Messages in this thread | | | Date | Thu, 14 Oct 2021 19:51:11 -0700 | From | "Andy Lutomirski" <> | Subject | Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C |
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On Wed, Oct 13, 2021, at 11:16 AM, Sami Tolvanen wrote: > The kernel has several assembly functions, which are not directly > callable from C but need to be referred to from C code. This change adds > the DECLARE_NOT_CALLED_FROM_C macro, which allows us to declare these > symbols using an opaque type, which makes misuse harder, and avoids the > need to annotate references to the functions for Clang's Control-Flow > Integrity (CFI). > > Suggested-by: Andy Lutomirski <luto@amacapital.net> > Suggested-by: Steven Rostedt <rostedt@goodmis.org> > Signed-off-by: Sami Tolvanen <samitolvanen@google.com> > Tested-by: Nick Desaulniers <ndesaulniers@google.com> > Tested-by: Sedat Dilek <sedat.dilek@gmail.com> > --- > include/linux/linkage.h | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/include/linux/linkage.h b/include/linux/linkage.h > index dbf8506decca..f982d5f550ac 100644 > --- a/include/linux/linkage.h > +++ b/include/linux/linkage.h > @@ -48,6 +48,19 @@ > #define __PAGE_ALIGNED_DATA .section ".data..page_aligned", "aw" > #define __PAGE_ALIGNED_BSS .section ".bss..page_aligned", "aw" > > +/* > + * 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.)
So I think we should use incomplete structs, which can’t be dereferenced and will therefore be less error prone.
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