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112 lines
4.6 KiB
Text
112 lines
4.6 KiB
Text
POSIX and init:
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--------------
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POSIX.1 does not define 'init' but it mentions it in a few places.
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B.2.2.2, p205 line 873:
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This is part of the extensive 'job control' glossary entry.
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This specific reference says that 'init' must by default provide
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protection from job control signals to jobs it starts --
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it sets SIGTSTP, SIGTTIN and SIGTTOU to SIG_IGN.
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B.2.2.2, p206 line 889:
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Here is a reference to 'vhangup'. It says, 'POSIX.1 does
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not specify how controlling terminal access is affected by
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a user logging out (that is, by a controlling process
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terminating).' vhangup() is recognized as one way to handle
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the problem. I'm not clear what happens in Reno; I have
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the impression that when the controlling process terminates,
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references to the controlling terminal are converted to
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references to a 'dead' vnode. I don't know whether vhangup()
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is required.
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B.2.2.2, p206 line 921:
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Orphaned process groups bear indirectly on this issue. A
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session leader's process group is considered to be orphaned;
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that is, it's immune to job control signals from the terminal.
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B.2.2.2, p233 line 2055:
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'Historically, the implementation-dependent process that
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inherits children whose parents have terminated without
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waiting on them is called "init" and has a process ID of 1.'
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It goes on to note that it used to be the case that 'init'
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was responsible for sending SIGHUP to the foreground process
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group of a tty whose controlling process has exited, using
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vhangup(). It is now the responsibility of the kernel to
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do this when the controlling process calls _exit(). The
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kernel is also responsible for sending SIGCONT to stopped
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process groups that become orphaned. This is like old BSD
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but entire process groups are signaled instead of individual
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processes.
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In general it appears that the kernel now automatically
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takes care of orphans, relieving 'init' of any responsibility.
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Specifics are listed on the _exit() page (p50).
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On setsid():
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-----------
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It appears that neither getty nor login call setsid(), so init must
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do this -- seems reasonable. B.4.3.2 p 248 implies that this is the
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way that 'init' should work; it says that setsid() should be called
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after forking.
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Process group leaders cannot call setsid() -- another reason to
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fork! Of course setsid() causes the current process to become a
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process group leader, so we can only call setsid() once. Note that
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the controlling terminal acquires the session leader's process
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group when opened.
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Controlling terminals:
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---------------------
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B.7.1.1.3 p276: 'POSIX.1 does not specify a mechanism by which to
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allocate a controlling terminal. This is normally done by a system
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utility (such as 'getty') and is considered ... outside the scope
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of POSIX.1.' It goes on to say that historically the first open()
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of a tty in a session sets the controlling terminal. P130 has the
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full details; nothing particularly surprising.
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The glossary p12 describes a 'controlling process' as the first
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process in a session that acquires a controlling terminal. Access
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to the terminal from the session is revoked if the controlling
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process exits (see p50, in the discussion of process termination).
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Design notes:
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------------
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your generic finite state machine
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we are fascist about which signals we elect to receive,
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even signals purportedly generated by hardware
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handle fatal errors gracefully if possible (we reboot if we goof!!)
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if we get a segmentation fault etc., print a message on the console
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and spin for a while before rebooting
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(this at least decreases the amount of paper consumed :-)
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apply hysteresis to rapidly exiting gettys
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check wait status of children we reap
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don't wait for stopped children
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don't use SIGCHILD, it's too expensive
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but it may close windows and avoid races, sigh
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look for EINTR in case we need to change state
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init is responsible for utmp and wtmp maintenance (ick)
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maybe now we can consider replacements? maintain them in parallel
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init only removes utmp and closes out wtmp entries...
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necessary states and state transitions (gleaned from the man page):
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1: single user shell (with password checking?); on exit, go to 2
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2: rc script: on exit 0, go to 3; on exit N (error), go to 1
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3: read ttys file: on completion, go to 4
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4: multi-user operation: on SIGTERM, go to 7; on SIGHUP, go to 5;
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on SIGTSTP, go to 6
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5: clean up mode (re-read ttys file, killing off controlling processes
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on lines that are now 'off', starting them on lines newly 'on')
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on completion, go to 4
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6: boring mode (no new sessions); signals as in 4
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7: death: send SIGHUP to all controlling processes, reap for 30 seconds,
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then go to 1 (warn if not all processes died, i.e. wait blocks)
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Given the -s flag, we start at state 1; otherwise state 2
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