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From:   Stephan =?ISO-8859-1?Q?M=FCller?= <smueller@chronox.de>
To:     herbert@gondor.apana.org.au
Cc:     linux-crypto@vger.kernel.org,
        Marcelo Cerri <marcelo.cerri@canonical.com>,
        Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Subject: [PATCH 2/3] lib/mpi: Add mpi_sub_ui()
Date:   Fri, 10 Jul 2020 12:10:40 +0200
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Add mpi_sub_ui() based on Gnu PG mpz_sub_ui() from mpz/aors_ui.h
adapting the code to the kernel's structures and coding style and also
removing the defines used to produce mpz_sub_ui() and mpz_add_ui()
from the same code.

Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
---
 include/linux/mpi.h  |  3 +++
 lib/mpi/Makefile     |  1 +
 lib/mpi/mpi-sub-ui.c | 60 ++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 64 insertions(+)
 create mode 100644 lib/mpi/mpi-sub-ui.c

diff --git a/include/linux/mpi.h b/include/linux/mpi.h
index 7bd6d8af0004..5d906dfbf3ed 100644
--- a/include/linux/mpi.h
+++ b/include/linux/mpi.h
@@ -63,6 +63,9 @@ int mpi_powm(MPI res, MPI base, MPI exp, MPI mod);
 int mpi_cmp_ui(MPI u, ulong v);
 int mpi_cmp(MPI u, MPI v);
 
+/*-- mpi-sub-ui.c --*/
+int mpi_sub_ui(MPI w, MPI u, unsigned long vval);
+
 /*-- mpi-bit.c --*/
 void mpi_normalize(MPI a);
 unsigned mpi_get_nbits(MPI a);
diff --git a/lib/mpi/Makefile b/lib/mpi/Makefile
index d5874a7f5ff9..43b8fce14079 100644
--- a/lib/mpi/Makefile
+++ b/lib/mpi/Makefile
@@ -16,6 +16,7 @@ mpi-y = \
 	mpicoder.o			\
 	mpi-bit.o			\
 	mpi-cmp.o			\
+	mpi-sub-ui.o			\
 	mpih-cmp.o			\
 	mpih-div.o			\
 	mpih-mul.o			\
diff --git a/lib/mpi/mpi-sub-ui.c b/lib/mpi/mpi-sub-ui.c
new file mode 100644
index 000000000000..fa6b085bac36
--- /dev/null
+++ b/lib/mpi/mpi-sub-ui.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* mpi-sub-ui.c  -  MPI functions
+ *      Copyright 1991, 1993, 1994, 1996, 1999-2002, 2004, 2012, 2013, 2015
+ *      Free Software Foundation, Inc.
+ *
+ * This file is part of GnuPG.
+ *
+ * Note: This code is heavily based on the GNU MP Library.
+ *	 Actually it's the same code with only minor changes in the
+ *	 way the data is stored; this is to support the abstraction
+ *	 of an optional secure memory allocation which may be used
+ *	 to avoid revealing of sensitive data due to paging etc.
+ *	 The GNU MP Library itself is published under the LGPL;
+ *	 however I decided to publish this code under the plain GPL.
+ */
+
+#include "mpi-internal.h"
+
+int mpi_sub_ui(MPI w, MPI u, unsigned long vval)
+{
+	if (u->nlimbs == 0) {
+		if (mpi_resize(w, 1) < 0)
+			return -ENOMEM;
+		w->d[0] = vval;
+		w->nlimbs = (vval != 0);
+		w->sign = (vval != 0);
+		return 0;
+	}
+
+	/* If not space for W (and possible carry), increase space. */
+	if (mpi_resize(w, u->nlimbs + 1))
+		return -ENOMEM;
+
+	if (u->sign) {
+		mpi_limb_t cy;
+
+		cy = mpihelp_add_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
+		w->d[u->nlimbs] = cy;
+		w->nlimbs = u->nlimbs + cy;
+		w->sign = 1;
+	} else {
+		/* The signs are different.  Need exact comparison to determine
+		 * which operand to subtract from which.
+		 */
+		if (u->nlimbs == 1 && u->d[0] < vval) {
+			w->d[0] = vval - u->d[0];
+			w->nlimbs = 1;
+			w->sign = 1;
+		} else {
+			mpihelp_sub_1(w->d, u->d, u->nlimbs, (mpi_limb_t) vval);
+			/* Size can decrease with at most one limb. */
+			w->nlimbs = (u->nlimbs - (w->d[u->nlimbs - 1] == 0));
+			w->sign = 0;
+		}
+	}
+
+	mpi_normalize(w);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(mpi_sub_ui);
-- 
2.26.2