Re: [RFC PATCH 00/21] KCFI support

Peter Zijlstra
2022-05-04T07:35:32Z

On Tue, May 03, 2022 at 03:35:34PM -0700, Peter Collingbourne wrote:
> On Mon, May 2, 2022 at 1:02 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Mon, May 02, 2022 at 08:22:57AM -0700, Sami Tolvanen wrote:
> >
> > > > Anyway, I think I hate that __builtin, I'd *much* rather see a variable
> > > > attribute or qualifier for this, such that one can mark a function
> > > > pointer as not doing CFI.
> > > >
> > > > I simply doesn't make sense to have a builtin that operates on an
> > > > expression. The whole thing is about indirect calls, IOW function
> > > > pointers.
> > >
> > > I also thought an attribute would be more convenient, but the compiler
> > > folks prefer a built-in:
> > >
> > > https://reviews.llvm.org/D122673
> >
> > That seems to mostly worry about C++ things (overload sets, template
> > specialization, name mangling) we kernel folks don't seem to much care
> > about.
> >
> > I'll stick with saying type system makes more sense to me though.
>
> I'd say it's not only the C++ issues but more the "action at a
> distance" that's implied by having this be part of the type system.
> With this being in the function type it's hard to tell whether any
> particular call will have CFI disabled, without needing to go and look
> at how the function pointer is defined.

Look at how we use volatile:

*(volatile int *)(&foo)

we don't use volatile on actual variable definitions (much), but instead
cast it in at the usage site. Same can be done with this if so desired.

> On the other hand, if we
> explicitly mark up the calls with CFI disabled, the code becomes
> easier to audit (think Rust "unsafe" blocks).

I don't know any Rust. To me Rust still looks like line noise.

> Does it seem any better to you to have this be marked up via the
> function expression, rather than the call? The idea is that this would
> always compile to a check-free function call, no matter what "func"
> is:
>
> __builtin_kcfi_call_unchecked(func)(args)
>
> We already have this, to some degree, with KCFI as implemented: CFI
> checks are disabled if the function expression refers to a declared
> function. The builtin would allow overriding the decision to also
> disable CFI checks for function expressions that use the builtin. It
> also wouldn't preclude a type based system later on (the builtin would
> become effectively a cast to the "unchecked" type).

That's still a bit naf; you've effectively made that builtin a type-cast.