Call only asynchronous-safe functions within signal handlers.

According to Section 7.14.1.1 of the C Rationale \[[ISO/IEC 03|AA. C References#ISO/IEC 03]\]:

When a signal occurs, the normal flow of control of a program is interrupted. If a signal occurs that is being trapped by a signal handler, that handler is invoked. When it is finished, execution continues at the point at which the signal occurred. This arrangement could cause problems if the signal handler invokes a library function that was being executed at the time of the signal. Since library functions are not guaranteed to be reentrant, they should not be called from a signal handler that returns.

Similarly, Section 7.14.1 paragraph 5 of C99 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] states that:

If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler refers to any object with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t, or the signal handler calls any function in the standard library other than the abort function, the _Exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler.

Non-Compliant Code Example

In this non-compliant code example, storage is dynamically allocated to hold a copy of a string. A string literal is copied into the allocated memory, which is then printed and the memory freed. The program also registers the signal handler int_handler() to handle the terminal interrupt signal SIGINT, which also calls free().

#include <signal.h>

char *foo;

void int_handler() {
  free(foo);
  _Exit(0);
}

int main(void) {
  foo = (char *)malloc(sizeof("Hello World."));
  if (foo == NULL) {
    /* handle error condition */
  }
  signal(SIGINT, int_handler);
  strcpy(foo, "Hello World.");
  puts(foo);
  free(foo);
  return 0;
}

This program has two potential problems. The first is that the free() function is not asynchronous-safe and its invocation from within a signal handler is a violation of this rule. If an interrupt signal is received during the free() call in main(), the heap may be corrupted.

The second problem is if SIGINT occurs after the call to free(), resulting in the memory referenced by foo() being freed twice. This is a violation of MEM31-C. Free dynamically allocated memory exactly once and also SIG31-C. Do not access or modify shared objects in signal handlers.

The _Exit() function called from within the int_handler() signal handler causes immediate program termination, and is asynchronous-safe, whereas exit() may call cleanup routines first, and is consequently not asynchronous-safe.

Implementation Details

POSIX

The following table from the the Open Group Base Specifications \[[Open Group 04|AA. C References#Open Group 04]\] defines a set of functions that are asynchronous-signal-safe. Applications may consequently invoke them, without restriction, from signal-catching functions:

All functions not in the this table are considered to be unsafe with respect to signals. In the presence of signals, all functions defined by IEEE Std 1003.1-2001 behave as defined when called from or interrupted by a signal-catching function, with a single exception: when a signal interrupts an unsafe function and the signal-catching function calls an unsafe function, the behavior is undefined.

Note that while raise() is on the list of asynchronous-safe functions, it is specifically covered by SIG33-C. Do not recursively invoke the raise() function.

OpenBSD

The OpenBSD signal() man page identifies functions that are asynchronous-signal safe. Applications may consequently invoke them, without restriction, from signal-catching functions.

The OpenBSD signal() man page also says:

A few other functions are signal race safe in OpenBSD but
     probably not on other systems:

           snprintf()    Safe.
           vsnprintf()   Safe.
           syslog_r()    Safe if the syslog_data struct is initialized
                         as a local variable.

Note that, in general, I/O functions are not safe to invoke inside signal handlers. Check your system's asynchronous-safe functions before using them in signal handlers.

Compliant Solution

Signal handlers should be as concise as possible, ideally unconditionally setting a flag and returning. They may also call the _Exit() function.

#include <signal.h>

void int_handler() {
  _Exit(0);
}

int main(void) {
  char *foo = (char *)malloc(sizeof("Hello World."));
  if (foo == NULL) {
    /* handle error condition */
  }
  signal(SIGINT, int_handler);
  strcpy(foo, "Hello World.");
  puts(foo);
  free(foo);
  return 0;
}

Risk Assessment

Invoking functions that are not asynchronous-safe from within a signal handler may result in privilege escalation and other attacks. For an overview of some software vulnerabilities, see Zalewski's paper on understanding, exploiting and preventing signal-handling related vulnerabilities \[[Zalewski 01|AA. C References#Zalewski 01]\]. [VU #834865|http://www.kb.cert.org/vuls/id/834865] describes a vulnerability resulting from a violation of this rule.

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

Automated Detection

The tool Compass Rose can detect violations of the rule for single-file programs.

References

\[[Dowd 06|AA. C References#Dowd 06]\] Chapter 13, "Synchronization and State"
\[[ISO/IEC 03|AA. C References#ISO/IEC 03]\] Section 5.2.3, "Signals and interrupts"
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 7.14, "Signal handling <signal.h>"
\[[Open Group 04|AA. C References#Open Group 04]\] [longjmp|http://www.opengroup.org/onlinepubs/000095399/functions/longjmp.html]
\[OpenBSD\] [{{signal()}} Man Page|http://www.openbsd.org/cgi-bin/man.cgi?query=signal]
\[[Zalewski 01|AA. C References#Zalewski 01]\]

SIG02-A. Avoid using signals to implement normal functionality      11. Signals (SIG)       SIG31-C. Do not access or modify shared objects in signal handlers