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signal.c

/*
 * Copyright (C) 2004 PathScale, Inc
 * Copyright (C) 2004 - 2007 Jeff Dike (jdike@{addtoit,linux.intel}.com)
 * Licensed under the GPL
 */

#include <stdlib.h>
#include <stdarg.h>
#include <errno.h>
#include <signal.h>
#include <strings.h>
#include "os.h"
#include "sysdep/barrier.h"
#include "sysdep/sigcontext.h"
#include "user.h"

/*
 * These are the asynchronous signals.  SIGPROF is excluded because we want to
 * be able to profile all of UML, not just the non-critical sections.  If
 * profiling is not thread-safe, then that is not my problem.  We can disable
 * profiling when SMP is enabled in that case.
 */
#define SIGIO_BIT 0
#define SIGIO_MASK (1 << SIGIO_BIT)

#define SIGVTALRM_BIT 1
#define SIGVTALRM_MASK (1 << SIGVTALRM_BIT)

/*
 * These are used by both the signal handlers and
 * block/unblock_signals.  I don't want modifications cached in a
 * register - they must go straight to memory.
 */
static volatile int signals_enabled = 1;
static volatile int pending = 0;

void sig_handler(int sig, struct sigcontext *sc)
{
      int enabled;

      enabled = signals_enabled;
      if (!enabled && (sig == SIGIO)) {
            pending |= SIGIO_MASK;
            return;
      }

      block_signals();

      sig_handler_common_skas(sig, sc);

      set_signals(enabled);
}

static void real_alarm_handler(struct sigcontext *sc)
{
      struct uml_pt_regs regs;

      if (sc != NULL)
            copy_sc(&regs, sc);
      regs.is_user = 0;
      unblock_signals();
      timer_handler(SIGVTALRM, &regs);
}

void alarm_handler(int sig, struct sigcontext *sc)
{
      int enabled;

      enabled = signals_enabled;
      if (!signals_enabled) {
            pending |= SIGVTALRM_MASK;
            return;
      }

      block_signals();

      real_alarm_handler(sc);
      set_signals(enabled);
}

void timer_init(void)
{
      set_handler(SIGVTALRM, (__sighandler_t) alarm_handler,
                SA_ONSTACK | SA_RESTART, SIGUSR1, SIGIO, SIGWINCH, -1);
}

void set_sigstack(void *sig_stack, int size)
{
      stack_t stack = ((stack_t) { .ss_flags    = 0,
                             .ss_sp = (__ptr_t) sig_stack,
                             .ss_size     = size - sizeof(void *) });

      if (sigaltstack(&stack, NULL) != 0)
            panic("enabling signal stack failed, errno = %d\n", errno);
}

void remove_sigstack(void)
{
      stack_t stack = ((stack_t) { .ss_flags    = SS_DISABLE,
                             .ss_sp = NULL,
                             .ss_size     = 0 });

      if (sigaltstack(&stack, NULL) != 0)
            panic("disabling signal stack failed, errno = %d\n", errno);
}

void (*handlers[_NSIG])(int sig, struct sigcontext *sc);

void handle_signal(int sig, struct sigcontext *sc)
{
      unsigned long pending = 1UL << sig;

      do {
            int nested, bail;

            /*
             * pending comes back with one bit set for each
             * interrupt that arrived while setting up the stack,
             * plus a bit for this interrupt, plus the zero bit is
             * set if this is a nested interrupt.
             * If bail is true, then we interrupted another
             * handler setting up the stack.  In this case, we
             * have to return, and the upper handler will deal
             * with this interrupt.
             */
            bail = to_irq_stack(&pending);
            if (bail)
                  return;

            nested = pending & 1;
            pending &= ~1;

            while ((sig = ffs(pending)) != 0){
                  sig--;
                  pending &= ~(1 << sig);
                  (*handlers[sig])(sig, sc);
            }

            /*
             * Again, pending comes back with a mask of signals
             * that arrived while tearing down the stack.  If this
             * is non-zero, we just go back, set up the stack
             * again, and handle the new interrupts.
             */
            if (!nested)
                  pending = from_irq_stack(nested);
      } while (pending);
}

extern void hard_handler(int sig);

void set_handler(int sig, void (*handler)(int), int flags, ...)
{
      struct sigaction action;
      va_list ap;
      sigset_t sig_mask;
      int mask;

      handlers[sig] = (void (*)(int, struct sigcontext *)) handler;
      action.sa_handler = hard_handler;

      sigemptyset(&action.sa_mask);

      va_start(ap, flags);
      while ((mask = va_arg(ap, int)) != -1)
            sigaddset(&action.sa_mask, mask);
      va_end(ap);

      action.sa_flags = flags;
      action.sa_restorer = NULL;
      if (sigaction(sig, &action, NULL) < 0)
            panic("sigaction failed - errno = %d\n", errno);

      sigemptyset(&sig_mask);
      sigaddset(&sig_mask, sig);
      if (sigprocmask(SIG_UNBLOCK, &sig_mask, NULL) < 0)
            panic("sigprocmask failed - errno = %d\n", errno);
}

int change_sig(int signal, int on)
{
      sigset_t sigset, old;

      sigemptyset(&sigset);
      sigaddset(&sigset, signal);
      sigprocmask(on ? SIG_UNBLOCK : SIG_BLOCK, &sigset, &old);
      return !sigismember(&old, signal);
}

void block_signals(void)
{
      signals_enabled = 0;
      /*
       * This must return with signals disabled, so this barrier
       * ensures that writes are flushed out before the return.
       * This might matter if gcc figures out how to inline this and
       * decides to shuffle this code into the caller.
       */
      mb();
}

void unblock_signals(void)
{
      int save_pending;

      if (signals_enabled == 1)
            return;

      /*
       * We loop because the IRQ handler returns with interrupts off.  So,
       * interrupts may have arrived and we need to re-enable them and
       * recheck pending.
       */
      while(1) {
            /*
             * Save and reset save_pending after enabling signals.  This
             * way, pending won't be changed while we're reading it.
             */
            signals_enabled = 1;

            /*
             * Setting signals_enabled and reading pending must
             * happen in this order.
             */
            mb();

            save_pending = pending;
            if (save_pending == 0) {
                  /*
                   * This must return with signals enabled, so
                   * this barrier ensures that writes are
                   * flushed out before the return.  This might
                   * matter if gcc figures out how to inline
                   * this (unlikely, given its size) and decides
                   * to shuffle this code into the caller.
                   */
                  mb();
                  return;
            }

            pending = 0;

            /*
             * We have pending interrupts, so disable signals, as the
             * handlers expect them off when they are called.  They will
             * be enabled again above.
             */

            signals_enabled = 0;

            /*
             * Deal with SIGIO first because the alarm handler might
             * schedule, leaving the pending SIGIO stranded until we come
             * back here.
             */
            if (save_pending & SIGIO_MASK)
                  sig_handler_common_skas(SIGIO, NULL);

            if (save_pending & SIGVTALRM_MASK)
                  real_alarm_handler(NULL);
      }
}

int get_signals(void)
{
      return signals_enabled;
}

int set_signals(int enable)
{
      int ret;
      if (signals_enabled == enable)
            return enable;

      ret = signals_enabled;
      if (enable)
            unblock_signals();
      else block_signals();

      return ret;
}

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