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According to the C Standard, subclause 7.14.1.1 [ISO/IEC 9899:2011], returning from a SIGSEGV, SIGILL, or SIGFPE signal handler is undefined behavior:

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.

Furthermore, SIGFPE may not be caught for a significant number of instructions after the floating-point instruction that creates it.

Noncompliant Code Example

In this noncompliant code example, if the given user input is 0, the division operation sends 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 = 0;
    
  if (argc < 2) {
    return 0;
  }
  denom = (sig_atomic_t)strtol(argv[1], NULL, 10);
    
  signal(SIGFPE,(*sighandle));

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

This noncompliant code example will loop infinitely on input 0 when compiled with some implementations. 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

In the compliant solution, the only portably safe way to leave a SIGFPE, SIGILL, or SIGSEGV handler is through abort() or /_Exit(). In the case of SIGFPE, the default handler calls abort(), so no user-defined handler is actually needed. The handler shown is only for consistency.

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

volatile sig_atomic_t denom;

void sighandle(int s){
  /* Recovery is impossible */
  abort();
}

int main(int argc, char *argv[]){
  int result = 0;
    
  if (argc < 2) {
    return 0;
  }
  denom = (sig_atomic_t)strtol(argv[1], NULL, 10);
    
  signal(SIGFPE,(*sighandle));

  result = 100 / (int)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

Code that attempts to handle SIGSEGV, SIGILL, or SIGFPE signals is rare. However, code that does rely on handling these signals will usually require a redesign to fix the problem.

Related Vulnerabilities

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

Bibliography