Message-ID: <CAH5fLgjjnDZG2BBApmiss43+gOs85xjJM9DP8RLO+y+UXm1cOg@mail.gmail.com>
Date: Tue, 27 Aug 2024 13:36:41 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: Catalin Marinas <catalin.marinas@....com>, Jamie Cunliffe <Jamie.Cunliffe@....com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Nathan Chancellor <nathan@...nel.org>, 
	Conor Dooley <conor@...nel.org>, Masahiro Yamada <masahiroy@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Ard Biesheuvel <ardb@...nel.org>, Marc Zyngier <maz@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, Mark Brown <broonie@...nel.org>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, 
	Valentin Obst <kernel@...entinobst.de>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	rust-for-linux@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v5] rust: support for shadow call stack sanitizer

On Fri, Aug 23, 2024 at 3:21 PM Will Deacon <will@...nel.org> wrote:
>
> 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@...nel.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@...nel.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@...nel.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.

Done, see:
https://lore.kernel.org/all/20240826-shadow-call-stack-v6-1-495a7e3eb0ef@google.com/

I took the opportunity to incorporate new developments in rustc
changes into the Kconfig rules.

Alice

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