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Date:   Mon, 16 May 2022 09:04:44 -0700
From:   Sami Tolvanen <>
To:     Kees Cook <>
Cc:, Josh Poimboeuf <>,
        Peter Zijlstra <>,,
        Catalin Marinas <>,
        Will Deacon <>,
        Mark Rutland <>,
        Nathan Chancellor <>,
        Nick Desaulniers <>,
        Joao Moreira <>,
        Sedat Dilek <>,
        Steven Rostedt <>,,,
Subject: Re: [RFC PATCH v2 07/21] cfi: Add type helper macros

On Sat, May 14, 2022 at 2:49 PM Kees Cook <> wrote:
> On Fri, May 13, 2022 at 01:21:45PM -0700, Sami Tolvanen wrote:
> > With CONFIG_CFI_CLANG, assembly functions called indirectly
> > from C code must be annotated with type identifiers to pass CFI
> > checking. The compiler emits a __kcfi_typeid_<function> symbol for
> > each address-taken function declaration in C, which contains the
> > expected type identifier. Add typed versions of SYM_FUNC_START and
> > SYM_FUNC_START_ALIAS, which emit the type identifier before the
> > function.
> >
> > Signed-off-by: Sami Tolvanen <>
> And the reason to not make this change universally (i.e. directly in
> SYM_FUNC_START) is to minimize how many of these symbol annotations get
> emitted? (And to more directly indicate which asm is called indirectly?)

The reason not to add this to SYM_FUNC_START is that the compiler
doesn't emit the type symbols for all functions. It currently emits
them for all address-taken function declarations in each translation
unit. We could potentially further limit this by emitting them only
for function declarations with a specific attribute, for example, but
that's something we can optimize later.

> What happens if an asm function is called indirectly and it doesn't have
> this annotation?

It will fail the CFI check.

> (Is this case detectable at compile-time?)

It's not. I'll update the commit message in the next version to
clarify these points.


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