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According to the C Standard, 7.14.1.1 [ISO/IEC 9899:2011], 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).

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 <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[]) {
  int result;
    
  if (argc < 2) {
    return 0;
  }
  denom = (sig_atomic_t)strtol(argv[1], NULL, 10);   
  signal(SIGFPE, sighandle);
  result = 100 / (int)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 <signal.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
  int result;
  int denom;

  if (argc < 2) {
    return 0;
  }
  denom = strtol(argv[1], NULL, 10);
  result = 100 / denom;
  return 0;
}

Assigning the result of strtol() to denom without checking to make sure it fits violates INT31-C. Ensure that integer conversions do not result in lost or misinterpreted data. See INT06-C. Use strtol() or a related function to convert a string token to an integer for a proper example of how to convert a string token to an integer and ensure that the value is in the range of int.

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.

Related Vulnerabilities

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

Bibliography