Re: [RFC PATCH v2 07/21] cfi: Add type helper macros

Rasmus Villemoes
2022-05-16T12:28:26Z

On 14/05/2022 23.49, 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 <samitolvanen@google.com>
>
> 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?)
>
> What happens if an asm function is called indirectly and it doesn't have
> this annotation?

Presumably that's a fail.

I'm also interested in how this works at the asm/linker level. I assume
that the .o file generated from the asm input has
__kcfi_typeid_<function> as an undefined symbol; the compiler emits that
symbol as an absolute one upon taking the address of <function>, and the
linker then has the info it needs to patch things up.

But what then happens if we have some function implemented in assembly
which for whatever .config reason never has its address taken in any .c
translation unit that gets linked in? Does the __kcfi_typeid_<function>
symbol silently resolve to 0, or does the link fail?

I can't really imagine the compiler emitting __kcfi_typeid_<function>
symbols for each and every function it sees merely declared in some header.

Two different .c files both taking the address of <function> should of
course emit the same value for __kcfi_typeid_<function>. Is there any
sanity check anywhere that that's actually the case?

Can we please have some objdump/readelf output from some .o files
involved here?

Rasmus