Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C | Date | Fri, 15 Oct 2021 17:55:22 +0200 |
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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.
Thanks,
tglx
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