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From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date: Tue, 5 Mar 2024 13:14:19 +0100
Message-ID: <CANiq72mCzRBW7_H5Q4VQF8PGRFwaKJzQwOe8LOP2NbStz4husg@mail.gmail.com>
Subject: Re: [PATCH v2] rust: add flags for shadow call stack sanitizer
To: Alice Ryhl <aliceryhl@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
	Jamie Cunliffe <Jamie.Cunliffe@arm.com>, Sami Tolvanen <samitolvanen@google.com>, 
	Masahiro Yamada <masahiroy@kernel.org>, Nathan Chancellor <nathan@kernel.org>, 
	Nicolas Schier <nicolas@fjasle.eu>, Ard Biesheuvel <ardb@kernel.org>, Marc Zyngier <maz@kernel.org>, 
	Mark Rutland <mark.rutland@arm.com>, Mark Brown <broonie@kernel.org>, 
	Nick Desaulniers <ndesaulniers@google.com>, Kees Cook <keescook@chromium.org>, 
	Miguel Ojeda <ojeda@kernel.org>, Alex Gaynor <alex.gaynor@gmail.com>, 
	Wedson Almeida Filho <wedsonaf@gmail.com>, Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>, 
	=?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>, 
	Benno Lossin <benno.lossin@proton.me>, Andreas Hindborg <a.hindborg@samsung.com>, 
	Valentin Obst <kernel@valentinobst.de>, linux-kbuild@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, 
	rust-for-linux@vger.kernel.org
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On Tue, Mar 5, 2024 at 12:58=E2=80=AFPM Alice Ryhl <aliceryhl@google.com> w=
rote:
>
> Add flags to support the shadow call stack sanitizer, both in the
> dynamic and non-dynamic modes.
>
> Right now, the compiler will emit the warning "unknown feature specified
> for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
> passes it to the codegen backend, so the flag will work just fine. Once
> rustc starts recognizing the flag (or provides another way to enable the
> feature), it will stop emitting this warning. See [1] for the relevant
> issue.
>
> Currently, the compiler thinks that the aarch64-unknown-none target
> doesn't support -Zsanitizer=3Dshadow-call-stack, so the build will fail i=
f
> you enable shadow call stack in non-dynamic mode. However, I still think
> it is reasonable to add the flag now, as it will at least fail the build
> when using an invalid configuration, until the Rust compiler is fixed to
> list -Zsanitizer=3Dshadow-call-stack as supported for the target. See [2]
> for the feature request to add this.
>
> I have tested this change with Rust Binder on an Android device using
> CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=3D+reserve-x18 flag, the
> phone crashes immediately on boot, and with the flag, the phone appears
> to work normally.
>
> This contains a TODO to add the -Zuse-sync-unwind=3Dn flag. The flag
> defaults to n, so it isn't a problem today, but the flag is unstable, so
> the default could change in a future compiler release.
>
> Link: https://github.com/rust-lang/rust/issues/121970 [1]
> Link: https://github.com/rust-lang/rust/issues/121972 [2]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> This patch raises the question of whether we should change the Rust
> aarch64 support to use a custom target.json specification. If we do
> that, then we can fix both the warning for dynamic SCS and the
> build-failure for non-dynamic SCS without waiting for a new version of
> rustc with the mentioned issues fixed.

If the arm64 maintainers are OK with the warning being triggered in that ca=
se:

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Otherwise partially reverting to the `target.json` approach sounds good too=
.

I added the `-Zuse-sync-unwind=3Dn` 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.

Cheers,
Miguel