difference between gcc -D_FORTIFY_SOURCE=1 and -D_FORTIFY_SOURCE=2

From the manual page for the Feature Test Macros (man 7 feature_test_macros)

_FORTIFY_SOURCE (since glibc 2.3.4)

Defining this macro causes some lightweight checks to be performed to detect some buffer overflow errors when employing various string and memory manipulation functions (for example, memcpy, memset, stpcpy, strcpy, strncpy, strcat, strncat, sprintf, snprintf, vsprintf, vsnprintf, gets, and wide character variants thereof). For some functions, argument consistency is checked; for example, a check is made that open has been supplied with a mode argument when the specified flags include O_CREAT. Not all problems are detected, just some common cases.

If _FORTIFY_SOURCE is set to 1, with compiler optimization level 1 (gcc -O1) and above, checks that shouldn't change the behavior of conforming programs are performed.

With _FORTIFY_SOURCE set to 2, some more checking is added, but some conforming programs might fail.

Some of the checks can be performed at compile time (via macros logic implemented in header files), and result in compiler warnings; other checks take place at run time, and result in a run-time error if the check fails.

Use of this macro requires compiler support, available with gcc since version 4.0.

Moreover, the article Enhance application security with FORTIFY_SOURCE (March 2014) says:

• gcc -D_FORTIFY_SOURCE=1 adds checks at compile-time only (some headers are necessary as #include <string.h>)
• gcc -D_FORTIFY_SOURCE=2 also adds checks at run-time (detected buffer overflow terminates the program)

Essentially, _FORTIFY_SOURCE level 2 is more secure, but is a slightly riskier compilation strategy; if you use it, make sure you have very strong regression tests for your compiled code to prove the compiler hasn't introduced any unexpected behaviour.

http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html goes into more detail than feature_test_macros(7).

Here's the relevant excerpt, lightly edited/reformatted for clarity:

The difference between -D_FORTIFY_SOURCE=1 and -D_FORTIFY_SOURCE=2 is e.g. for

  struct S {
      struct T {
        char buf[5];
        int x;
      } t;
      char buf[20];
  } var;

With -D_FORTIFY_SOURCE=1,

  strcpy (&var.t.buf[1], "abcdefg");

is not considered an overflow (object is whole var), while with -D_FORTIFY_SOURCE=2

  strcpy (&var.t.buf[1], "abcdefg");

will be considered a buffer overflow.

Another difference is that with -D_FORTIFY_SOURCE=2, %n in format strings of the most common *printf family functions is allowed only if it is stored in read-only memory (usually string literals, gettext's _("%s string %n") is fine too), but usually when an attacker attempts to exploit a format string vulnerability, %n will be somewhere where the attacker could write it into.