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

[Andy Lutomirski]

On Fri, Oct 15, 2021, at 9:47 AM, Sami Tolvanen wrote:
> 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.

If you were to call (with a regular C function call using ()) clear_page_erms, what happens?  IMO it should either work or fail to compile. Crashing is no good.

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

Yes, as they are not C functions.

> Sami