x86/kcfi: Don't emit KCFI_REFERENCE() in modules

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.

scripts/module.lds.S does not help here: modpost checks <module>.o
before the final link, and even after it the discarded section only
loses its relocations, the undefined symbols stay in .symtab, where
the module loader resolves them with the same license check.

Only emit the reference when building vmlinux.

Tested on x86_64 with clang 23.1.3 and CONFIG_CFI=y, with both
CONFIG_LTO_CLANG_FULL=y (the configuration from the Fixes: commit) and
CONFIG_LTO_CLANG_THIN=y: 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
without CFI failures, in QEMU and on a desktop with CFI enforcing.

Fixes: 9b041a4b66b3 ("x86/mm: Ensure clear_page() variants always have __kcfi_typeid_ symbols")
Assisted-by: LLM
Signed-off-by: Ridge Lomont <cyridge@proton.me>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>
Link: https://github.com/openzfs/zfs/issues/18092
Link: https://patch.msgid.link/20261027-kcfi-reference-modules-v2-1-3a3247f15806@proton.me

0 files changed, 0 insertions, 0 deletions