elfmalloc: Detect or eliminate frees from other allocators

[joshlf]

When dynamically loading on Mac, the loader seems to free a pointer to us that was not allocated by us (presumably it was allocated by malloc before our symbols were loaded). As I see it, we have two options:

• Figure out how to load on Mac so that this doesn't happen
• Figure out how to detect whether a pointer is ours; if it isn't, just leak it (or maybe free it to libc free?)

@davidtgoldblatt Have you folks run into this problem with jemalloc?

[davidtgoldblatt]

I don't remember all the details, but related is the OS X "malloc zone" API (jemalloc stuff is in src/zone.c).

I think Firefox may need to deal with this as well -- I remember that they have some wonkiness in dealing with the mixture of allocators.

[davidtgoldblatt]

My memory is that this is fundamental on OS X; there will be a mixture of pointers allocated by your malloc and the libc malloc no matter what, so that if you want to replace libc malloc you need to be able to handle this issue. I'm less confident about that one though.

[joshlf]

Ah that's very helpful, thanks! I'll take a look at the malloc zone API and see what we can do about it.

EDIT: Useful links for posterity:

• jemalloc's src/zone.c
• Apple's Tips for Allocating Memory (see "Using Malloc Memory Zones" section)