[PATCH] x86/mce: Gate copy_mc_fragile() export by CONFIG_COPY_MC_TEST=y

[Dan Williams]

It appears that modpost is not happy about exporting assembly symbols
that are not consumed in the same build. As Boris reports:

    WARNING: modpost: EXPORT symbol "copy_mc_fragile" [vmlinux] version generation failed, symbol will not be versioned.

The export is only consumed in the CONFIG_COPY_MC_TEST=y case, and even
then not in a way that modpost could see. CONFIG_COPY_MC_TEST uses a
module built in tools/testing/nvdimm/ to exercise the copy_mc_fragile()
corner cases.  Given the test already requires manually editing the
config entry for CONFIG_COPY_MC_TEST to make it "def_bool y" the
additional dependency to require is CONFIG_MODVERSIONS=n is not too
onerous.

Alternatively, COPY_MC_TEST and its related infrastructure could just be
ripped out because it has served its purpose. For now, just stop
exporting the symbol by default, and add the MODVERSIONS dependency to
the test.

Fixes: ec6347bb4339 ("x86, powerpc: Rename memcpy_mcsafe() to copy_mc_to_{user, kernel}()")
Reported-by: Borislav Petkov <bp@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
 arch/x86/Kconfig.debug    |    1 +
 arch/x86/lib/copy_mc_64.S |    2 ++
 2 files changed, 3 insertions(+)

diff --git a/arch/x86/Kconfig.debug b/arch/x86/Kconfig.debug
index 27b5e2bc6a01..6f0f5d8ac62e 100644
--- a/arch/x86/Kconfig.debug
+++ b/arch/x86/Kconfig.debug
@@ -63,6 +63,7 @@ config EARLY_PRINTK_USB_XDBC
 	  crashes or need a very simple printk logging facility.
 
 config COPY_MC_TEST
+	depends on !MODVERSIONS
 	def_bool n
 
 config EFI_PGT_DUMP
diff --git a/arch/x86/lib/copy_mc_64.S b/arch/x86/lib/copy_mc_64.S
index 892d8915f609..50fb05256751 100644
--- a/arch/x86/lib/copy_mc_64.S
+++ b/arch/x86/lib/copy_mc_64.S
@@ -88,7 +88,9 @@ SYM_FUNC_START(copy_mc_fragile)
 .L_done:
 	ret
 SYM_FUNC_END(copy_mc_fragile)
+#ifdef CONFIG_COPY_MC_TEST
 EXPORT_SYMBOL_GPL(copy_mc_fragile)
+#endif
 
 	.section .fixup, "ax"
 	/*