4 ms·

If I recall, AIX is one system which always returns 0 if NULL is dereferenced - the word at address 0 is guaranteed to a) be addressable, and b) contain a 0. Th

If I recall, AIX is one system which always returns 0 if NULL is dereferenced - the word at address 0 is guaranteed to a) be addressable, and b) contain a 0. These days I guess it's a security vulnerability, but the original intention was to allow wizards to optimize away the need for certain null checks. IBM actually has a number of patents related to techniques such as array boundary checking under this system.

jacquesm 17y ago
I'm all for making the 0 page non-existent and to make it a processor feature rather than an os feature that the 0th address being referenced in any way will generate a hardware exception no matter what. Otherwise we'll be playing null pointer dereference cat&mouse for ever.
- tptacek 17y ago
  It doesn't matter what you do with the zero page if the address is getting offset with an attacker-controlled caddr_t. I'm skeptical about NULL pointer countermeasures.
  - jacquesm 17y ago
    That's true. That would defeat such a check because the 0th location would never be hit.