linux-kernel - [PATCH] arm64: rust: clean Rust 1.85.0 warning using softfloat target

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Message-ID: <20250210163732.281786-1-ojeda@kernel.org>
Date: Mon, 10 Feb 2025 17:37:32 +0100
From: Miguel Ojeda <ojeda@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>
Cc: linux-arm-kernel@...ts.infradead.org (moderated for non-subscribers),
	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@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	patches@...ts.linux.dev,
	stable@...r.kernel.org,
	Matthew Maurer <mmaurer@...gle.com>,
	Ralf Jung <post@...fj.de>,
	Jubilee Young <workingjubilee@...il.com>
Subject: [PATCH] arm64: rust: clean Rust 1.85.0 warning using softfloat target

Starting with Rust 1.85.0 (to be released 2025-02-20), `rustc` warns
[1] about disabling neon in the aarch64 hardfloat target:

    warning: target feature `neon` cannot be toggled with
             `-Ctarget-feature`: unsound on hard-float targets
             because it changes float ABI
      |
      = note: this was previously accepted by the compiler but
              is being phased out; it will become a hard error
              in a future release!
      = note: for more information, see issue #116344
              <https://github.com/rust-lang/rust/issues/116344>

Thus, instead, use the softfloat target instead.

While trying it out, I found that the kernel sanitizers were not enabled
for that built-in target [2]. Upstream Rust agreed to backport
the enablement for the current beta so that it is ready for
the 1.85.0 release [3] -- thanks!

However, that still means that before Rust 1.85.0, we cannot switch
since sanitizers could be in use. Thus conditionally do so.

Cc: <stable@...r.kernel.org> # Needed in 6.12.y and 6.13.y only (Rust is pinned in older LTSs).
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>
Cc: Matthew Maurer <mmaurer@...gle.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>
Cc: Ralf Jung <post@...fj.de>
Cc: Jubilee Young <workingjubilee@...il.com>
Link: https://github.com/rust-lang/rust/pull/133417 [1]
Link: https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/arm64.20neon.20.60-Ctarget-feature.60.20warning/near/495358442 [2]
Link: https://github.com/rust-lang/rust/pull/135905 [3]
Link: https://github.com/rust-lang/rust/issues/116344
Signed-off-by: Miguel Ojeda <ojeda@...nel.org>
---
Matthew: if you could please give this a go and confirm that it is fine
for Android (with sanitizers enabled, for 1.84.1 and 1.85.0), then a
Tested-by tag would be great. Thanks!

I would like to get this one into -rc3 if possible so that by the time
the compiler releases we are already clean.

I am sending another patch to introduce `rustc-min-version` and use it
here, but I am doing so after this one rather than before so that this
fix can be as simple as possible.

 arch/arm64/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 358c68565bfd..2b25d671365f 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -48,7 +48,11 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_NO_FPU) \
 KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
 KBUILD_AFLAGS	+= $(compat_vdso)

+ifeq ($(call test-ge, $(CONFIG_RUSTC_VERSION), 108500),y)
+KBUILD_RUSTFLAGS += --target=aarch64-unknown-none-softfloat
+else
 KBUILD_RUSTFLAGS += --target=aarch64-unknown-none -Ctarget-feature="-neon"
+endif

 KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)
 KBUILD_AFLAGS	+= $(call cc-option,-mabi=lp64)

base-commit: a64dcfb451e254085a7daee5fe51bf22959d52d3
--
2.48.1