Re: [PATCH v5] rust: support for shadow call stack sanitizer

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Aug 20, 2024 at 4:35 PM Will Deacon <will@kernel.org> wrote:
>
> On Tue, Aug 06, 2024 at 10:01:44AM +0000, Alice Ryhl wrote:
> > This patch adds all of the flags that are needed to support the shadow
> > call stack (SCS) sanitizer with Rust, and updates Kconfig to allow
> > configurations that work.
>
> Minor nit, but some folks have allergic reactions to "This patch".
> See:
>
> https://docs.kernel.org/process/submitting-patches.html#describe-your-changes
>
> I think the commit message is much better now, though, so thank you for
> adding so much more detail for v5. If you end up respinning anyway, you
> could move this all to the imperative.

Ah, yeah, I keep forgetting about this. I'll change it to imperative
if I send another version.

> >  Makefile            | 1 +
> >  arch/arm64/Makefile | 3 +++
> >  init/Kconfig        | 2 +-
> >  3 files changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 44c02a6f60a1..eb01a26d8354 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -927,6 +927,7 @@ ifdef CONFIG_SHADOW_CALL_STACK
> >  ifndef CONFIG_DYNAMIC_SCS
> >  CC_FLAGS_SCS := -fsanitize=shadow-call-stack
> >  KBUILD_CFLAGS        += $(CC_FLAGS_SCS)
> > +KBUILD_RUSTFLAGS += -Zsanitizer=shadow-call-stack
> >  endif
> >  export CC_FLAGS_SCS
> >  endif
> > diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> > index f6bc3da1ef11..b058c4803efb 100644
> > --- a/arch/arm64/Makefile
> > +++ b/arch/arm64/Makefile
> > @@ -57,9 +57,11 @@ KBUILD_AFLAGS      += $(call cc-option,-mabi=lp64)
> >  ifneq ($(CONFIG_UNWIND_TABLES),y)
> >  KBUILD_CFLAGS        += -fno-asynchronous-unwind-tables -fno-unwind-tables
> >  KBUILD_AFLAGS        += -fno-asynchronous-unwind-tables -fno-unwind-tables
> > +KBUILD_RUSTFLAGS += -Cforce-unwind-tables=n
> >  else
> >  KBUILD_CFLAGS        += -fasynchronous-unwind-tables
> >  KBUILD_AFLAGS        += -fasynchronous-unwind-tables
> > +KBUILD_RUSTFLAGS += -Cforce-unwind-tables=y -Zuse-sync-unwind=n
> >  endif
> >
> >  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
> > @@ -114,6 +116,7 @@ endif
> >
> >  ifeq ($(CONFIG_SHADOW_CALL_STACK), y)
> >  KBUILD_CFLAGS        += -ffixed-x18
> > +KBUILD_RUSTFLAGS += -Zfixed-x18
> >  endif
> >
> >  ifeq ($(CONFIG_CPU_BIG_ENDIAN), y)
> > diff --git a/init/Kconfig b/init/Kconfig
> > index fe76c5d0a72e..d857f6f90885 100644
> > --- a/init/Kconfig
> > +++ b/init/Kconfig
> > @@ -1909,7 +1909,7 @@ config RUST
> >       depends on !MODVERSIONS
> >       depends on !GCC_PLUGINS
> >       depends on !RANDSTRUCT
> > -     depends on !SHADOW_CALL_STACK
> > +     depends on !SHADOW_CALL_STACK || RUSTC_VERSION >= 108000 && UNWIND_PATCH_PAC_INTO_SCS
>
> Sorry, I didn't spot this in v4, but since UNWIND_PATCH_PAC_INTO_SCS is
> specific to arm64 and the only other architecture selecting
> ARCH_SUPPORTS_SHADOW_CALL_STACK is riscv, I can't help but feel it would
> be cleaner to move this logic into the arch code selecting HAVE_RUST.
>
> That is, it's up to the architecture to make sure that it has whatever
> it needs for SCS to work with Rust if it claims to support Rust.
>
> What do you think?

The `select RUST if ...` is going to get really complicated if we
apply that rule in general. Having options here allows us to split
them across several `depends on` clauses. I'm not sure it will even
work, I had issues with cyclic Kconfig errors previously. I also don't
think it's unreasonable for the architecture to say it supports both
options when it really does support both; they are just mutually
exclusive. I also think there is value in having all of the options
that Rust doesn't work with in one place.

So I'd like to keep it as-is.

Alice