crypto: memneq - move into lib/

This is used by code that doesn't need CONFIG_CRYPTO, so move this into
lib/ with a Kconfig option so that it can be selected by whatever needs
it.

This fixes a linker error Zheng pointed out when
CRYPTO_MANAGER_DISABLE_TESTS!=y and CRYPTO=m:

  lib/crypto/curve25519-selftest.o: In function `curve25519_selftest':
  curve25519-selftest.c:(.init.text+0x60): undefined reference to `__crypto_memneq'
  curve25519-selftest.c:(.init.text+0xec): undefined reference to `__crypto_memneq'
  curve25519-selftest.c:(.init.text+0x114): undefined reference to `__crypto_memneq'
  curve25519-selftest.c:(.init.text+0x154): undefined reference to `__crypto_memneq'

Reported-by: Zheng Bin <zhengbin13@huawei.com>
Cc: Eric Biggers <ebiggers@kernel.org>
Cc: stable@vger.kernel.org
Fixes: aa12796 ("crypto: lib/curve25519 - re-add selftests")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: zx2c4

crypto/Kconfig

@@ -15,6 +15,7 @@ source "crypto/async_tx/Kconfig"
 #
 menuconfig CRYPTO
 	tristate "Cryptographic API"
+	select LIB_MEMNEQ
 	help
 	  This option provides the core Cryptographic API.
 

crypto/Makefile

@@ -4,7 +4,7 @@
 #
 
 obj-$(CONFIG_CRYPTO) += crypto.o
-crypto-y := api.o cipher.o compress.o memneq.o
+crypto-y := api.o cipher.o compress.o
 
 obj-$(CONFIG_CRYPTO_ENGINE) += crypto_engine.o
 obj-$(CONFIG_CRYPTO_FIPS) += fips.o

lib/Kconfig

@@ -120,6 +120,9 @@ config INDIRECT_IOMEM_FALLBACK
 
 source "lib/crypto/Kconfig"
 
+config LIB_MEMNEQ
+	bool
+
 config CRC_CCITT
 	tristate "CRC-CCITT functions"
 	help

lib/Makefile

@@ -251,6 +251,7 @@ obj-$(CONFIG_DIMLIB) += dim/
 obj-$(CONFIG_SIGNATURE) += digsig.o
 
 lib-$(CONFIG_CLZ_TAB) += clz_tab.o
+lib-$(CONFIG_LIB_MEMNEQ) += memneq.o
 
 obj-$(CONFIG_GENERIC_STRNCPY_FROM_USER) += strncpy_from_user.o
 obj-$(CONFIG_GENERIC_STRNLEN_USER) += strnlen_user.o

lib/crypto/Kconfig

@@ -71,6 +71,7 @@ config CRYPTO_LIB_CURVE25519
 	tristate "Curve25519 scalar multiplication library"
 	depends on CRYPTO_ARCH_HAVE_LIB_CURVE25519 || !CRYPTO_ARCH_HAVE_LIB_CURVE25519
 	select CRYPTO_LIB_CURVE25519_GENERIC if CRYPTO_ARCH_HAVE_LIB_CURVE25519=n
+	select LIB_MEMNEQ
 	help
 	  Enable the Curve25519 library interface. This interface may be
 	  fulfilled by either the generic implementation or an arch-specific