MAP_STACK is a no-op on linux

[lolbinarycat]

https://www.man7.org/linux/man-pages/man2/mmap.2.html

on linux, memmap2 should use MAP_GROWSDOWN, or possibly MAP_GROWSDOWN | MAP_STACK, for future proofing (glibc uses MAP_STACK despite it doing nothing).

[adamreichold]

If we do decide to set GROWSDOWN, we need to ensure the rest of the code is able to handle the automatic growth and adjusted return value properly. I think this is quite the project compared to just setting the flag.

[lolbinarycat]

MAP_GROWSDOWN will not cause the mapping region to grow beyond the requested bounds. it will cause it to start at one page, and then whenever the next "guard page" is touched, it grows by a page, up to the size initially requested. it is equivalent to MAP_STACK on bsd systems.

[adamreichold]

Isn't that behaviour also tied to stack probing, i.e. it does not happen if the fault lies beyond the guard page?