cosmopolitan/libc/calls/sigaction.c

/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/assert.h"
#include "libc/bits/bits.h"
#include "libc/calls/calls.h"
#include "libc/calls/internal.h"
#include "libc/calls/sigbits.h"
#include "libc/calls/strace.internal.h"
#include "libc/calls/struct/sigaction-freebsd.internal.h"
#include "libc/calls/struct/sigaction-linux.internal.h"
#include "libc/calls/struct/sigaction-netbsd.h"
#include "libc/calls/struct/sigaction-openbsd.internal.h"
#include "libc/calls/struct/sigaction-xnu.internal.h"
#include "libc/calls/struct/sigaction.h"
#include "libc/calls/typedef/sigaction_f.h"
#include "libc/calls/ucontext.h"
#include "libc/dce.h"
#include "libc/intrin/asan.internal.h"
#include "libc/intrin/spinlock.h"
#include "libc/limits.h"
#include "libc/macros.internal.h"
#include "libc/mem/mem.h"
#include "libc/runtime/runtime.h"
#include "libc/str/str.h"
#include "libc/sysv/consts/sa.h"
#include "libc/sysv/consts/sig.h"
#include "libc/sysv/errfuns.h"

#define SA_RESTORER 0x04000000

#ifndef SWITCHEROO
#define SWITCHEROO(S1, S2, A, B, C, D)                     \
  do {                                                     \
    autotype((S2).A) a = (typeof((S2).A))(S1).A;           \
    autotype((S2).B) b = (typeof((S2).B))(S1).B;           \
    autotype((S2).C) c = (typeof((S2).C))(S1).C;           \
    typeof((S2).D) d;                                      \
    bzero(&d, sizeof(d));                                  \
    memcpy(&d, &((S1).D), MIN(sizeof(d), sizeof((S1).D))); \
    (S2).A = a;                                            \
    (S2).B = b;                                            \
    (S2).C = c;                                            \
    bzero(&((S2).D), sizeof((S2).D));                      \
    memcpy(&((S2).D), &d, MIN(sizeof(d), sizeof((S2).D))); \
  } while (0);
#endif

union metasigaction {
  struct sigaction cosmo;
  struct sigaction_linux linux;
  struct sigaction_freebsd freebsd;
  struct sigaction_openbsd openbsd;
  struct sigaction_netbsd netbsd;
  struct sigaction_xnu_in xnu_in;
  struct sigaction_xnu_out xnu_out;
};

void __sigenter_netbsd(int, void *, void *);
void __sigenter_freebsd(int, void *, void *);
void __sigenter_openbsd(int, void *, void *);

static void sigaction_cosmo2native(union metasigaction *sa) {
  if (!sa) return;
  switch (__hostos) {
    case LINUX:
      SWITCHEROO(sa->cosmo, sa->linux, sa_handler, sa_flags, sa_restorer,
                 sa_mask);
      break;
    case XNU:
      SWITCHEROO(sa->cosmo, sa->xnu_in, sa_handler, sa_flags, sa_restorer,
                 sa_mask);
      break;
    case FREEBSD:
      SWITCHEROO(sa->cosmo, sa->freebsd, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    case OPENBSD:
      SWITCHEROO(sa->cosmo, sa->openbsd, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    case NETBSD:
      SWITCHEROO(sa->cosmo, sa->netbsd, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    default:
      break;
  }
}

static void sigaction_native2cosmo(union metasigaction *sa) {
  if (!sa) return;
  switch (__hostos) {
    case LINUX:
      SWITCHEROO(sa->linux, sa->cosmo, sa_handler, sa_flags, sa_restorer,
                 sa_mask);
      break;
    case XNU:
      SWITCHEROO(sa->xnu_out, sa->cosmo, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    case FREEBSD:
      SWITCHEROO(sa->freebsd, sa->cosmo, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    case OPENBSD:
      SWITCHEROO(sa->openbsd, sa->cosmo, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    case NETBSD:
      SWITCHEROO(sa->netbsd, sa->cosmo, sa_handler, sa_flags, sa_flags,
                 sa_mask);
      break;
    default:
      break;
  }
}

static int __sigaction(int sig, const struct sigaction *act,
                       struct sigaction *oldact) {
  _Static_assert((sizeof(struct sigaction) > sizeof(struct sigaction_linux) &&
                  sizeof(struct sigaction) > sizeof(struct sigaction_xnu_in) &&
                  sizeof(struct sigaction) > sizeof(struct sigaction_xnu_out) &&
                  sizeof(struct sigaction) > sizeof(struct sigaction_freebsd) &&
                  sizeof(struct sigaction) > sizeof(struct sigaction_openbsd) &&
                  sizeof(struct sigaction) > sizeof(struct sigaction_netbsd)),
                 "sigaction cosmo abi needs tuning");
  int64_t arg4, arg5;
  int rc, rva, oldrva;
  struct sigaction *ap, copy;
  if (IsMetal()) return enosys(); /* TODO: Signals on Metal */
  if (!(0 < sig && sig < NSIG)) return einval();
  if (sig == SIGKILL || sig == SIGSTOP) return einval();
  if (IsAsan() && ((act && !__asan_is_valid(act, sizeof(*act))) ||
                   (oldact && !__asan_is_valid(oldact, sizeof(*oldact))))) {
    return efault();
  }
  if (!act) {
    rva = (int32_t)(intptr_t)SIG_DFL;
  } else if ((intptr_t)act->sa_handler < kSigactionMinRva) {
    rva = (int)(intptr_t)act->sa_handler;
  } else if ((intptr_t)act->sa_handler >= (intptr_t)&_base + kSigactionMinRva &&
             (intptr_t)act->sa_handler < (intptr_t)&_base + INT_MAX) {
    rva = (int)((uintptr_t)act->sa_handler - (uintptr_t)&_base);
  } else {
    return efault();
  }
  if (__vforked && rva != (intptr_t)SIG_DFL && rva != (intptr_t)SIG_IGN) {
    return einval();
  }
  if (!IsWindows()) {
    if (act) {
      memcpy(&copy, act, sizeof(copy));
      ap = &copy;
      if (IsXnu()) {
        ap->sa_restorer = (void *)&__sigenter_xnu;
        ap->sa_handler = (void *)&__sigenter_xnu;
      } else if (IsLinux()) {
        if (!(ap->sa_flags & SA_RESTORER)) {
          ap->sa_flags |= SA_RESTORER;
          ap->sa_restorer = &__restore_rt;
        }
      } else if (IsNetbsd()) {
        ap->sa_sigaction = (sigaction_f)__sigenter_netbsd;
      } else if (IsFreebsd()) {
        ap->sa_sigaction = (sigaction_f)__sigenter_freebsd;
      } else if (IsOpenbsd()) {
        ap->sa_sigaction = (sigaction_f)__sigenter_openbsd;
      } else {
        return enosys();
      }
      sigaction_cosmo2native((union metasigaction *)ap);
    } else {
      ap = NULL;
    }
    if (IsXnu()) {
      arg4 = (int64_t)(intptr_t)oldact; /* from go code */
      arg5 = 0;
    } else if (IsNetbsd()) {
      if (ap) {
        arg4 = (int64_t)(intptr_t)&__restore_rt_netbsd;
        arg5 = 2; /* netbsd/lib/libc/arch/x86_64/sys/__sigtramp2.S */
      } else {
        arg4 = 0;
        arg5 = 0; /* netbsd/lib/libc/arch/x86_64/sys/__sigtramp2.S */
      }
    } else {
      arg4 = 8; /* or linux whines */
      arg5 = 0;
    }
    if ((rc = sys_sigaction(sig, ap, oldact, arg4, arg5)) != -1) {
      sigaction_native2cosmo((union metasigaction *)oldact);
    }
  } else {
    if (oldact) {
      bzero(oldact, sizeof(*oldact));
    }
    rc = 0;
  }
  if (rc != -1 && !__vforked) {
    cthread_spinlock(&__sig_lock);
    if (oldact) {
      oldrva = __sighandrvas[sig];
      oldact->sa_sigaction = (sigaction_f)(
          oldrva < kSigactionMinRva ? oldrva : (intptr_t)&_base + oldrva);
    }
    if (act) {
      __sighandrvas[sig] = rva;
      __sighandflags[sig] = act->sa_flags;
    }
    cthread_spunlock(&__sig_lock);
  }
  return rc;
}

/**
 * Installs handler for kernel interrupt, e.g.:
 *
 *     void GotCtrlC(int sig, siginfo_t *si, ucontext_t *ctx);
 *     struct sigaction sa = {.sa_sigaction = GotCtrlC,
 *                            .sa_flags = SA_RESETHAND|SA_RESTART|SA_SIGINFO};
 *     CHECK_NE(-1, sigaction(SIGINT, &sa, NULL));
 *
 * The following flags are supported across platforms:
 *
 * - `SA_SIGINFO`: Causes the `siginfo_t` and `ucontext_t` parameters to
 *   be passed. This not only gives you more information about the
 *   signal, but also allows your signal handler to change the CPU
 *   registers. That's useful for recovering from crashes. If you don't
 *   use this attribute, then signal delivery will go a little faster.
 *
 * - `SA_RESTART`: Enables BSD signal handling semantics. Normally i/o
 *   entrypoints check for pending signals to deliver. If one gets
 *   delivered during an i/o call, the normal behavior is to cancel the
 *   i/o operation and return -1 with EINTR in errno. If you use the
 *   `SA_RESTART` flag then that behavior changes, so that any function
 *   that's been annotated with @restartable will not return `EINTR` and
 *   will instead resume the i/o operation. This makes coding easier but
 *   it can be an anti-pattern if not used carefully, since poor usage
 *   can easily result in latency issues. It also requires one to do
 *   more work in signal handlers, so special care needs to be given to
 *   which C library functions are @asyncsignalsafe.
 *
 * - `SA_RESETHAND`: Causes signal handler to be single-shot. This means
 *   that, upon entry of delivery to a signal handler, it's reset to the
 *   `SIG_DFL` handler automatically. You may use the alias `SA_ONESHOT`
 *   for this flag, which means the same thing.
 *
 * - `SA_NODEFER`: Disables the reentrancy safety check on your signal
 *   handler. Normally that's a good thing, since for instance if your
 *   `SIGSEGV` signal handler happens to segfault, you're going to want
 *   your process to just crash rather than looping endlessly. But in
 *   some cases it's desirable to use `SA_NODEFER` instead, such as at
 *   times when you wish to `longjmp()` out of your signal handler and
 *   back into your program. This is only safe to do across platforms
 *   for non-crashing signals such as `SIGCHLD` and `SIGINT`. Crash
 *   handlers should use Xed instead to recover execution, because on
 *   Windows a `SIGSEGV` or `SIGTRAP` crash handler might happen on a
 *   separate stack and/or a separate thread. You may use the alias
 *   `SA_NOMASK` for this flag, which means the same thing.
 *
 * - `SA_NOCLDWAIT`: Changes `SIGCHLD` so the zombie is gone and you
 *   can't call `wait()` anymore; similar to SIGCHLD + SIG_IGN but may
 *   still deliver the SIGCHLD.
 *
 * - `SA_NOCLDSTOP`: Lets you set `SIGCHLD` handler that's only notified
 *   on exit/termination and not notified on `SIGSTOP`, `SIGTSTP`,
 *   `SIGTTIN`, `SIGTTOU`, or `SIGCONT`.
 *
 * Here's an example of the most professional way to handle signals in
 * an i/o event loop. It's generally a best practice to have signal
 * handlers do the fewest number of things possible. The trick is to
 * have your signals work hand-in-glove with the EINTR errno. This
 * obfuscates the need for having to worry about @asyncsignalsafe.
 *
 *    static volatile bool gotctrlc;
 *
 *    void OnCtrlC(int sig) {
 *      gotctrlc = true;
 *    }
 *
 *    int main() {
 *      size_t got;
 *      ssize_t rc;
 *      char buf[1];
 *      struct sigaction oldint;
 *      struct sigaction saint = {.sa_handler = GotCtrlC};
 *      if (sigaction(SIGINT, &saint, &oldint) == -1) {
 *        perror("sigaction");
 *        exit(1);
 *      }
 *      for (;;) {
 *        rc = read(0, buf, sizeof(buf));
 *        if (rc == -1) {
 *          if (errno == EINTR) {
 *            if (gotctrlc) {
 *              break;
 *            }
 *          } else {
 *            perror("read");
 *            exit(2);
 *          }
 *        }
 *        if (!(got = rc)) {
 *          break;
 *        }
 *        for (;;) {
 *          rc = write(1, buf, got);
 *          if (rc != -1) {
 *            assert(rc == 1);
 *            break;
 *          } else if (errno != EINTR) {
 *            perror("write");
 *            exit(3);
 *          }
 *        }
 *      }
 *      sigaction(SIGINT, &oldint, 0);
 *    }
 *
 * Please note that you can't do the above if you use SA_RESTART. Since
 * the purpose of SA_RESTART is to restart i/o operations whose docs say
 * that they're @restartable and read() is one such function. Here's
 * some even better news: if you don't install any signal handlers at
 * all, then your i/o calls will never be interrupted!
 *
 * Here's an example of the most professional way to recover from
 * `SIGSEGV`, `SIGFPE`, and `SIGILL`.
 *
 *     void ContinueOnCrash(void);
 *
 *     void SkipOverFaultingInstruction(struct ucontext *ctx) {
 *       struct XedDecodedInst xedd;
 *       xed_decoded_inst_zero_set_mode(&xedd, XED_MACHINE_MODE_LONG_64);
 *       xed_instruction_length_decode(&xedd, (void *)ctx->uc_mcontext.rip, 15);
 *       ctx->uc_mcontext.rip += xedd.length;
 *     }
 *
 *     void OnCrash(int sig, struct siginfo *si, struct ucontext *ctx) {
 *       SkipOverFaultingInstruction(ctx);
 *       ContinueOnCrash();  // reinstall here in case *rip faults
 *     }
 *
 *     void ContinueOnCrash(void) {
 *       struct sigaction sa = {.sa_handler = OnSigSegv,
 *                              .sa_flags = SA_SIGINFO | SA_RESETHAND};
 *       sigaction(SIGSEGV, &sa, 0);
 *       sigaction(SIGFPE, &sa, 0);
 *       sigaction(SIGILL, &sa, 0);
 *     }
 *
 *     int main() {
 *       ContinueOnCrash();
 *       // ...
 *     }
 *
 * You may also edit any other CPU registers during the handler. For
 * example, you can use the above technique so that division by zero
 * becomes defined to a specific value of your choosing!
 *
 * Please note that Xed isn't needed to recover from `SIGTRAP` which can
 * be raised at any time by embedding `DebugBreak()` or `asm("int3")` in
 * your program code. Your signal handler will automatically skip over
 * the interrupt instruction, assuming your signal handler returns.
 *
 * The important signals supported across all platforms are:
 *
 * - `SIGINT`: When you press Ctrl-C this signal gets broadcasted to
 *   your process session group. This is the normal way to terminate
 *   console applications.
 *
 * - `SIGQUIT`: When you press CTRL-\ this signal gets broadcasted to
 *   your process session group. This is the irregular way to kill an
 *   application in cases where maybe your `SIGINT` handler is broken
 *   although, Cosmopolitan Libc ShowCrashReports() should program it
 *   such as to attach a debugger to the process if possible, or else
 *   show a crash report. Also note that in New Technology you should
 *   press CTRL+BREAK rather than CTRL+\ to get this signal.
 *
 * - `SIGHUP`: This gets sent to your non-daemon processes when you
 *   close your terminal session.
 *
 * - `SIGTERM` is what the `kill` command sends by default. It's the
 *   choice signal for terminating daemons.
 *
 * - `SIGUSR1` and `SIGUSR2` can be anything you want. Their default
 *   action is to kill the process. By convention `SIGUSR1` is usually
 *   used by daemons to reload the config file.
 *
 * - `SIGCHLD` is sent when a process terminates and it takes a certain
 *   degree of UNIX mastery to address sanely.
 *
 * - `SIGALRM` is invoked by `setitimer()` and `alarm()`. It can be
 *   useful for interrupting i/o operations like `connect()`.
 *
 * - `SIGTRAP`: This happens when an INT3 instruction is encountered.
 *
 * - `SIGILL` happens on illegal instructions, e.g. `UD2`.
 *
 * - `SIGABRT` happens when you call `abort()`.
 *
 * - `SIGFPE` happens when you divide ints by zero, among other things.
 *
 * - `SIGSEGV` and `SIGBUS` indicate memory access errors and they have
 *   inconsistent semantics across platforms like FreeBSD.
 *
 * - `SIGWINCH` is sent when your terminal window is resized.
 *
 * - `SIGXCPU` and `SIGXFSZ` may be raised if you run out of resources,
 *   which can happen if your process, or the parent process that
 *   spawned your process, happened to call `setrlimit()`. Doing this is
 *   a wonderful idea.
 *
 * @return 0 on success or -1 w/ errno
 * @see xsigaction() for a much better api
 * @asyncsignalsafe
 * @vforksafe
 */
int(sigaction)(int sig, const struct sigaction *act, struct sigaction *oldact) {
  int rc;
  char buf[2][128];
  if (sig == SIGKILL || sig == SIGSTOP) {
    rc = einval();
  } else {
    rc = __sigaction(sig, act, oldact);
  }
  STRACE("sigaction(%G, %s, [%s]) → %d% m", sig,
         __strace_sigaction(buf[0], sizeof(buf[0]), 0, act),
         __strace_sigaction(buf[1], sizeof(buf[1]), rc, oldact), rc);
  return rc;
}