The last time I checked there was no such check in the unlink macro (no matter if debug mode or not).
Ah, ok, I see what you meant. The check I was referring to wasn't in the unlink macro, but in one of dlmalloc's debugging routines. If you move it into unlink itself, then it does indeed prevent all unlink exploits, as you say. I agree that a combination of the two techniques would theoretically be stronger than each on its own, but I also believe that using properly randomized magic numbers in practice guarantees that chunk headers cannot be tampered with. However, you do get a lot for this simple check, so it makes sense to include it.
Thanks for pointing that out.
Stefan Esser
-- William Robertson Reliable Software Group, UC Santa Barbara http://www.cs.ucsb.edu/~wkr/
