MSC22-C. Use the setjmp(), longjmp() facility securely

The setjmp() macro should be invoked from only one of the contexts listed in §7.13.1.1 of [ISO/IEC 9899:2011]. Invoking setjmp() outside of one of these contexts results in undefined behavior. (See undefined behavior 125.)

After invoking longjmp(), non–volatile-qualified local objects should not be accessed if their values could have changed since the invocation of setjmp(). Their value in this case is considered indeterminate, and accessing them is undefined behavior. (See undefined behavior 127, 10.)

The longjmp() function should never be used to return control to a function that has terminated execution. (See Undefined Behavior 126.)

Signal masks, floating-point status flags, and the state of open files are not saved by the setjmp() function. If signal masks need to be saved, the sigsetjmp() function should be used.

This recommendation is related to SIG32-C. Do not call longjmp() from inside a signal handler and ENV32-C. All atexit handlers must return normally.

Noncompliant Code Example

This noncompliant code example calls setjmp() in an assignment statement, resulting in undefined behavior.

jmp_buf buf;

void f(void) {
  int i = setjmp(buf);
  if (i == 0) {
    g();
  } else {
    /* longjmp was invoked */
  }
}

void g(void) {
  /* ... */
  longjmp(buf, 1);
}

Compliant Solution

Placing the call to setjmp() in the if statement and, optionally, comparing it with a constant integer removes the undefined behavior, as shown in this compliant solution.

jmp_buf buf;

void f(void) {
  if (setjmp(buf) == 0) {
    g();
  } else {
    /* longjmp was invoked */
  }
}

void g(void) {
  /* ... */
  longjmp(buf, 1);
}

Noncompliant Code Example

Any attempt to invoke the longjmp() function to transfer control to a function that has completed execution results in undefined behavior.

jmp_buf buf;
unsigned char b[] = {0xe5, 0x06, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00};

int main(void) {
  setup();
  do_stuff();
  return 0;
}

void setup(void) {
  f();
}

void f(void) {
  g();
}

void g(void) {
  if (setjmp(buf) == 0) {
    printf("setjmp() invoked\n");
  } else {
    printf("longjmp() invoked\n");
  }
}

void do_stuff(void) {
  char a[8];
  memcpy(a, b, 8);
  /* ... stuff ... */
  longjmp(buf, 1);
}

void bad(void) {
  printf("Should not be called!\n");
  exit(1);
}

Implementation Details

When compiled for x86-64 using GCC v4.1.2 on Linux, the above example outputs the following when run:

setjmp() invoked
longjmp() invoked
Should not be called!

Because g() has finished executing at the time longjmp() is called, it is no longer on the stack. When do_stuff() is invoked, its stackframe occupies the same memory as the old stackframe of g(). In this case, a was located in the same location as the return address of function g(). The call to memcpy() overwrites the return address, so, when longjmp() sends control back to function g(), the function returns to the wrong address (in this case, to function bad()).

If the array b were user-specified, the user would be able to set the return address of function g() to any location.

Compliant Solution

The longjmp() function should only be used when the function containing the corresponding setjmp() is guaranteed not to have completed execution, as in the following example:

jmp_buf buf;
unsigned char b[] = {0xe5, 0x06, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00};

int main(void) {
  if (setjmp(buf) == 0) {
    printf("setjmp() invoked\n");
  } else {
    printf("longjmp() invoked\n");
  }
  do_stuff();
  return 0;
}

void do_stuff(void) {
  char a[8];
  memcpy(a, b, 8);
  /* ... stuff ... */
  longjmp(buf, 1);
}

void bad(void) {
  printf("Should not be called!\n");
  exit(1);
}

There is no risk of overwriting a return address because the stack frame of main() (the function that invoked setjmp()) is still on the stack; so when do_stuff() is invoked, the two stack frames will not overlap.

Noncompliant Code Example

In this noncompliant example, non-volatile-qualified objects local to the function that invoked the corresponding setjmp() have indeterminate values after longjmp() has been executed if their value has been changed since the invocation of setjmp().

jmp_buf buf;

void f(void) {
  int i = 0;
  if (setjmp(buf) != 0) {
    printf("%i\n", i);
    /* ... */
  }
  i = 2;
  g();
}

void g(void) {
  /* ... */
  longjmp(buf, 1);
}

Compliant Solution

If an object local to the function that invoked setjmp() needs to be accessed after longjmp() returns control to the function, the object should be volatile-qualified.

jmp_buf buf;

void f(void) {
  volatile int i = 0;
  if (setjmp(buf) != 0) {
    printf("%i\n", i);
    /* ... */
  }
  i = 2;
  g();
}

void g(void) {
  /* ... */
  longjmp(buf, 1);
}

Risk Assessment

Related Guidelines

ISO/IEC 9899:1999 Section 7.13, "Nonlocal jumps <setjmp.h>", Section J.2, "Portability issues"

Bibliography