Re: randomize_kstack: To init or not to init?

Segher Boessenkool
2021-12-09T21:04:52Z

On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote:
> Clang supports CONFIG_INIT_STACK_ALL_ZERO, which appears to be the
> default since dcb7c0b9461c2, which is why this came on my radar. And
> Clang also performs auto-init of allocas when auto-init is on
> (https://reviews.llvm.org/D60548), with no way to skip. As far as I'm
> aware, GCC 12's upcoming -ftrivial-auto-var-init= doesn't yet auto-init
> allocas.

The space allocated by alloca is not an automatic variable, so of course
it is not affected by this compiler flag. And it should not, this flag
is explicitly for *small fixed-size* stack variables (initialising
others can be much too expensive).

> C. Introduce a new __builtin_alloca_uninitialized().

That is completely backwards. That is the normal behaviour of alloca
already. Also you can get __builtin_alloca inserted by the compiler
(for a variable length array for example), and you typically do not want
those initialised either, for the same reasons.

Segher