Re: randomize_kstack: To init or not to init?

[Marco Elver]

On Thu, 9 Dec 2021 at 11:27, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote:
[...]
> > There are several options:
> >
> >       A. Make memset (and probably all other mem-transfer functions)
> >          noinstr compatible, if that is even possible. This only solves
> >          problem #2.
>
> While we can shut up objtool real easy, the bigger problem is that
> noinstr also excludes things like kprobes and breakpoints and other such
> goodness from being placed in the text.
>
> >       B. A workaround could be using a VLA with
> >          __attribute__((uninitialized)), but requires some restructuring
> >          to make sure the VLA remains in scope and other trickery to
> >          convince the compiler to not give up that stack space.
> >
> >       C. Introduce a new __builtin_alloca_uninitialized().
> >
> > I think #C would be the most robust solution, but means this would
> > remain as-is for a while.
> >
> > Preferences?
>
> I'm with you on C.

Seems simple enough, so I've sent https://reviews.llvm.org/D115440 --
either way, there needs to be support to not initialize alloca'd
memory. Let's see where we end up.