Messages in this thread | | | Date | Fri, 15 Oct 2021 09:22:27 -0700 | From | "Andy Lutomirski" <> | Subject | Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C |
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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?).
I think #2 is far better. I complained about this quite a few versions ago, and, sorry, the word “canonical” is pretty much a non-starter. There needs to be a way to annotate a function pointer type and an extern function declaration that says “the callee follows the ABI *without CFI*” and the compiler needs to do the right thing. And whatever attribute or keyword gets used needs to give the reader at least some chance of understanding.
(If there is a technical reason why function *pointers* of this type can’t be called, perhaps involving IBT, so be it. But the type system should really be aware of C-ABI functions that come from outside the CFI world.)
It looks like clear_page might be improved by using static_call some day, and then proper typing will be a requirement.
Would it help if I file a clang bug about this?
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