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

[Will Deacon <will@kernel.org>]

On Fri, Aug 23, 2024 at 03:09:40PM +0200, Alice Ryhl wrote:
> On Fri, Aug 23, 2024 at 2:57 PM Will Deacon <will@kernel.org> wrote:
> >
> > On Fri, Aug 23, 2024 at 02:38:20PM +0200, Alice Ryhl wrote:
> > > On Fri, Aug 23, 2024 at 2:24 PM Will Deacon <will@kernel.org> wrote:
> > > >
> > > > On Tue, Aug 20, 2024 at 05:13:58PM +0200, Alice Ryhl wrote:
> > > > > 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:
> > > > > > > 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.
> > > >
> > > > I'm not sure I follow why this will get really complicated. Isn't it as
> > > > straightforward as the diff below, or did I miss something?
> > >
> > > Hmm. I tried this but I wasn't able to enable Rust with this setup.
> > > Even though the deps of RUSTC_SUPPORTS_ARM64 are ok, it doesn't seem
> > > to be enabled and I can't find it in menuconfig. I think we need to
> > > have a `select RUSTC_SUPPORTS_ARM64` somewhere.
> >
> > Sorry, yes, my diff was a little half-arsed:
> >
> > > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > > index a2f8ff354ca6..2f5702cb9dac 100644
> > > > --- a/arch/arm64/Kconfig
> > > > +++ b/arch/arm64/Kconfig
> > > > @@ -231,7 +231,7 @@ config ARM64
> > > >         select HAVE_FUNCTION_ARG_ACCESS_API
> > > >         select MMU_GATHER_RCU_TABLE_FREE
> > > >         select HAVE_RSEQ
> > > > -       select HAVE_RUST if CPU_LITTLE_ENDIAN
> > > > +       select HAVE_RUST if RUSTC_SUPPORTS_ARM64
> > > >         select HAVE_STACKPROTECTOR
> > > >         select HAVE_SYSCALL_TRACEPOINTS
> > > >         select HAVE_KPROBES
> > > > @@ -265,6 +265,11 @@ config ARM64
> > > >         help
> > > >           ARM 64-bit (AArch64) Linux support.
> > > >
> > > > +config RUSTC_SUPPORTS_ARM64
> > > > +       bool
> >
> > This line ^^^ should be 'def_bool y'.
> 
> Ah, I see, I guess I learned something today. It also seems to work if
> I add `default y`.
> 
> I can change it if you think this is better. I still think there's
> some value in having everything in one place, but it's not a big deal.
> Either way, it should be temporary for a few kernel releases as we'll
> eventually only support compiler versions where this works.

I do like moving the reference to UNWIND_PATCH_PAC_INTO_SCS into the
arch code, so if you could respin along these lines then that would be
great.

Thanks,

Will