KVM: x86: Enforce use of EXPORT_SYMBOL_FOR_KVM_INTERNAL

Add a (gnarly) inline "script" in the Makefile to fail the build if there
is EXPORT_SYMBOL_GPL or EXPORT_SYMBOL usage in virt/kvm or arch/x86/kvm
beyond the known-good/expected exports for other modules.  Remembering to
use EXPORT_SYMBOL_FOR_KVM_INTERNAL is surprisingly difficult, and hoping
to detect "bad" exports via code review is not a robust long-term strategy.

Jump through a pile of hoops to coerce make into printing a human-friendly
error message, with the offending files+lines cleanly separated.

E.g. where <srctree> is the resolution of $(srctree), i.e. '.' for in-tree
builds, and the absolute path for out-of-tree-builds:

  <srctree>/arch/x86/kvm/Makefile:97: *** ERROR ***
  found 2 unwanted occurrences of EXPORT_SYMBOL_GPL:
    <srctree>/arch/x86/kvm/x86.c:686:EXPORT_SYMBOL_GPL(__kvm_set_user_return_msr);
    <srctree>/arch/x86/kvm/x86.c:703:EXPORT_SYMBOL_GPL(kvm_set_user_return_msr);
  in directories:
    <srctree>/arch/x86/kvm
    <srctree>/virt/kvm
  Use EXPORT_SYMBOL_FOR_KVM_INTERNAL, not EXPORT_SYMBOL_GPL.  Stop.

and

  <srctree>/arch/x86/kvm/Makefile:98: *** ERROR ***
  found 1 unwanted occurrences of EXPORT_SYMBOL:
    <srctree>/arch/x86/kvm/x86.c:709:EXPORT_SYMBOL(kvm_get_user_return_msr);
  in directories:
    <srctree>/arch/x86/kvm
    <srctree>/virt/kvm
  Use EXPORT_SYMBOL_FOR_KVM_INTERNAL, not EXPORT_SYMBOL.  Stop.

Put the enforcement in x86's Makefile even though the rule itself applies
to virt/kvm, as putting the enforcement in virt/kvm/Makefile.kvm would
effectively require exempting every architecture except x86.  PPC is the
only other architecture with sub-modules, and PPC hasn't been switched to
use EXPORT_SYMBOL_FOR_KVM_INTERNAL (and given its nearly-orphaned state,
likely never will).  And for KVM architectures without sub-modules, that
means that, barring truly spurious exports, the exports are intended for
non-KVM usage and thus shouldn't be using EXPORT_SYMBOL_FOR_KVM_INTERNAL.

Tested-by: Chao Gao <chao.gao@intel.com>
Link: https://patch.msgid.link/20251121190514.293385-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/Makefile

@@ -47,3 +47,52 @@ $(obj)/kvm-asm-offsets.h: $(obj)/kvm-asm-offsets.s FORCE
 
 targets += kvm-asm-offsets.s
 clean-files += kvm-asm-offsets.h
+
+
+# Fail the build if there is unexpected EXPORT_SYMBOL_GPL (or EXPORT_SYMBOL)
+# usage.  All KVM-internal exports should use EXPORT_SYMBOL_FOR_KVM_INTERNAL.
+# Only a handful of exports intended for other modules (VFIO, KVMGT) should
+# use EXPORT_SYMBOL_GPL, and EXPORT_SYMBOL should never be used.
+ifdef CONFIG_KVM_X86
+# Search recursively for whole words and print line numbers.  Filter out the
+# allowed set of exports, i.e. those that are intended for external usage.
+exports_grep_trailer := --include='*.[ch]' -nrw $(srctree)/virt/kvm $(srctree)/arch/x86/kvm | \
+			grep -v -e kvm_page_track_register_notifier \
+				-e kvm_page_track_unregister_notifier \
+				-e kvm_write_track_add_gfn \
+				-e kvm_write_track_remove_gfn \
+				-e kvm_get_kvm \
+				-e kvm_get_kvm_safe \
+				-e kvm_put_kvm
+
+# Force grep to emit a goofy group separator that can in turn be replaced with
+# the above newline macro (newlines in Make are a nightmare).  Note, grep only
+# prints the group separator when N lines of context are requested via -C,
+# a.k.a. --NUM.  Simply request zero lines.  Print the separator only after
+# filtering out expected exports to avoid extra newlines in the error message.
+define get_kvm_exports
+$(shell grep "$(1)" -C0 $(exports_grep_trailer) | grep "$(1)" -C0 --group-separator="!SEP!")
+endef
+
+define check_kvm_exports
+nr_kvm_exports := $(shell grep "$(1)" $(exports_grep_trailer) | wc -l)
+
+ifneq (0,$$(nr_kvm_exports))
+$$(error ERROR ***\
+$$(newline)found $$(nr_kvm_exports) unwanted occurrences of $(1):\
+$$(newline)  $(subst !SEP!,$$(newline) ,$(call get_kvm_exports,$(1)))\
+$$(newline)in directories:\
+$$(newline)  $(srctree)/arch/x86/kvm\
+$$(newline)  $(srctree)/virt/kvm\
+$$(newline)Use EXPORT_SYMBOL_FOR_KVM_INTERNAL, not $(1))
+endif # nr_kvm_exports != 0
+undefine nr_kvm_exports
+endef # check_kvm_exports
+
+$(eval $(call check_kvm_exports,EXPORT_SYMBOL_GPL))
+$(eval $(call check_kvm_exports,EXPORT_SYMBOL))
+
+undefine check_kvm_exports
+undefine get_kvm_exports
+undefine exports_grep_trailer
+endif # CONFIG_KVM_X86