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

Sami Tolvanen
2022-05-16T16:23:42Z

On Mon, May 16, 2022 at 5:28 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> 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.

Correct. The generated code looks like this:

00000000000003f7 <__cfi_blowfish_dec_blk>:
3f7: cc int3
3f8: cc int3
3f9: 8b 04 25 00 00 00 00 mov 0x0,%eax
3fc: R_X86_64_32S __kcfi_typeid_blowfish_dec_blk
400: cc int3
401: cc int3

0000000000000402 <blowfish_dec_blk>:

And the symbol table in the file that takes the address has this:

45: ffffffffef478db5 0 NOTYPE WEAK DEFAULT ABS
__kcfi_typeid_blowfish_dec_blk

> 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?

It will fail to link in that case.

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

The compiler emits these only for address-taken declarations.

> 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?

Not at the moment. I suppose we could warn about mismatches in the
linker though.

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

Sure, I'll add examples to the commit message.

Sami