[PATCH] Dynamic growable arrays for internal use

[Paul Eggert]

Florian Weimer wrote:
> Does anyone have comments about the interface and implementation?

It suffers from a common problem: it can allocate objects containing more than 
PTRDIFF_MAX bytes. Such objects do not work in C, because pointer subtraction 
has undefined behavior (and in practice does not work even if the correct result 
would be representable as ptrdiff_t).

At a minimum I suggest adding a ptrdiff_t overflow check to 
__check_mul_overflow_size_t so that it reports overflow if the result exceeds 
PTRDIFF_MAX.

In applications I am starting to take the lead of the GNU Emacs internals, and 
to prefer ptrdiff_t to size_t when calculating sizes. This allows for better 
automated checking of integer overflow in size calculations, since ptrdiff_t 
overflow has undefined behavior and can be caught by -fsanitize=undefined. I 
suggest doing this in this new module, and if it works well we should consider 
doing it elsewhere in glibc.

Of course this problem affects other code in glibc, but one thing at a time.