On Thu, Feb 1, 2024 at 9:00 PM Theo de Raadt deraadt@openbsd.org wrote:
Jeff Xu jeffxu@chromium.org wrote:
Even without free. I personally do not like the heap getting sealed like that.
Component A. p=malloc(4096); writing something to p.
Component B: mprotect(p,4096, RO) mseal(p,4096)
This will split the heap VMA, and prevent the heap from shrinking, if this is in a frequent code path, then it might hurt the process's memory usage.
The existing code is more likely to use malloc than mmap(), so it is easier for dev to seal a piece of data belonging to another component. I hope this pattern is not wide-spreading.
The ideal way will be just changing the library A to use mmap.
I think you are lacking some test programs to see how it actually behaves; the effect is worse than you think, and the impact is immediately visible to the programmer, and the lesson is clear:
you can only seal objects which you gaurantee never get recycled. Pushing a sealed object back into reuse is a disasterous bug. Noone should call this interface, unless they understand that.I'll say again, you don't have a test program for various allocators to understand how it behaves. The failure modes described in your docuemnts are not correct.
I understand what you mean: I will add that part to the document: Try to recycle a sealed memory is disastrous, e.g. p=malloc(4096); mprotect(p,4096,RO) mseal(p,4096) free(p);
My point is: I think sealing an object from the heap is a bad pattern in general, even dev doesn't free it. That was one of the reasons for the sealable flag, I hope saying this doesn't be perceived as looking for excuses.