alloca avoidance patches

[Joseph Myers]

On Mon, 19 Jun 2017, Szabolcs Nagy wrote:

> in any case i think it's more productive to
> fix the stack usage bugs, instead of hardening
> for this class of exploitable stack usage bugs,
> even if the guard page catches the issue it
> is an unwanted crash.

Which gets back to wanting to use appropriate warning options, even if 
they don't catch all cases - and to needing an ABI-defined size it's safe 
to allocate, possibly more than a page if you rely on kernel fixes.

(I expect strtold has one of the larger static stack allocations in glibc.  
I can see such amounts, possibly more, being needed for fixing cpow{,f,l} 
inaccuracy as well, on the assumption we should avoid libm functions 
calling malloc.)

-- 
Joseph S. Myers
joseph@codesourcery.com