Re: randomize_kstack: To init or not to init?

Marco Elver
2021-12-09T12:58:48Z

On Thu, 9 Dec 2021 at 11:27, Peter Zijlstra <peterz@infradead.org> 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.