Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C

[Sami Tolvanen]

On Fri, Oct 15, 2021 at 9:22 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
>
>
> On Fri, Oct 15, 2021, at 8:55 AM, Thomas Gleixner wrote:
> > On Thu, Oct 14 2021 at 19:51, Andy Lutomirski wrote:
> >> On Wed, Oct 13, 2021, at 11:16 AM, Sami Tolvanen wrote:
> >>>
> >>> +/*
> >>> + * Declares a function not callable from C using an opaque type. Defined as
> >>> + * an array to allow the address of the symbol to be taken without '&'.
> >>> + */
> >> I’m not convinced that taking the address without using & is a
> >> laudable goal.  The magical arrays-are-pointers-too behavior of C is a
> >> mistake, not a delightful simplification.
> >
> >>> +#ifndef DECLARE_NOT_CALLED_FROM_C
> >>> +#define DECLARE_NOT_CALLED_FROM_C(sym) \
> >>> +   extern const u8 sym[]
> >>> +#endif
> >>
> >
> >> The relevant property of these symbols isn’t that they’re not called
> >> from C.  The relevant thing is that they are just and not objects of a
> >> type that the programmer cares to tell the compiler about. (Or that
> >> the compiler understands, for that matter. On a system with XO memory
> >> or if they’re in a funny section, dereferencing them may fail.)
> >
> > I agree.
> >
> >> So I think we should use incomplete structs, which can’t be
> >> dereferenced and will therefore be less error prone.
> >
> > While being late to that bike shed painting party, I really have to ask
> > the question _why_ can't the compiler provide an annotation for these
> > kind of things which:
> >
> >     1) Make the build fail when invoked directly
> >
> >     2) Tell CFI that this is _NOT_ something it can understand
> >
> > -void clear_page_erms(void *page);
> > +void __bikeshedme clear_page_erms(void *page);
> >
> > That still tells me:
> >
> >     1) This is a function
> >
> >     2) It has a regular argument which is expected to be in RDI
> >
> > which even allows to do analyis of e.g. the alternative call which
> > invokes that function.
> >
> > DECLARE_NOT_CALLED_FROM_C(clear_page_erms);
> >
> > loses these properties and IMO it's a tasteless hack.
> >
>
>
> Ah, but clear_page_erms is a different beast entirely as compared to, say, the syscall entry. It *is* a C function.  So I see two ways to handle it:
>
> 1. Make it completely opaque.  Tglx doesn’t like it, and I agree, but it would *work*.
>
> 2. Make it a correctly typed function. In clang CFI land, this may or may not be “canonical” (or non canonical?).

Technically speaking the clear_page_* declarations don't need to be
changed for CFI, they do work fine as is, but I included them in the
patch as they're not actually called from C code right now. But you're
right, we should use a proper function declarations for these. I'll
drop the changes to this file in the next version.

I wouldn't mind having a consensus on how to deal with exception
handlers etc. though. Should I still use opaque types for those?

Sami