From: Peter Zijlstra <peterz@infradead.org>
To: cyridge@proton.me
Cc: x86@kernel.org, Thomas Gleixner <tglx@kernel.org>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
Chris Li <sparse@chrisli.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Bill Wendling <morbo@google.com>,
Justin Stitt <justinstitt@google.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Nathan Chancellor <nathan@kernel.org>,
Sami Tolvanen <samitolvanen@google.com>,
Kees Cook <kees@kernel.org>,
llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
linux-sparse@vger.kernel.org
Subject: Re: [PATCH] x86/kcfi: Don't emit KCFI_REFERENCE() in modules
Date: Mon, 26 Oct 2026 11:37:43 +0100 [thread overview]
Message-ID: <20261026103743.GD2668200@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <20261024-kcfi-reference-modules-v1-1-43c2a4a9e657@proton.me>
On Sat, Oct 24, 2026 at 06:46:33PM -0700, Cytops via B4 Relay wrote:
> From: Cytops <cyridge@proton.me>
>
> KCFI_REFERENCE() takes the address of a symbol so that the compiler
> emits its __kcfi_typeid_ symbol, which the SYM_TYPED_FUNC_START()
> assembly implementing it needs. It is used from arch/x86 headers for
> __clear_pages_unrolled(), copy_page(), __memset() and __memmove(), and
> all of that assembly is built into vmlinux.
>
> Modules include the same headers, so with CONFIG_CFI=y every module
> also gets an __ADDRESSABLE() reference to each of these symbols in
> .discard.addressable. This serves no purpose there, and since
> __clear_pages_unrolled() is EXPORT_SYMBOL_GPL, modpost now rejects any
> GPL-incompatible module that includes <linux/mm.h>:
>
> ERROR: modpost: GPL-incompatible module repro.ko uses GPL-only
> symbol '__clear_pages_unrolled'
>
> even though the module never uses the symbol. This makes OpenZFS
> impossible to build against CFI kernels; its configure checks fail
> in the same way and are misreported as CONFIG_DEBUG_LOCK_ALLOC.
>
> Only emit the reference when building vmlinux.
>
> Tested on x86_64 with clang 23.1.3, CONFIG_CFI=y and
> CONFIG_LTO_CLANG_FULL=y (the configuration from the Fixes: commit):
> vmlinux links without warnings and still contains
> __kcfi_typeid___clear_pages_unrolled, __kcfi_typeid_copy_page,
> __kcfi_typeid___memset and __kcfi_typeid___memmove. It boots in QEMU
> with no CFI failures. A minimal module with MODULE_LICENSE("Proprietary")
> including <linux/mm.h> fails modpost before this change and builds and
> loads after it. OpenZFS builds against a CFI kernel with this change,
> and its pool, encryption, checksum and send/recv operations run with
> CONFIG_CFI_PERMISSIVE=y without CFI failures.
>
> Fixes: 9b041a4b66b3 ("x86/mm: Ensure clear_page() variants always have __kcfi_typeid_ symbols")
> Link: https://github.com/openzfs/zfs/issues/18092
> Assisted-by: LLM
> Signed-off-by: Cytops <cyridge@proton.me>
Yeah, I suppose this will do. Thanks.
next prev parent reply other threads:[~2026-10-26 10:37 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-25 1:46 Cytops via B4 Relay
2026-10-26 10:37 ` Peter Zijlstra [this message]
2026-10-26 21:43 ` Nathan Chancellor
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