lib/adds.[ch]: Add addsl() and addsl3()

These functions add 2 or 3 longs, saturating to LONG_{MIN,MAX} instead
of overflowing.

Cc: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/Makefile.am

@@ -23,6 +23,8 @@ libshadow_la_CFLAGS = $(LIBBSD_CFLAGS) $(LIBCRYPT_PAM) $(LIBSYSTEMD)
 
 libshadow_la_SOURCES = \
 	addgrps.c \
+	adds.c \
+	adds.h \
 	age.c \
 	agetpass.c \
 	agetpass.h \

lib/adds.c

@@ -0,0 +1,13 @@
+// SPDX-FileCopyrightText: 2023, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#include <config.h>
+
+#include "adds.h"
+
+
+extern inline long addsl(long a, long b);
+extern inline long addsl3(long a, long b, long c);
+
+extern inline int cmpl(const void *p1, const void *p2);

lib/adds.h

@@ -0,0 +1,72 @@
+// SPDX-FileCopyrightText: 2023, Alejandro Colomar <alx@kernel.org>
+// SPDX-License-Identifier: BSD-3-Clause
+
+
+#ifndef SHADOW_INCLUDE_LIB_ADDS_H_
+#define SHADOW_INCLUDE_LIB_ADDS_H_
+
+
+#include <config.h>
+
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+
+#include "sizeof.h"
+
+
+inline long addsl(long a, long b);
+inline long addsl3(long a, long b, long c);
+
+inline int cmpl(const void *p1, const void *p2);
+
+
+inline long
+addsl(long a, long b)
+{
+	if (a > 0 && b > LONG_MAX - a) {
+		errno = EOVERFLOW;
+		return LONG_MAX;
+	}
+	if (a < 0 && b < LONG_MIN - a) {
+		errno = EOVERFLOW;
+		return LONG_MIN;
+	}
+	return a + b;
+}
+
+
+inline long
+addsl3(long a, long b, long c)
+{
+	int   e;
+	long  sum;
+	long  n[3] = {a, b, c};
+
+	e = errno;
+	qsort(n, NITEMS(n), sizeof(n[0]), cmpl);
+
+	errno = 0;
+	sum = addsl(n[0], n[2]);
+	if (errno == EOVERFLOW)
+		return sum;
+	errno = e;
+	return addsl(sum, n[1]);
+}
+
+
+inline int
+cmpl(const void *p1, const void *p2)
+{
+	const long  *l1 = p1;
+	const long  *l2 = p2;
+
+	if (*l1 < *l2)
+		return -1;
+	if (*l1 > *l2)
+		return +1;
+	return 0;
+}
+
+
+#endif  // include guard