VM_GROWSDOWN

I'm not sure there are that many applications that rely on "unlimited stack" meaning "allow to fill the entire address-space", but that's why I would not change the default behavior and pick a new elf flag instead (and for anyone needing ridiculously large stacks, split stacks are IMHO the better long-term answer, see http://gcc.gnu.org/wiki/SplitStacks).

There is certainly the practical question of what it means to run a process with stacksize == RLIMIT_INFINITY when the stack vma is supposed to be fully expanded -- I'd say pick some random really large value like 512M, just enough to get sysvinit/upstart/systemd/whatever running, demand that sane limits be set afterwards and have admins suffer really if they do not.

In any case, apparently nothing breaks with my distribution's default 8MB stack rlimit, so I would expect that gradually converting the whole system over to use pre-allocated stack VMAs would not hit too many obstacles.