On 03/07/2013 08:12 PM, Mcdaniel, Daryl wrote:
> ...
> Another interesting bit of trivia about null pointers from the
> ISO/IEC 9899:199409 C language specification (C95, 1995):
> "A null pointer is guaranteed to compare unequal to a pointer to any
> object or function."
>
> "The C Programming Language", 1978, by Kernighan and Ritchie states:
> "C guarantees that no pointer that validly points at data will
> contain zero, so a return value of zero can be used to signal an
> abnormal event."
>
> This is only a problem if you attempt to create a pointer using
> constants, such as (char *)0x00000000. By definition, this is a NULL
> pointer and cannot point at data. But, any pointer that becomes zero
> (NULL) during program operation can be dereferenced. "Safe" systems
> sometimes put a trapping instruction at 0x00000000 so that if someone
> dereferences a NULL function pointer it will be trapped before things
> disintegrate. If the trapping instruction is read as data, it is a
> known value that if seen during debug is an indication that you might
> be dereferencing a null pointer.
We've implemented a similar "safety" check in our BIOS by modifying the
segment descriptors (in 32-bit mode) and the 1:1 page tables (in 64-bit
mode) to prohibit access to addresses 0-4095. That caught some bugs and
unforseen end cases in our code.
Unfortunately this check interfered with the CSM, so we were not able to
use it in production. But it might be a handy option (under a PCD
feature flag, of course) to have available in EDK2.
--
Brian J. Johnson
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