Improve cancellations, randomness, and time

- Exhaustively document cancellation points - Rename SIGCANCEL to SIGTHR just like BSDs - Further improve POSIX thread cancellations - Ensure asynchronous cancellations work correctly - Elevate the quality of getrandom() and getentropy() - Make futexes cancel correctly on OpenBSD 6.x and 7.x - Add reboot.com and shutdown.com to examples directory - Remove underscore prefix from awesome timespec_*() APIs - Create assertions that help verify our cancellation points - Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)
2025-07-04 02:08:30 +00:00 · 2022-11-05 19:49:41 -07:00 · 2022-11-05 19:49:41 -07:00 · 3f0bcdc3ef
commit 3f0bcdc3ef
parent 0d7c265392
173 changed files with 1599 additions and 782 deletions
--- a/libc/thread/posixthread.internal.h
+++ b/libc/thread/posixthread.internal.h
@ -6,11 +6,13 @@
 #include "libc/thread/thread.h"
 #include "libc/thread/tls.h"

-#define PT_OWNSTACK   1
-#define PT_MAINTHREAD 2
-#define PT_ASYNC      4
-#define PT_NOCANCEL   8
-#define PT_MASKED     16
+#define PT_OWNSTACK       1
+#define PT_MAINTHREAD     2
+#define PT_ASYNC          4
+#define PT_NOCANCEL       8
+#define PT_MASKED         16
+#define PT_INCANCEL       32
+#define PT_OPENBSD_KLUDGE 64

 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 COSMOPOLITAN_C_START_
--- a/libc/thread/pthread_atfork.c
+++ b/libc/thread/pthread_atfork.c
@ -18,6 +18,7 @@
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
 #include "libc/errno.h"
+#include "libc/intrin/atomic.h"
 #include "libc/intrin/kmalloc.h"
 #include "libc/runtime/memtrack.internal.h"
 #include "libc/str/str.h"
@ -65,8 +66,14 @@ void _pthread_onfork_parent(void) {
 }

 void _pthread_onfork_child(void) {
+  struct CosmoTib *tib;
+  struct PosixThread *pt;
  pthread_mutexattr_t attr;
  extern pthread_mutex_t __mmi_lock_obj;
+  tib = __get_tls();
+  pt = (struct PosixThread *)tib->tib_pthread;
+  atomic_store_explicit(&pt->cancelled, false, memory_order_relaxed);
+  pt->tid = atomic_load_explicit(&tib->tib_tid, memory_order_relaxed);
  pthread_mutexattr_init(&attr);
  pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
  pthread_mutex_init(&__mmi_lock_obj, &attr);
--- a/libc/thread/pthread_cancel.c
+++ b/libc/thread/pthread_cancel.c
@ -21,7 +21,9 @@
 #include "libc/calls/struct/sigaction.h"
 #include "libc/calls/struct/siginfo.h"
 #include "libc/calls/struct/sigset.h"
+#include "libc/calls/syscall_support-sysv.internal.h"
 #include "libc/calls/ucontext.h"
+#include "libc/dce.h"
 #include "libc/errno.h"
 #include "libc/intrin/atomic.h"
 #include "libc/intrin/kprintf.h"
@ -45,7 +47,7 @@ int _pthread_cancel_sys(void) {
  if (!(pt->flags & (PT_NOCANCEL | PT_MASKED)) || (pt->flags & PT_ASYNC)) {
    pthread_exit(PTHREAD_CANCELED);
  }
-  pt->flags |= PT_NOCANCEL;
+  pt->flags |= PT_NOCANCEL | PT_OPENBSD_KLUDGE;
  return ecanceled();
 }

@ -56,12 +58,14 @@ static void OnSigCancel(int sig, siginfo_t *si, void *ctx) {
  if (pt && !(pt->flags & PT_NOCANCEL) &&
      atomic_load_explicit(&pt->cancelled, memory_order_acquire)) {
    sigaddset(&uc->uc_sigmask, sig);
-    if ((pt->flags & PT_ASYNC) ||
-        (systemfive_cancellable <= (char *)uc->uc_mcontext.rip &&
-         (char *)uc->uc_mcontext.rip < systemfive_cancellable_end)) {
+    if (systemfive_cancellable <= (char *)uc->uc_mcontext.rip &&
+        (char *)uc->uc_mcontext.rip < systemfive_cancellable_end) {
      uc->uc_mcontext.rip = (intptr_t)systemfive_cancel;
+    } else if (pt->flags & PT_ASYNC) {
+      pthread_exit(PTHREAD_CANCELED);
    } else {
-      tkill(atomic_load_explicit(&tib->tib_tid, memory_order_relaxed), sig);
+      __tkill(atomic_load_explicit(&tib->tib_tid, memory_order_relaxed), sig,
+              tib);
    }
  }
 }
@ -71,7 +75,7 @@ static void ListenForSigCancel(void) {
  sa.sa_sigaction = OnSigCancel;
  sa.sa_flags = SA_SIGINFO | SA_RESTART | SA_ONSTACK;
  memset(&sa.sa_mask, -1, sizeof(sa.sa_mask));
-  _npassert(!sigaction(SIGCANCEL, &sa, 0));
+  _npassert(!sigaction(SIGTHR, &sa, 0));
 }

 /**
@ -79,28 +83,142 @@ static void ListenForSigCancel(void) {
 *
 * When a thread is cancelled, it'll interrupt blocking i/o calls,
 * invoke any cleanup handlers that were pushed on the thread's stack
- * as well as key destructors, and then the thread exits.
+ * before the cancellation occurred, in addition to destructing pthread
+ * keys, before finally, the thread shall abruptly exit.
 *
- * By default, pthread_cancel() can only take effect when a thread is
- * blocked on a @cancellationpoint, which is any system call that's
- * specified as raising `EINTR`. For example, `openat`, `poll`, `ppoll`,
- * `select`, `pselect`, `read`, `readv`, `pread`, `preadv`, `write`,
- * `writev`, `pwrite`, `pwritev`, `accept`, `connect`, `recvmsg`,
- * `sendmsg`, `recv`, `send`, `tcdrain`, `clock_nanosleep`, `fsync`,
- * `fdatasync`, `fcntl(F_SETLKW)`, `epoll`, `sigsuspend`, `msync`,
- * `wait4`, `getrandom`, `pthread_cond_timedwait`, and `sem_timedwait`
- * are most cancellation points, plus many userspace libraries that call
- * the above functions, unless they're using pthread_setcancelstate() to
- * temporarily disable the cancellation mechanism. Some userspace
- * functions, e.g. system() will eagerly call pthread_testcancel_np() to
- * help avoid the potential for resource leaks later on.
+ * By default, pthread_cancel() can only take effect when a thread
+ * reaches a cancellation point. Such functions are documented with
+ * @cancellationpoint. They check the cancellation state before the
+ * underlying system call is issued. If the system call is issued and
+ * blocks, then pthread_cancel() will interrupt the operation in which
+ * case the syscall wrapper will check the cancelled state a second
+ * time, only if the raw system call returned EINTR.
+ *
+ * The following system calls are implemented as cancellation points.
+ *
+ * - `accept4`
+ * - `accept`
+ * - `clock_nanosleep`
+ * - `connect`
+ * - `copy_file_range`
+ * - `creat`
+ * - `epoll_wait`
+ * - `fcntl(F_OFD_SETLKW)`
+ * - `fcntl(F_SETLKW)`
+ * - `fdatasync`
+ * - `flock`
+ * - `fstatfs`
+ * - `fsync`
+ * - `ftruncate`
+ * - `getrandom`
+ * - `msync`
+ * - `nanosleep`
+ * - `open`
+ * - `openat`
+ * - `pause`
+ * - `poll`
+ * - `ppoll`
+ * - `pread`
+ * - `preadv`
+ * - `pselect`
+ * - `pwrite`
+ * - `pwritev`
+ * - `read`
+ * - `readv`
+ * - `recvfrom`
+ * - `recvmsg`
+ * - `select`
+ * - `sendmsg`
+ * - `sendto`
+ * - `sigsuspend`
+ * - `sigtimedwait`
+ * - `sigwaitinfo`
+ * - `statfs`
+ * - `tcdrain`
+ * - `truncate`
+ * - `wait3`
+ * - `wait4`
+ * - `wait`
+ * - `waitpid`
+ * - `write`
+ * - `writev`
+ *
+ * The following library calls are implemented as cancellation points.
+ *
+ * - `fopen`
+ * - `gzopen`, `gzread`, `gzwrite`, etc.
+ * - `lockf(F_LOCK)`
+ * - `nsync_cv_wait_with_deadline`
+ * - `nsync_cv_wait`
+ * - `opendir`
+ * - `pclose`
+ * - `popen`
+ * - `fwrite`, `printf`, `fprintf`, `putc`, etc.
+ * - `pthread_cond_timedwait`
+ * - `pthread_cond_wait`
+ * - `pthread_join`
+ * - `sem_timedwait`
+ * - `sem_wait`
+ * - `sleep`
+ * - `timespec_sleep_until`
+ * - `tmpfd`
+ * - `tmpfile`
+ * - `usleep`
+ *
+ * Other userspace libraries provided by Cosmopolitan Libc that call the
+ * cancellation points above will block cancellations while running. The
+ * following are examples of functions that *aren't* cancellation points
+ *
+ * - `INFOF()`, `WARNF()`, etc.
+ * - `getentropy`
+ * - `gmtime_r`
+ * - `kprintf`
+ * - `localtime_r`
+ * - `nsync_mu_lock`
+ * - `nsync_mu_unlock`
+ * - `openpty`
+ * - `pthread_getname_np`
+ * - `pthread_mutex_lock`
+ * - `pthread_mutex_unlock`
+ * - `pthread_setname_np`
+ * - `sem_open`
+ * - `system`
+ * - `timespec_sleep`
+ * - `touch`
+ *
+ * The way to block cancellations temporarily is:
+ *
+ *     int cs;
+ *     pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
+ *     // ...
+ *     pthread_setcancelstate(cs, 0);
+ *
+ * In order to support cancellations all your code needs to be rewritten
+ * so that when resources such as file descriptors are managed they must
+ * have a cleanup crew pushed to the stack. For example even malloc() is
+ * technically unsafe w.r.t. leaks without doing something like this:
+ *
+ *     void *p = malloc(123);
+ *     pthread_cleanup_push(free, p);
+ *     read(0, p, 123);
+ *     pthread_cleanup_pop(1);
+ *
+ * Consider using Cosmopolitan Libc's garbage collector since it will be
+ * executed when a thread exits due to a cancellation.
+ *
+ *     void *p = _gc(malloc(123));
+ *     read(0, p, 123);
 *
 * It's possible to put a thread in asynchronous cancellation mode using
- * pthread_setcanceltype(), thus allowing a cancellation to occur at any
- * assembly opcode. Please be warned that doing so is risky since it can
- * easily result in resource leaks. For example, a cancellation might be
- * triggered between calling open() and pthread_cleanup_push(), in which
- * case the application will leak a file descriptor.
+ *
+ *     pthread_setcancelstate(PTHREAD_CANCEL_ASYNCHRONOUS, 0);
+ *     for (;;) donothing;
+ *
+ * In which case a thread may be cancelled at any assembly opcode. This
+ * is useful for immediately halting threads that consume cpu and don't
+ * use any system calls. It shouldn't be used on threads that will call
+ * cancellation points since in that case asynchronous mode could cause
+ * resource leaks to happen, in such a way that can't be worked around.
 *
 * If none of the above options seem savory to you, then a third way is
 * offered for doing cancellations. Cosmopolitan Libc supports the Musl
@ -109,6 +227,33 @@ static void ListenForSigCancel(void) {
 * be abruptly destroyed upon cancellation and have their stack unwound;
 * instead, cancellation points will simply raise an `ECANCELED` error,
 * which can be more safely and intuitively handled for many use cases.
+ * For example:
+ *
+ *     pthread_setcancelstate(PTHREAD_CANCEL_MASKED, 0);
+ *     void *p = malloc(123);
+ *     int rc = read(0, p, 123);
+ *     free(p);
+ *     if (rc == ECANCELED) {
+ *       pthread_exit(0);
+ *     }
+ *
+ * Shows how the masked cancellations paradigm can be safely used. Note
+ * that it's so important that cancellation point error return codes be
+ * checked. Code such as the following:
+ *
+ *     pthread_setcancelstate(PTHREAD_CANCEL_MASKED, 0);
+ *     void *p = malloc(123);
+ *     write(2, "log\n", 4); // XXX: fails to check result
+ *     int rc = read(0, p, 123);
+ *     free(p);
+ *     if (rc == ECANCELED) {
+ *       pthread_exit(0); // XXX: not run if write() was cancelled
+ *     }
+ *
+ * Isn't safe to use in masked mode. That's because if a cancellation
+ * occurs during the write() operation then cancellations are blocked
+ * while running read(). Masked mode doesn't have second chances. You
+ * must rigorously check the results of each cancellation point call.
 *
 * @return 0 on success, or errno on error
 * @raise ESRCH if thread isn't alive
@ -117,7 +262,6 @@ errno_t pthread_cancel(pthread_t thread) {
  int e, rc, tid;
  static bool once;
  struct PosixThread *pt;
-  __require_tls();
  if (!once) ListenForSigCancel(), once = true;
  pt = (struct PosixThread *)thread;
  switch (atomic_load_explicit(&pt->status, memory_order_acquire)) {
@ -127,18 +271,18 @@ errno_t pthread_cancel(pthread_t thread) {
    default:
      break;
  }
-  atomic_exchange_explicit(&pt->cancelled, 1, memory_order_release);
+  atomic_store_explicit(&pt->cancelled, 1, memory_order_release);
  if (thread == __get_tls()->tib_pthread) {
    if (!(pt->flags & (PT_NOCANCEL | PT_MASKED)) && (pt->flags & PT_ASYNC)) {
      pthread_exit(PTHREAD_CANCELED);
    }
    return 0;
  }
-  if (IsWindows()) return 0;  // TODO(jart): Should we do this?
+  if (IsWindows()) return 0;  // no true cancellations on Windows yet
  tid = atomic_load_explicit(&pt->tib->tib_tid, memory_order_acquire);
-  if (tid <= 0) return 0;  // TODO(jart): Do we need this?
+  if (tid <= 0) return 0;  // -1 means still starting, 0 means exited
  e = errno;
-  if (!tkill(tid, SIGCANCEL)) {
+  if (!__tkill(tid, SIGTHR, pt->tib)) {
    return 0;
  } else {
    rc = errno;
--- a/libc/thread/pthread_cond_timedwait.c
+++ b/libc/thread/pthread_cond_timedwait.c
@ -27,7 +27,7 @@
 * Waits for condition with optional time limit, e.g.
 *
 *     struct timespec ts;  // one second timeout
- *     ts = _timespec_add(_timespec_real(), _timespec_frommillis(1000));
+ *     ts = timespec_add(timespec_real(), timespec_frommillis(1000));
 *     if (pthread_cond_timedwait(cond, mutex, &ts) == ETIMEDOUT) {
 *       // handle timeout...
 *     }
--- a/libc/thread/pthread_kill.c
+++ b/libc/thread/pthread_kill.c
@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/calls/calls.h"
+#include "libc/calls/syscall_support-sysv.internal.h"
 #include "libc/errno.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
@ -34,7 +35,7 @@
 errno_t pthread_kill(pthread_t thread, int sig) {
  int rc, e = errno;
  struct PosixThread *pt = (struct PosixThread *)thread;
-  if (!tkill(pt->tid, sig)) {
+  if (!__tkill(pt->tid, sig, pt->tib)) {
    rc = 0;
  } else {
    rc = errno;
--- a/libc/thread/pthread_setcanceltype.c
+++ b/libc/thread/pthread_setcanceltype.c
@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/dce.h"
 #include "libc/errno.h"
 #include "libc/thread/posixthread.internal.h"
 #include "libc/thread/thread.h"
@ -26,17 +27,22 @@
 *
 * @param type may be one of:
 *     - `PTHREAD_CANCEL_DEFERRED` (default)
- *     - `PTHREAD_CANCEL_ASYNCHRONOUS` (cray cray)
+ *     - `PTHREAD_CANCEL_ASYNCHRONOUS`
 * @param oldtype optionally receives old value
 * @return 0 on success, or errno on error
+ * @raise ENOTSUP on Windows if asynchronous
 * @raise EINVAL if `type` has bad value
 * @see pthread_cancel() for docs
 */
 errno_t pthread_setcanceltype(int type, int *oldtype) {
  struct PosixThread *pt;
  switch (type) {
-    case PTHREAD_CANCEL_DEFERRED:
    case PTHREAD_CANCEL_ASYNCHRONOUS:
+      if (IsWindows()) {
+        return ENOTSUP;
+      }
+      // fallthrough
+    case PTHREAD_CANCEL_DEFERRED:
      pt = (struct PosixThread *)__get_tls()->tib_pthread;
      if (oldtype) {
        if (pt->flags & PT_ASYNC) {
--- a/libc/thread/sem_timedwait.c
+++ b/libc/thread/sem_timedwait.c
@ -17,6 +17,7 @@
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
 #include "libc/assert.h"
+#include "libc/calls/cp.internal.h"
 #include "libc/calls/struct/timespec.h"
 #include "libc/calls/struct/timespec.internal.h"
 #include "libc/errno.h"
@ -42,11 +43,11 @@ static struct timespec *sem_timeout(struct timespec *memory,
    *memory = *abstime;
    return memory;
  } else {
-    now = _timespec_real();
-    if (_timespec_cmp(now, *abstime) > 0) {
+    now = timespec_real();
+    if (timespec_cmp(now, *abstime) > 0) {
      *memory = (struct timespec){0};
    } else {
-      *memory = _timespec_sub(*abstime, now);
+      *memory = timespec_sub(*abstime, now);
    }
    return memory;
  }
@ -87,6 +88,7 @@ int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
    }
  }

+  BEGIN_CANCELLATION_POINT;
  _unassert(atomic_fetch_add_explicit(&sem->sem_waiters, +1,
                                      memory_order_acq_rel) >= 0);
  pthread_cleanup_push(sem_timedwait_cleanup, sem);
@ -102,7 +104,7 @@ int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
        rc = 0;
      } else if (rc == -ETIMEDOUT) {
        _npassert(abstime);
-        if (_timespec_cmp(*abstime, _timespec_real()) <= 0) {
+        if (timespec_cmp(*abstime, timespec_real()) <= 0) {
          rc = etimedout();
        } else {
          rc = 0;
@ -122,6 +124,7 @@ int sem_timedwait(sem_t *sem, const struct timespec *abstime) {
                             memory_order_relaxed)));

  pthread_cleanup_pop(1);
+  END_CANCELLATION_POINT;

  return rc;
 }
--- a/libc/thread/wait0.c
+++ b/libc/thread/wait0.c
@ -16,6 +16,7 @@
 │ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
 │ PERFORMANCE OF THIS SOFTWARE.                                                │
 ╚─────────────────────────────────────────────────────────────────────────────*/
+#include "libc/calls/cp.internal.h"
 #include "libc/dce.h"
 #include "libc/errno.h"
 #include "libc/intrin/atomic.h"
@ -33,13 +34,17 @@
 *
 * @return 0 on success, or errno on error
 * @raise ECANCELED if calling thread was cancelled in masked mode
+ * @cancellationpoint
 */
 errno_t _wait0(const atomic_int *ctid) {
-  int x, rc;
+  int x, rc = 0;
+  BEGIN_CANCELLATION_POINT;
  while ((x = atomic_load_explicit(ctid, memory_order_acquire))) {
    if (nsync_futex_wait_(ctid, x, !IsWindows(), 0) == -ECANCELED) {
-      return ECANCELED;
+      rc = ECANCELED;
+      break;
    }
  }
-  return 0;
+  END_CANCELLATION_POINT;
+  return rc;
 }