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

On Fri, Oct 15, 2021 at 9:22 AM Andy Lutomirski [off-list ref] wrote:

quoted

On Fri, Oct 15, 2021, at 8:55 AM, Thomas Gleixner wrote:

quoted

On Thu, Oct 14 2021 at 19:51, Andy Lutomirski wrote:

quoted

On Wed, Oct 13, 2021, at 11:16 AM, Sami Tolvanen wrote:

quoted

+/*
+ * 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.

quoted

quoted

+#ifndef DECLARE_NOT_CALLED_FROM_C
+#define DECLARE_NOT_CALLED_FROM_C(sym) \
+   extern const u8 sym[]
+#endif

quoted

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.

quoted

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