Date: Tue, 4 Jun 2024 16:53:16 +0200
From: Alice Ryhl <aliceryhl@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: Jamie.Cunliffe@....com, a.hindborg@...sung.com, alex.gaynor@...il.com, 
	ardb@...nel.org, benno.lossin@...ton.me, bjorn3_gh@...tonmail.com, 
	boqun.feng@...il.com, broonie@...nel.org, catalin.marinas@....com, 
	gary@...yguo.net, keescook@...omium.org, kernel@...entinobst.de, 
	linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, mark.rutland@....com, masahiroy@...nel.org, 
	maz@...nel.org, miguel.ojeda.sandonis@...il.com, nathan@...nel.org, 
	ndesaulniers@...gle.com, nicolas@...sle.eu, ojeda@...nel.org, 
	rust-for-linux@...r.kernel.org, samitolvanen@...gle.com, wedsonaf@...il.com
Subject: Re: [PATCH v2] rust: add flags for shadow call stack sanitizer

On Tue, Jun 4, 2024 at 4:29 PM Will Deacon <will@...nel.org> wrote:
>
> On Tue, Apr 30, 2024 at 11:09:25AM +0000, Alice Ryhl wrote:
> > Will Deacon <will@...nel.org> wrote:
> > > On Tue, Mar 05, 2024 at 01:14:19PM +0100, Miguel Ojeda wrote:
> > >> Otherwise partially reverting to the `target.json` approach sounds good too.
> > >>
> > >> I added the `-Zuse-sync-unwind=n` to the list at
> > >> https://github.com/Rust-for-Linux/linux/issues/2. Given the default is
> > >> what we want, I have put it in the "Good to have" section.
> > >
> > > I think we have time to do this properly, like we did for the clang
> > > enablement a few years ago. In hindsight, avoiding hacks for the early
> > > toolchains back then was a really good idea because it meant we could
> > > rely on a solid baseline set of compiler features from the start.
> > >
> > > So, please can we fix this in rustc and just have SCS dependent on that?
> >
> > Just to keep you in the loop, I've posted a PR to make rustc recognize
> > the reserve-x18 target feature, so that the -Ctarget-feature=+reserve-x18
> > flag stops emitting a warning.
> >
> > This should be sufficient for adding support for CONFIG_DYNAMIC_SCS.
> >
> > You can find it here:
> > https://github.com/rust-lang/rust/pull/124323
> >
> > As for non-dynamic SCS, I plan to tackle that after the PR is merged.
> > See the "Future possibilities" section in the linked PR for more info on
> > that.
>
> Thanks for persevering with this, Alice. I read the pull request above,
> but it looks like you went with:
>
> https://github.com/rust-lang/rust/pull/124655
>
> instead, which was merged (hurrah!). Do we need anything else?

Yeah, it took a while, but I've managed to get a -Zfixed-x18 flag in.
It will be available starting with Rust 1.80, which will be released
on the 25th of July.

A few things:

1. The -Zsanitizer=shadow-call-stack flag still doesn't work because
the compiler thinks that the target doesn't support it. I'll fix this
eventually, but at least CONFIG_DYNAMIC_SCS works now.

2. I haven't convinced the Rust maintainers that -Zfixed-x18 is the
way to go long term (flags starting with -Z are unstable and may
change). Some of the maintainers want to instead add a x18-available
target feature (that is, the inverse of the current reserve-x18 target
feature), that you can disable with -Ctarget-feature=-x18-available.

And a few questions for you:

By the time support for 1.80 goes in, we are probably supporting more
than one Rust compiler. For pre-1.80 compilers, should we fall back to
-Ctarget-feature=+reserve-x18 (which emits a warning, but works), or
fail compilation?

Similarly, we should probably submit a fix to the stable branches so
that SCS+Rust doesn't silently break in a hard-to-debug way. Do you
prefer a backport with -Ctarget-feature=+reserve-x18 or one that fails
compilation?

Alice

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