crypto: curve25519 - Fix selftest build error

If CRYPTO_CURVE25519 is y, CRYPTO_LIB_CURVE25519_GENERIC will be
y, but CRYPTO_LIB_CURVE25519 may be set to m, this causes build
errors:

lib/crypto/curve25519-selftest.o: In function `curve25519':
curve25519-selftest.c:(.text.unlikely+0xc): undefined reference to `curve25519_arch'
lib/crypto/curve25519-selftest.o: In function `curve25519_selftest':
curve25519-selftest.c:(.init.text+0x17e): undefined reference to `curve25519_base_arch'

This is because the curve25519 self-test code is being controlled
by the GENERIC option rather than the overall CURVE25519 option,
as is the case with blake2s.  To recap, the GENERIC and ARCH options
for CURVE25519 are internal only and selected by users such as
the Crypto API, or the externally visible CURVE25519 option which
in turn is selected by wireguard.  The self-test is specific to the
the external CURVE25519 option and should not be enabled by the
Crypto API.

This patch fixes this by splitting the GENERIC module from the
CURVE25519 module with the latter now containing just the self-test.

Reported-by: Hulk Robot <[email protected]>
Fixes: aa12796 ("crypto: lib/curve25519 - re-add selftests")
Signed-off-by: Herbert Xu <[email protected]>
Reviewed-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>

lib/crypto/Makefile

@@ -19,9 +19,12 @@ libblake2s-y					+= blake2s.o
 obj-$(CONFIG_CRYPTO_LIB_CHACHA20POLY1305)	+= libchacha20poly1305.o
 libchacha20poly1305-y				+= chacha20poly1305.o
 
-obj-$(CONFIG_CRYPTO_LIB_CURVE25519_GENERIC)	+= libcurve25519.o
-libcurve25519-y					:= curve25519-fiat32.o
-libcurve25519-$(CONFIG_ARCH_SUPPORTS_INT128)	:= curve25519-hacl64.o
+obj-$(CONFIG_CRYPTO_LIB_CURVE25519_GENERIC)	+= libcurve25519-generic.o
+libcurve25519-generic-y				:= curve25519-fiat32.o
+libcurve25519-generic-$(CONFIG_ARCH_SUPPORTS_INT128)	:= curve25519-hacl64.o
+libcurve25519-generic-y				+= curve25519-generic.o
+
+obj-$(CONFIG_CRYPTO_LIB_CURVE25519)		+= libcurve25519.o
 libcurve25519-y					+= curve25519.o
 
 obj-$(CONFIG_CRYPTO_LIB_DES)			+= libdes.o

lib/crypto/curve25519-generic.c

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0 OR MIT
+/*
+ * Copyright (C) 2015-2019 Jason A. Donenfeld <[email protected]>. All Rights Reserved.
+ *
+ * This is an implementation of the Curve25519 ECDH algorithm, using either
+ * a 32-bit implementation or a 64-bit implementation with 128-bit integers,
+ * depending on what is supported by the target compiler.
+ *
+ * Information: https://cr.yp.to/ecdh.html
+ */
+
+#include <crypto/curve25519.h>
+#include <linux/module.h>
+
+const u8 curve25519_null_point[CURVE25519_KEY_SIZE] __aligned(32) = { 0 };
+const u8 curve25519_base_point[CURVE25519_KEY_SIZE] __aligned(32) = { 9 };
+
+EXPORT_SYMBOL(curve25519_null_point);
+EXPORT_SYMBOL(curve25519_base_point);
+EXPORT_SYMBOL(curve25519_generic);
+
+MODULE_LICENSE("GPL v2");
+MODULE_DESCRIPTION("Curve25519 scalar multiplication");
+MODULE_AUTHOR("Jason A. Donenfeld <[email protected]>");

lib/crypto/curve25519.c

@@ -15,13 +15,6 @@
 
 bool curve25519_selftest(void);
 
-const u8 curve25519_null_point[CURVE25519_KEY_SIZE] __aligned(32) = { 0 };
-const u8 curve25519_base_point[CURVE25519_KEY_SIZE] __aligned(32) = { 9 };
-
-EXPORT_SYMBOL(curve25519_null_point);
-EXPORT_SYMBOL(curve25519_base_point);
-EXPORT_SYMBOL(curve25519_generic);
-
 static int __init mod_init(void)
 {
 	if (!IS_ENABLED(CONFIG_CRYPTO_MANAGER_DISABLE_TESTS) &&