D - Unsafe and doomed

[deadalnix]

On Monday, 6 January 2014 at 00:13:19 UTC, Walter Bright wrote:
> On 1/5/2014 3:59 PM, deadalnix wrote:
>>>> because it is known to fool optimizer and cause really
>>>> nasty bugs (typically, a pointer is dereferenced, so the 
>>>> optimizer assume it
>>>> isn't null and remove null check after the dereference, and 
>>>> then the dereference
>>>> is removed as it is dead.
>>>
>>> I'd like to see a case where this is nasty. I can't think of 
>>> one.
>>>
>>
>> A recent linux kernel exploit was caused by this. Reread 
>> carefully, this nasty
>> behavior is created by the optimizer, and avoiding it mean 
>> preventing the
>> optimizer to optimize aways loads, unless it can prove the 
>> pointer is non null.
>> As D is meant to be fast, this limitation in the optimizer is 
>> highly undesirable.
>
> I'd still like to see an example, even a contrived one.

void foo(int* ptr) {
     *ptr;
     if (ptr is null) {
         // do stuff
     }

     // do stuff.
}

The code look stupid, but this is quite common after a first pass 
of optimization/inlining, do end up with something like that when 
a null check if forgotten.

The problem here is that the if can be removed, as you can't 
reach that point if the pointer is null, but *ptr can also be 
removed later as it is a dead load.

The resulting code won't crash and do random shit instead.