Re: [PATCH] cfi: rust: pass -Zpatchable-function-entry on all architectures

[Sami Tolvanen <samitolvanen@google.com>]

On Wed, Oct 9, 2024 at 8:15 PM Alice Ryhl <aliceryhl@google.com> wrote:
>
> On Wed, Oct 9, 2024 at 7:43 PM Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > There's an existing incompatibility between CFI and pre-function NOPs
> > for C code, because we override -fpatchable-function-entry on a
> > per-function basis (e.g. for noinstr and notrace), and we don't
> > currently have a mechanism to ensure the CFI tag is in the same place
> > regardless. This is why arm64 has CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > depend on !CFI.
> >
> > For C code at least, just using regular -fpatchable-function-entry=M or
> > -fpatchable-function-entry=M,0 shouldn't change the location of the CFI
> > tag relative to the function entrypoint, and so should have no adverse
> > effect on CFI.
> >
> > Is Rust any different here?
>
> Ah, no it shouldn't be. Sami can you confirm?

KCFI is implemented in the LLVM back-end, so the behavior is exactly
the same for both C and Rust.

> > As above, I suspect this isn't necessary to make CFI work, for any case
> > that works with C today, due to -fpatchable-funtion-entry being
> > overridden on a per-function basis. Are you seeing a problem in
> > practice, or was this found by inspection?
> >
[..]
> Well, I was told that it's a problem and was able to trigger a failure
> on x86. I didn't manage to trigger one on arm64, but I wasn't sure
> whether that was me doing something wrong, or whether the problem only
> exists on x86. We already have the flag on x86 for FINEIBT, but I
> thought on the off chance that it's not a problem in practice on arm,
> it still doesn't hurt to add the flag.

This only impacts KCFI on x86 at the moment. However, we should
nevertheless pass the same patchable-function-entry flags to both
compilers on other architectures too.

> Regarding the AAPCS64 calling convention thing ... rustc uses the Rust
> calling convention for functions internally in Rust code and I don't
> know whether that changes anything relevant for what you mention.
> Matthew/Sami do you know?

AFAIK this shouldn't be a problem, but Matt knows this much better, so
I'll let him explain.

Sami