Re: [PATCH] x86: Enable IBT in Rust if enabled in C

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Oct 10, 2023 at 07:34:45AM -0700, Matthew Maurer wrote:
> On Tue, Oct 10, 2023 at 7:24 AM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Oct 10, 2023 at 07:06:32AM -0700, Matthew Maurer wrote:
> >
> > > > > +KBUILD_RUSTFLAGS += -Zcf-protection=branch -Zno-jump-tables
> > > >
> > > > One question, -Zcf-protection=branch, will that ever emit NOTRACK
> > > > prefix? The kernel very explicitly does not support (enable) NOTRACK.
> >
> > > rustc does this via LLVM, so its code generation works very similarly to clang.
> > > It does not create its own explicit NOTRACKs, but LLVM will by default
> > > with just -Zcf-protection-branch.
> > > I've linked a godbolt showing that at least for the basic case, your
> > > no-jump-tables approach from clang ports over.
> > > https://godbolt.org/z/bc4n6sq5q
> > > Whether rust generates NOTRACK should end up being roughly equivalent
> > > to whether clang generates it, and if LLVM gains a code generation
> > > flag for NOTRACK being disallowed some day, we can pass that through
> > > as well.
> >
> > IIRC C++ will also emit NOTRACK for things like catch/throw and other
> > stack/scope unwinds. Obviously C doesn't have that, but does Rust? (as
> > might be obvious, I *really* don't know the language).
> >
> That's fine - Rust does have stack/scope unwinds with the
> `panic=unwind` strategy. In the kernel, we use `panic=abort` and are
> unlikely to ever change this approach. There are a host of other
> complications that come from unwinding without NOTRACK getting
> involved :)
> 
> In case you find `catch_unwind` - this function only has an effect
> with `panic=unwind`. When `panic=abort`, there's nothing analogous to
> catch/throw anymore, and `catch_unwind` becomes a no-op.
> 
> Are there other features you expect might trigger NOTRACK?

I'm not sure -- if they happen, objtool should warn about them. So I
suppose we'll take it from there.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>