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According to the C Standard, 7.14.1.1 1 paragraph 3 [ISO/IEC 9899:20112024], if a signal handler returns when it has been entered as a result of a computational exception (that is, with the value of its argument of SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to such an exception) returns, then the behavior is undefined. (see See undefined behavior 130.)

When a signal occurs and func points to a function, it is implementation-defined whether the equivalent of signal (sig, SIG_DFL); is executed or the implementation prevents some implementation- defined set of signals (at least including sig) from occurring until the current signal handling has completed; in the case of SIGILL, the implementation may alternatively define that no action is taken. Then the equivalent of (*func)(sig); is executed. If and when the function returns, if the value of sig is SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a computational exception, the behavior is undefined; otherwise the program will resume execution at the point it was interrupted.

The Portable Operating System Interface (POSIX^®), Base Specifications, Issue 7 [IEEE Std 1003.1:2013], adds SIGBUS to the list of computational exception signal handlers,:

The behavior of a process is undefined after it returns normally from a signal-catching function for a SIGBUS, SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(), sigqueue(), or raise().

Do not return from SIGFPE, SIGILL, SIGSEGV, or any other implementation-defined value corresponding to a computational exception, such as SIGBUS on POSIX systems, regardless of how the signal was generated.

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In this noncompliant code example, the division operation has undefined behavior if denom equals 0. (see See INT33-C. Ensure that division and remainder operations do not result in divide-by-zero errors)  and may result in a SIGFPE signal to the program:.)

#include <errno.h>
#include <limits.h>
#include <signal.h>
#include <stdlib.h>

volatile sig_atomic_t denom;

void sighandle(int s) {
  /* Fix the offending volatile */
  if (denom == 0) {
    denom = 1;
  }
}

int main(int argc, char *argv[]) {
  if (argc < 2) {
    return 0;
  }
 
  char *end = NULL;
  long temp = strtol(argv[1], &end, 10);
 
  if (end == argv[1] || 0 != *end ||
      ((LONG_MIN == temp || LONG_MAX == temp) && errno == ERANGE)) {
    /* Handle error */
  }
 
  denom = (sig_atomic_t)temp;
  signal(SIGFPE, sighandle);

  long result = 100 / (long)denom;
  return 0;
}

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Returning from a computational exception signal handler is undefined behavior.

Automated Detection

Related Vulnerabilities

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

Bibliography

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