Re: [PATCH] x86/kcfi: Don't emit KCFI_REFERENCE() in modules

[Peter Zijlstra]

On Mon, Oct 26, 2026 at 11:37:43AM +0100, Peter Zijlstra wrote:
> On Sat, Oct 24, 2026 at 06:46:33PM -0700, Cytops via B4 Relay wrote:
> > From: Cytops <cyridge@xxxxxxxxx>
> > 
> > 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@xxxxxxxxx>
> 
> Yeah, I suppose this will do. Thanks.

Borislav reminded me that we need a real identity for SoB.