* Daniel Micay: >> It makes a lot of sense on x86_64 where it means the canary is >> still 56 bits. Also, you want -fstack-check for protecting again >> stack overflows rather than stack *buffer* overflow. SSP won't >> really help you in that regard. Sadly, while -fstack-check now >> works well in GCC 6 with little performance cost, it's not really a
I think GCC still does not treat the return address push on architectures which have such a CALL instruction as an implicit stack probe. >> complete feature (and Clang impls it as a no-op!). How many guard pages at the end of the stack does the kernel guarantee? I saw some -fstack-check-generated code which seemed to jump over a single guard page. The other thing I've seen which could impact the effectiveness of -fstack-check: mmap *without* MAP_FIXED and a hint within stack allocation can create a mapping inside the stack. That's rather surprising, and I'm not sure if the net result is that there actually is a guard page in all cases. > Note: talking about userspace after the entropy bit. The kernel doesn't > really -fstack-check, at least in even slightly sane code... There used to be lots of discussions about kernel stack sizes ...
