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Re: gcc 4.1 bug miscompiles pointer range checks, may place you at risk
From: "Forrest J. Cavalier III" <mibsoft () mibsoftware com>
Date: Mon, 17 Apr 2006 18:26:46 -0400

Felix von Leitner wrote:
I wrote a small library of functions to do typical range checks as they
are needed in code that handles incoming packets or messages from
untrusted sources.  My impetus was SMB code, in case you want to know.

Here is one of my functions:

static inline int range_ptrinbuf(const void* buf,unsigned long len,const void* ptr) {
  register const char* c=(const char*)buf;      /* no pointer arithmetic on void* */
  return (c && c+len>c && (const char*)ptr-c<len);
}

Of course, when developing security critical code like this, you also
write a good test suite for it, that exercises all the cases.  Here is
part of my test suite:

  assert(range_ptrinbuf(buf,(unsigned long)-1,buf+1)==0);


Overflow tests are hard to get right in a platform-independent way.

What if your sizeof(ptrdiff_t) != sizeof(unsigned long)?

And what do think about this:

   http://c0x.coding-guidelines.com/6.5.6.html#1160

?  Can we be sure ptr-c is defined?  Even when ptr < buf?  Even when
ptr > c + len + 1?






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