linux-kernel - Re: [PATCH v1] rust: cfi: only 64-bit arm and x86 support CFI

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Message-ID: <aMAuhKfrYRlkTHSy@google.com>
Date: Tue, 9 Sep 2025 13:41:24 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Kees Cook <kees@...nel.org>
Cc: Conor Dooley <conor@...nel.org>, linux-kernel@...r.kernel.org, 
	Conor Dooley <conor.dooley@...rochip.com>, stable@...r.kernel.org, 
	Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Alexandre Ghiti <alex@...ti.fr>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Matthew Maurer <mmaurer@...gle.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, 
	linux-riscv@...ts.infradead.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v1] rust: cfi: only 64-bit arm and x86 support CFI_CLANG

On Mon, Sep 08, 2025 at 08:11:48PM -0700, Kees Cook wrote:
> On Mon, Sep 08, 2025 at 02:12:35PM +0100, Conor Dooley wrote:
> > From: Conor Dooley <conor.dooley@...rochip.com>
> > 
> > The kernel uses the standard rustc targets for non-x86 targets, and out
> > of those only 64-bit arm's target has kcfi support enabled. For x86, the
> > custom 64-bit target enables kcfi.
> > 
> > The HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC config option that allows
> > CFI_CLANG to be used in combination with RUST does not check whether the
> > rustc target supports kcfi. This breaks the build on riscv (and
> > presumably 32-bit arm) when CFI_CLANG and RUST are enabled at the same
> > time.
> > 
> > Ordinarily, a rustc-option check would be used to detect target support
> > but unfortunately rustc-option filters out the target for reasons given
> > in commit 46e24a545cdb4 ("rust: kasan/kbuild: fix missing flags on first
> > build"). As a result, if the host supports kcfi but the target does not,
> > e.g. when building for riscv on x86_64, the build would remain broken.
> > 
> > Instead, make HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC depend on the only
> > two architectures where the target used supports it to fix the build.
> 
> I'm generally fine with this, but normally we do arch-specific stuff
> only in arch/$arch/Kconfig, and expose some kind of
> ARCH_HAS_CFI_ICALL_NORMALIZE_INTEGERS that would get tested here. Should
> we do that here too?

I'm thinking in this case it makes sense to keep this patch simple as
it's a fix. Once rustc supports cfi on riscv (which should really just
be changing the target to list it as supported), we can reorganize it to
match what you're describing at that point.

> > CC: stable@...r.kernel.org
> > Fixes: ca627e636551e ("rust: cfi: add support for CFI_CLANG with Rust")
> > Signed-off-by: Conor Dooley <conor.dooley@...rochip.com>
> > ---
> > CC: Paul Walmsley <paul.walmsley@...ive.com>
> > CC: Palmer Dabbelt <palmer@...belt.com>
> > CC: Alexandre Ghiti <alex@...ti.fr>
> > CC: Miguel Ojeda <ojeda@...nel.org>
> > CC: Alex Gaynor <alex.gaynor@...il.com>
> > CC: Boqun Feng <boqun.feng@...il.com>
> > CC: Gary Guo <gary@...yguo.net>
> > CC: "Björn Roy Baron" <bjorn3_gh@...tonmail.com>
> > CC: Benno Lossin <lossin@...nel.org>
> > CC: Andreas Hindborg <a.hindborg@...nel.org>
> > CC: Alice Ryhl <aliceryhl@...gle.com>
> > CC: Trevor Gross <tmgross@...ch.edu>
> > CC: Danilo Krummrich <dakr@...nel.org>
> > CC: Kees Cook <kees@...nel.org>
> > CC: Sami Tolvanen <samitolvanen@...gle.com>
> > CC: Matthew Maurer <mmaurer@...gle.com>
> > CC: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> > CC: linux-kernel@...r.kernel.org
> > CC: linux-riscv@...ts.infradead.org
> > CC: rust-for-linux@...r.kernel.org
> > ---
> >  arch/Kconfig | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index d1b4ffd6e0856..880cddff5eda7 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -917,6 +917,7 @@ config HAVE_CFI_ICALL_NORMALIZE_INTEGERS_RUSTC
> >  	def_bool y
> >  	depends on HAVE_CFI_ICALL_NORMALIZE_INTEGERS_CLANG
> >  	depends on RUSTC_VERSION >= 107900
> > +	depends on ARM64 || X86_64
> >  	# With GCOV/KASAN we need this fix: https://github.com/rust-lang/rust/pull/129373
> >  	depends on (RUSTC_LLVM_VERSION >= 190103 && RUSTC_VERSION >= 108200) || \
> >  		(!GCOV_KERNEL && !KASAN_GENERIC && !KASAN_SW_TAGS)
> > -- 
> > 2.47.2
> > 
> 
> -- 
> Kees Cook