrupt.
• Just before blocking on an interruptible event
• Immediately after waking up from an interruptible
  event
• psig() dispatch the signal
• sendsig() invoke the user-defined handler

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Signal Handling
Signal delivered
Execute normal code
Resume normal code
Signal handler runs
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Signal Generation

• Signal sources
• Exceptions
• Other processes
• Terminal interrupts
• Job control
• Quotas
• Notifications
• Alarms

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Typical Scenarios

• C
• Exceptions
• Trap
• issig() when return to user mode.
• Pending signals processed one by one.

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Sleep and signals

• Interruptible sleep is waiting for an event with
  indefinite time.
• Uninterruptible sleep is waiting for a short
  term event such as disk I/O
• Pending the signal
• Recognizing it until returning to user mode or
  blocking on an event
• if (issig()) psig()

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4.3 Unreliable Signals

• Signal handlers are not persistent and do not
  mask recurring instances of the same
  signal.(SVR2)
• Racing two C.
• Performance SIG_DFL, SIG_IGN,
• Kernel does not know the u_signal
• If SIG_IGN Awake, check, and go back to sleep
  again(waste of time).

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Reinstalling a signal handler

• void sigint_handler(sig)
• int sig
• signal(SIGINT, sigint_handler)
• main()
• signal(SIGINT, sigint_handler)
• //sig.c

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Reliable Signals

• Primary features
• Persistent handlers need not to be reinstalled.
• Masking A signal can be masked
  temporarily.(remember it)
• Sleeping processes let the signal disposition
  info visible to the kernel.(kept in the proc)
• Unblock and wait sigpause()-automatically
  unmasks a signal and blocks the process.

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The SVR3 implementation

• int sig_received 0
• void handler (int sig)
• sig_received
• main()
• sigset (SIGQUIT, handler)
• sighold(SIGQUIT)
• while (sig_received 0) sigpause(SIGINT)
• //sig1.c

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4.5 Signals in SVR4

• sigprocmask(how, setp, osetp)
• SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK
• sigaltstack(stack, old_stack)
• Specify a new stack to handle the signal
• sigsuspend(sigmask)
• Set the blocked signals mask to sigmask and puts
  the process to sleep
• sigpending(setp)
• setp contains the set of signals pending to the
  process

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Signals in SVR4

• sigsendset(procset, sig)
• Sends the signal sig to the set of processes
  procset
• sigaction(signo, act, oact)
• Specify a handler for signal signo.
• act -gt a sigaction structure
• oact a function that returns the previous
  sigaction data
• Compatibility interface signal, sigset,
  sighold, sigrelse, sigignore, sigpause (sig.txt)

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Signal flags

• SA_NOCLDSTOP Do not generate SIGCHLD when a
  child is suspended
• SA_RESTART Restart system call automatically if
  interrupted by this signal
• SA_ONSTACK Handle this signal on the alternate
  stack, if one has been specified by sigaltstack
• SA_NOCLDWAIT sleep until all terminate
• SA_SIGINFO additional info to the handler.
• SA_NODEFER do not block this signal
• SA_RESETHAND reset the action to default

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4.6 Signals Implementation

• The info needed for kernel
• u_signal vector for handlers
• u_sigmask masks associated with handlers
• u_signalstack Pointer to the signal stack
• u_sigonstack mask of signals
• u_oldsig set of handlers exhibits the old
  behavior
• p_cursig The current signal being handled
• p_sig Pending signals mask
• p_hold Blocked signals mask
• p_ignore Ignored signals mask

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Signal Generation

1. Check(Kernel)
2. ignored?returnadds the signal to p_cursig.
3. Interruptible sleep !blocked? Wake up the
   process nothing.
4. SIGSTOP or SIGCONT suspend or resume

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Delivery and Handling

• Checks by issig()
• If (1 bit(p_cursig) set)
• if ( ! p_hold )
• store the signal number in p_sig, return TRUE
• else
• psig()
• psig()
• If (no handler) default
• else invoke a handler
• if (! SA_NODEFER) change p_hold
• If (SA_RESETHAND) u_signal is reset to SIG_DFL
• sendsig() //machine-dependent

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4.7 Exceptions

• An event when a program encounters an unusual
  condition(error).
• Using signals to notify.
• SIGSEGV
• Exceptions are used by debuggers.
• ptrace a system call
• Drawbacks
• Same context for signal handler and exception
• Signals are for single-threaded processes.
• ptrace-based debugger can control only its
  immediate children

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4.9 Process Groups and Terminal Management

• Common Concepts
• Process groups group-ID, leader
• Controlling terminal login terminal
• The /dev/tty file associated with the
  controlling terminal
• Controlling group the group associated with a
  terminal.
• Job control mechanism that can suspend or
  resume, or control the access to the terminal.

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The SVR3 Model

• Process groups
• inherits the process group id during fork
• Controlling terminal
• Owned by its controlling group.
• Typical scenario
• init fork a child, the login shell is the leader
• Terminal access no job control
• Terminal signals Z, C, fg only
• Detaching the terminal t_pgrp 0
• Death of group leader exit
• Implementation proc, u area,

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Process groups in SVR3 UNIX
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Limitations

• No way to close its controlling terminal and
  allocate another.
• No way to preserve a login session after
  disconnecting from its controlling terminal.
• No consistent way of handling loss of carrier
  by a controlling terminal.
• The kernel does not synchronize access to the
  terminal by different proceses in the group.
• When the group leader terminates, the kernel
  sends a SIGHUP signal to all processes in the
  group.
• If a process invokes setpgrp, it will be
  disconnected from the controlling terminal.
• No job control facilities.
• A terminal emulator has no way of receiving
  carrier loss notification.

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4.10 The SVR4 Sessions Architecture

• Motivation
• Supporting both login session and job abstraction
• Providing BSD style job control
• Retaining backward compatibility
• Allowing a login session to attach and detach
  several controlling terminals
• Making the session leader responsible fro
  maintaining the sessions integrity and security.
• Allowing terminal access based solely on file
  access permissions.
• Eliminating inconsistencies of earlier
  implementation.

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Sessions and Process Group

• Create a session setsid makes the caller the
  leader of both the session and the group.
• fg group and bg group
• Inherit group
• Change a session by setpgid(pid, pgid)
• Move within a session
• Leave a session by setsid

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SVR4 Sessions Architecture
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Data Structure

• setsid allocates a new session structure and
  resets the p_sessp p_pgidp in the proc.

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Controlling Terminals

• Login
• Calls setsid to become a session leader
• Close stdin, stdout, stderr
• Calls open to open a designated terminal
• Duplicates this descriptor elsewhere
• Opens /dev/tty as stdin, and duplicates stdout,
  and stderr
• Close the saved descriptor

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Controlling Terminals

• The session leader is a trusted process.
• The driver sends SIGTSTP to fg process group.
• A session leader may disconnect the current
  controlling terminal and open a new.
• When the session leader terminates, it ends the
  login session.