bugtraq - RE: gcc 4.1 bug miscompiles pointer range checks, may place you at risk

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Date: Mon, 17 Apr 2006 17:06:28 -0700
From: "Michael Wojcik" <Michael.Wojcik@...rofocus.com>
To: <bugtraq@...urityfocus.com>
Subject: RE: gcc 4.1 bug miscompiles pointer range checks, may place you at risk

> From: Felix von Leitner [mailto:felix-bugtraq@...e.de] 
> Sent: Monday, 17 April, 2006 16:04
> 
> 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);
> }
> 
> ...
>   assert(range_ptrinbuf(buf,(unsigned long)-1,buf+1)==0);
> 
> Imagine my surprise when this assertion failed.

As far as the C language is concerned, this isn't a compiler "bug".
You've created an invalid pointer in "c+len" (the result neither points
within the object that includes the location c points to, nor one past
it), which invokes Undefined Behavior (ISO 9899:1990 6.5.6 #8).  And
you've compared two pointers which do not point within the same object
("c+len>c"), which also invokes Undefined Behavior (6.5.8 #5).

The behavior of gcc 4.1 in this case might be infelicitous, but it is
not, properly speaking, a bug.

-- 
Michael Wojcik
Principal Software Systems Developer, Micro Focus