SIG35-C. Do not return from a computational exception signal handler

According to the C Standard, 7.14.1.1 paragraph 3 [ISO/IEC 9899:2024], 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 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.

Noncompliant Code Example

In this noncompliant code example, the division operation has undefined behavior if denom equals 0. (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;
}

When compiled with some implementations, this noncompliant code example will loop infinitely if given the input 0. It illustrates that even when a SIGFPE handler attempts to fix the error condition while obeying all other rules of signal handling, the program still does not behave as expected.

Compliant Solution

The only portably safe way to leave a SIGFPE, SIGILL, or SIGSEGV handler is to invoke abort(), quick_exit(), or _Exit(). In the case of SIGFPE, the default action is abnormal termination, so no user-defined handler is required:

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

int main(int argc, char *argv[]) {
  if (argc < 2) {
    return 0;
  }
 
  char *end = NULL;
  long denom = strtol(argv[1], &end, 10);
 
  if (end == argv[1] || 0 != *end ||
      ((LONG_MIN == denom || LONG_MAX == denom) && errno == ERANGE)) {
    /* Handle error */
  }
 
  long result = 100 / denom;
  return 0;
}

Implementation Details

Some implementations define useful behavior for programs that return from one or more of these signal handlers. For example, Solaris provides the sigfpe() function specifically to set a SIGFPE handler that a program may safely return from. Oracle also provides platform-specific computational exceptions for the SIGTRAP, SIGBUS, and SIGEMT signals. Finally, GNU libsigsegv takes advantage of the ability to return from a SIGSEGV handler to implement page-level memory management in user mode.

Risk Assessment

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