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A signal handler should not reassert its desire to handle its own signal. This is often done on nonpersistent platforms—that is, platforms that, upon receiving a signal, reset the handler for the signal to SIG_DFL before calling the bound signal handler. Calling signal() under these conditions presents a race condition. (see See SIG01-C. Understand implementation-specific details regarding signal handler persistence.).

A signal handler may call signal() only if it does not need to be asynchronous-safe (that is, if all relevant signals are masked so that the handler cannot be interrupted).

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#include <signal.h>
#include <stddef.h>
 
void handler(int signum) {
  /* Handle signal */
}

void func(void) {
  struct sigaction act;
  act.sa_handler = handler;
  act.sa_flags = 0;
  if (sigemptyset(&act.sa_mask) != 0) {
    /* Handle error */
  }
  if (sigaction(SIGUSR1, &act, NULL) != 0) {
    /* Handle error */
  }
}

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Compliant Solution (Windows)

There are two classes of signals in the Visual C++ implementation:

• SIGSEGV, SIGILL, and SIGFPE have per-thread handlers, so each thread may register its own handler for these signals.
• SIGABRT, SIGBREAK, SIGTERM, and SIGINT have global handlers and access to these handlers is synchronized via a global lock.

For the signals with global handlers, the handler is reset to SIG_DFL and the handler is called under a lock, so there is no race if signal() is called again from the handler to reassert itself as the handler. For the signals with per-thread handlers, the state is local to the thread and resultantly there is no opportunity for a race.

Exceptions

There is no safe way to implement persistent signal-handler behavior on Windows platforms, and it should not be attempted. If a design depends on this behavior, and the design cannot be altered, it may be necessary to claim a deviation from this rule after completing an appropriate risk analysis.

The reason for this is that Windows is a nonpersistent platform as discussed above.  Just before calling the current handler function, Windows resets the handler for the next occurrence of the same signal to SIG_DFL. If the handler calls signal() to reinstall itself, there is still a race window. A signal might occur between the start of the handler and the call to signal(), which would invoke the default behavior instead of the desired handler.

Exceptions

SIG34-CSIG34-EX1: For implementations with persistent signal handlers, it is safe for a handler to modify the behavior of its own signal. Behavior modifications include ignoring the signal, resetting to the default behavior, and having the signal handled by a different handler. A handler reasserting its binding is also safe but unnecessary.

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Two signals in quick succession can trigger a race condition on nonpersistent platforms, causing the signal's default behavior despite a handler's attempt to override it.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

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