[PATCH v1] rust: cfi: only 64-bit arm and x86 support CFI_CLANG

[Alice Ryhl]

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 at microchip.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 at vger.kernel.org
> > Fixes: ca627e636551e ("rust: cfi: add support for CFI_CLANG with Rust")
> > Signed-off-by: Conor Dooley <conor.dooley at microchip.com>
> > ---
> > CC: Paul Walmsley <paul.walmsley at sifive.com>
> > CC: Palmer Dabbelt <palmer at dabbelt.com>
> > CC: Alexandre Ghiti <alex at ghiti.fr>
> > CC: Miguel Ojeda <ojeda at kernel.org>
> > CC: Alex Gaynor <alex.gaynor at gmail.com>
> > CC: Boqun Feng <boqun.feng at gmail.com>
> > CC: Gary Guo <gary at garyguo.net>
> > CC: "Björn Roy Baron" <bjorn3_gh at protonmail.com>
> > CC: Benno Lossin <lossin at kernel.org>
> > CC: Andreas Hindborg <a.hindborg at kernel.org>
> > CC: Alice Ryhl <aliceryhl at google.com>
> > CC: Trevor Gross <tmgross at umich.edu>
> > CC: Danilo Krummrich <dakr at kernel.org>
> > CC: Kees Cook <kees at kernel.org>
> > CC: Sami Tolvanen <samitolvanen at google.com>
> > CC: Matthew Maurer <mmaurer at google.com>
> > CC: "Peter Zijlstra (Intel)" <peterz at infradead.org>
> > CC: linux-kernel at vger.kernel.org
> > CC: linux-riscv at lists.infradead.org
> > CC: rust-for-linux at vger.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