435 lines
12 KiB
C
435 lines
12 KiB
C
/*
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* This file is part of Jehanne.
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*
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* Copyright (C) 2017 Giacomo Tesio <giacomo@tesio.it>
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*
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* This is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as
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* published by the Free Software Foundation, version 3 of the License.
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*
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* Jehanne is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License
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* along with Jehanne. If not, see <http://www.gnu.org/licenses/>.
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*/
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/* POSIX signals have weird semantics that are hard to emulate in a
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* sane system without giving up on its sanity.
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*
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* We distinguish control signals (SIGKILL, SIGSTOP, SIGCONT,
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* SIGABRT, SIGIOT), SIGCHLD/SIGCLD, timers' wakeups
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* (SIGALRM, SIGPROF, SIGVTALRM) and all the others.
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*
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* TRASMISSION
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* -----------
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* Signal transmission depends on the relation between the sender
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* and the receiver:
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* 1) if sender and receiver have no relation the signal is translated
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* to a note and sent to the receiver(s);
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* 2) if sender == receiver the signal trampoline is directly invoked
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* for all signals except control ones and the default disposition occurs if
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* the trampoline does not handle the signal
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* 3) if sender is parent or child of receiver the transmision
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* differs from the default if libposix_emulate_SIGCHLD() has been
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* called during library initialization
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*
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* Since notes in Jehanne are not reentrant, signals translated to
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* notes will be enqueued in kernel. A special machinery is implemented
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* for timers, so that they can be used in signal handlers.
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*
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* For all the signals except SIGCONT, the burden of interpreting the
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* signal is on the receiver: the sender just send the signal.
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*
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* The receiver will translate the note back to the appropriate signal
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* number and invoke the trampoline: if trampoline returns 0 no function
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* registered with signal() handled the signal and the library will
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* invoke the default disposition associated to the signal.
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*
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* CONTROL MESSAGES
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* ----------------
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* Control messages are translated by the receiver to equivalent actions:
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* - SIGKILL => write "kill" to the control file of the current process
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* - SIGSTOP => write "stop" to the control file of the current process
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* - SIGABRT/SIGIOT => invoke the registered signal handlers and abort
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* the current process
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* - SIGCONT => invoke the registered signal handlers via the signal
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* trampoline and continue; note that (since a stopped
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* process cannot resume itself) the sender will write
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* "start" to the control file of the receiver before
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* sending the note (unless SIGCHLD emulation is enable).
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*
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* SIGCHLD/SIGCLD
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* --------------
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* Jehanne (like Plan 9) does not support a SIGCHLD equivalent.
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* The user space emulation provided here is quite expensive, so it's
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* disabled by default.
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* Calling libposix_emulate_SIGCHLD() during libposix initialization
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* will enable this machinery for the whole life of the process.
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*
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* Such emulation change the way fork and kill works.
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*
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* Each fork() will spawn two additional processes that are designed
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* to proxy signals between the parent and the desired child:
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*
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* parent
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* +- proxy from parent to child (P2C)
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* +- proxy from child to parent (C2P)
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* +- child
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*
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* Fork will return the pid of P2C to the parent, so that the
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* parent process will see the first proxy as its child; the child will
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* see the second as its parent. Each proxy waits for its child and
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* forwards the notes it receive to its designed target.
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* Finally fork in child will return 0.
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*
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* When the child exits, C2P will send a SIGCHLD note to parent and
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* then exit with the same status. Then P2C will exit with
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* the same status too.
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*
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* As the parent process will see P2C as its child, it will send any
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* signal to it via kill and will wait it on SIGCHLD.
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* However, when this machinary is enabled, kill will treat all signals
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* for forked children in the same way, sending them to P2C.
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* It's P2C's responsibility to translate control messages as required,
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* so that SIGCONT will work as expected.
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*
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* TIMERS
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* ------
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* The functions alarm() and settimer() generate SIGALRM, SIGPROF
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* or SIGVTALRM for the current process. We want timers to be able to
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* expire in a signal handler (interrupting a blocking syscall) but
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* without giving up the simplicity of notes.
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*
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* We allocate these timers on libposix initialization. When normal
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* code is running timers will be implemented via Jehanne's alarms,
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* producing a note on expiration that will be mapped to the proper
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* signal for the trampoline by the receiving process.
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*
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* However, when a signal handler is being executed, the timers will
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* be implemented using the awake syscall that will be able
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* to interrupt blocking syscalls in the signal handler.
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* When a signal handler returns, if the timer has not be cleared and
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* did not expired, the wakeup is cleared and replaced with an alarm
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* note.
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*/
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#include <u.h>
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#include <lib9.h>
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#include <posix.h>
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#include "internal.h"
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#define __POSIX_SIGNAL_PREFIX_LEN (sizeof(__POSIX_SIGNAL_PREFIX)-1)
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unsigned char *__signals_to_code_map;
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unsigned char *__code_to_signal_map;
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int *__handling_external_signal;
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static int __sigrtmin;
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static int __sigrtmax;
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static int __min_known_sig;
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static int __max_known_sig;
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static PosixSignalTrampoline __libposix_signal_trampoline;
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typedef enum PosixSignalDisposition
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{
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SignalHandled = 0, /* the application handled the signal */
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TerminateTheProcess,
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TerminateTheProcessAndCoreDump,
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StopTheProcess,
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ResumeTheProcess
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} PosixSignalDisposition;
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static int
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note_all_writable_processes(int *errnop, char *note)
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{
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// TODO: loop over writable note files and post note.
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*errnop = __libposix_get_errno(PosixEPERM);
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return -1;
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}
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static void
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terminated_by_signal(int sig)
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{
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char buf[64];
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__libposix_free_wait_list();
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snprint(buf, sizeof(buf), __POSIX_EXIT_SIGNAL_PREFIX "%d", sig);
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exits(buf);
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}
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int
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send_control_msg(int pid, char *msg)
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{
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int fd, n;
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char buf[256];
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n = snprint(buf, sizeof(buf), "/proc/%d/ctl", pid);
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if(n < 0)
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goto ErrorBeforeOpen;
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fd = open(buf, OWRITE);
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if(fd < 0)
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goto ErrorBeforeOpen;
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n = snprint(buf, sizeof(buf), "%s", msg);
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if(n < 0)
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goto ErrorAfterOpen;
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if(write(fd, buf, n) < n)
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goto ErrorAfterOpen;
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close(fd);
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return 1;
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ErrorAfterOpen:
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close(fd);
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ErrorBeforeOpen:
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return 0;
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}
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/* Executes a PosixSignalDisposition for pid.
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*
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* MUST be called by POSIX_kill for unblockable signals.
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*/
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static int
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execute_disposition(int sig, PosixSignalDisposition action, int pid)
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{
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switch(action){
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case SignalHandled:
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return 1;
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case TerminateTheProcess:
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case TerminateTheProcessAndCoreDump:
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terminated_by_signal(sig);
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break;
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case StopTheProcess:
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if(pid < 0)
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sysfatal("libposix: signal %d with stop disposition reached process %d", sig, pid);
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return send_control_msg(pid, "stop");
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case ResumeTheProcess:
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if(pid < 0)
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sysfatal("libposix: signal %d with continue disposition reached process %d", sig, pid);
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return send_control_msg(pid, "start");
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}
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return 0;
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}
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static int
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send_signal(int *errnop, int pid, int signal)
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{
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char msg[64], file[128];
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int mode;
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int ret;
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snprint(msg, sizeof(msg), __POSIX_SIGNAL_PREFIX "%d", signal);
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switch(pid){
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case 0:
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mode = PNGROUP;
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break;
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case -1:
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return note_all_writable_processes(errnop, msg);
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default:
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if(pid < 0){
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mode = PNGROUP;
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pid = -pid;
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} else {
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mode = PNPROC;
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}
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}
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snprint(file, sizeof(file), "/proc/%d/note", pid);
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if(access(file, AWRITE) != 0){
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if(access(file, AEXIST) == 0)
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*errnop = __libposix_get_errno(PosixEPERM);
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else
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*errnop = __libposix_get_errno(PosixESRCH);
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return -1;
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}
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ret = postnote(mode, pid, msg);
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if(ret != 0){
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*errnop = __libposix_translate_errstr((uintptr_t)POSIX_kill);
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return -1;
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}
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return 0;
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}
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static PosixSignalDisposition
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default_signal_disposition(int code)
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{
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// see http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html
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if(code >= __sigrtmin || code <= __sigrtmin)
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return TerminateTheProcess;
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switch(__code_to_signal_map[code]){
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default:
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sysfatal("libposix: undefined signal %d", code);
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case PosixSIGALRM:
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case PosixSIGHUP:
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case PosixSIGINT:
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case PosixSIGKILL:
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case PosixSIGPIPE:
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case PosixSIGTERM:
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case PosixSIGUSR1:
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case PosixSIGUSR2:
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case PosixSIGPOLL:
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case PosixSIGPROF:
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case PosixSIGVTALRM:
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return TerminateTheProcess;
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case PosixSIGSTOP:
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case PosixSIGTSTP:
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case PosixSIGTTIN:
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case PosixSIGTTOU:
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return StopTheProcess;
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case PosixSIGABRT:
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case PosixSIGILL:
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case PosixSIGFPE:
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case PosixSIGBUS:
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case PosixSIGQUIT:
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case PosixSIGSEGV:
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case PosixSIGSYS:
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case PosixSIGTRAP:
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case PosixSIGXCPU:
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case PosixSIGXFSZ:
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return TerminateTheProcessAndCoreDump;
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case PosixSIGCHLD:
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case PosixSIGURG:
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return SignalHandled;
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case PosixSIGCONT:
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return ResumeTheProcess;
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}
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}
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int
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POSIX_kill(int *errnop, int pid, int sig)
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{
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PosixSignals signal;
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PosixSignalDisposition action;
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signal = __code_to_signal_map[sig];
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if(signal == 0
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&&(sig < __sigrtmin || sig > __sigrtmax))
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sysfatal("libposix: undefined signal %d", sig);
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if(pid == getpid()){
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if(__libposix_signal_trampoline(sig))
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action = SignalHandled;
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else
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action = default_signal_disposition(sig);
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if(!execute_disposition(sig, action, -1)){
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*errnop = __libposix_get_errno(PosixEPERM);
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return -1;
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}
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}
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switch(signal){
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case PosixSIGKILL:
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if(pid > 0)
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if(!send_control_msg(pid, "kill")){
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*errnop = __libposix_get_errno(PosixEPERM);
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return -1;
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}
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break;
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case PosixSIGSTOP:
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if(pid > 0)
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if(!send_control_msg(pid, "stop")){
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*errnop = __libposix_get_errno(PosixEPERM);
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return -1;
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}
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break;
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case PosixSIGCONT:
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if(pid > 0)
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if(!send_control_msg(pid, "start")){
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*errnop = __libposix_get_errno(PosixEPERM);
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return -1;
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}
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break;
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default:
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break;
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}
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return send_signal(errnop, pid, sig);
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}
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static int
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translate_jehanne_note(char *note)
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{
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// TODO: implement
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return 0;
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}
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int
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__libposix_note_handler(void *ureg, char *note)
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{
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int sig, ret;
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PosixSignalDisposition action;
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if(strncmp(note, __POSIX_SIGNAL_PREFIX, __POSIX_SIGNAL_PREFIX_LEN) != 0)
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return translate_jehanne_note(note); // TODO: should we translate common notes?
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sig = atoi(note+__POSIX_SIGNAL_PREFIX_LEN);
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if(sig < __min_known_sig || sig > __max_known_sig)
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sysfatal("libposix: '%s' does not carry a signal", note);
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*__handling_external_signal = 1;
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if(__libposix_signal_trampoline(sig))
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action = SignalHandled;
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else
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action = default_signal_disposition(sig);
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ret = execute_disposition(sig, action, -1);
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*__handling_external_signal = 0;
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return ret;
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}
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int
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libposix_define_realtime_signals(int sigrtmin, int sigrtmax)
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{
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if(sigrtmin >= 256 || sigrtmin <=0)
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sysfatal("libposix: invalid SIGRTMIN %d (must be positive and less then 256)", sigrtmin);
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if(sigrtmax >= 256 || sigrtmax <=0)
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sysfatal("libposix: invalid SIGRTMAX %d (must be positive and less then 256)", sigrtmax);
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if(sigrtmax <= sigrtmin)
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sysfatal("libposix: invalid SIGRTMAX %d (must be greater than SIGRTMIN %d)", sigrtmax, sigrtmin);
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if(__libposix_initialized())
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return 0;
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__sigrtmin = sigrtmin;
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__sigrtmax = sigrtmax;
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if(sigrtmin < __min_known_sig || __min_known_sig == 0)
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__min_known_sig = sigrtmin;
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if(sigrtmax > __max_known_sig || __max_known_sig == 0)
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__max_known_sig = sigrtmax;
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return 1;
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}
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int
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libposix_define_signal(PosixSignals signal, int code)
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{
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if(signal >= PosixNumberOfSignals)
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sysfatal("libposix: unknown PosixSignal %d", signal);
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if(code >= 256 || code <=0)
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sysfatal("libposix: invalid signal number %d (must be positive and less then 256)", code);
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if(__libposix_initialized())
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return 0;
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__signals_to_code_map[signal] = (unsigned char)code;
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__code_to_signal_map[code] = (unsigned char)signal;
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if(code < __min_known_sig || __min_known_sig == 0)
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__min_known_sig = code;
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if(code > __max_known_sig || __max_known_sig == 0)
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__max_known_sig = code;
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return 1;
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}
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int
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libposix_set_signal_trampoline(PosixSignalTrampoline trampoline)
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{
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if(__libposix_initialized())
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return 0;
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if(trampoline == nil)
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return 0;
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__libposix_signal_trampoline = trampoline;
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return 1;
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}
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void
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__libposix_signal_check_conf(void)
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{
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if(__libposix_signal_trampoline == nil)
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sysfatal("libposix: no signal trampoline");
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}
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