_FORTIFY_SOURCE=2

[David Fifield <david () bamsoftware com>]

On Sun, Jan 04, 2009 at 07:49:18PM -0800, Fyodor wrote:
> On Sat, Jan 03, 2009 at 10:39:49AM +0000, doug () hcsw org wrote:
> > For anyone not familiar with this error message, it is from a fairly
> > recent addition to gcc called _FORTIFY_SOURCE. Kris observed this
> > error because Ubuntu enables _FORTIFY_SOURCE=2 by default on all
> > gcc compiles, unlike many other linux distros.
>
> Sounds like a great feature!  Anyone want to make a patch so that Nmap
> always uses this when available?  It would be good for security, and
> may help us identify bugs which might otherwise go unnoticed (as we
> saw in this case).  Ideally it would cover PCRE and OpenSSL too.

I defined _FORTIFY_SOURCE=2 in nmap, nbase, ncat, and nsock. A few tests
scans didn't show anything obviously failing. I think I'll be able to
test speed in an upcoming nmap-perf benchmark.

This feature is undocumented to a surprising degree. The best web page
I found is the one Doug linked, a mailing list post from 2004:
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg02055.html
It adds extra security checks in the source code, looking for things
like ignoring return values that shouldn't be ignored, and overflowing a
buffer with a strcpy. Some checks happen at compile time and others
happen at run time. _FORTIFY_SOURCE=1 is not supposed to cause any
"conforming" program to fail. _FORTIFY_SOURCE=2 adds more checks on top
of that.

To get an idea of the kinds of source code changes effected, try
        g++ -E -Ilibdnet-stripped/include  -Inbase -Insock/include -O2 -DHAVE_CONFIG_H nmap.cc > nmap-fortify-0.E
        g++ -E -Ilibdnet-stripped/include  -Inbase -Insock/include -O2 -DHAVE_CONFIG_H -D_FORTIFY_SOURCE=2 nmap.cc > 
nmap-fortify-2.E
        diff -u nmap-fortify-0.E nmap-fortify-2.E

This change created two new compiler warnings, which I'll check out:

nbase_rnd.c: In function ‘nrand_init’:
nbase_rnd.c:195: warning: ignoring return value of ‘read’, declared with attribute warn_unused_result

tcpip.cc: In function ‘sys_route* getsysroutes_proc(FILE*, int*)’:
tcpip.cc:3022: warning: ignoring return value of ‘char* fgets(char*, int, FILE*)’, declared with attribute 
warn_unused_result

David Fifield

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