8319174: Enhance robustness of some j.m.BigInteger constructors

src/java.base/share/classes/java/math/BigInteger.java

@@ -351,12 +351,18 @@ public BigInteger(byte[] val, int off, int len) {
             throw new NumberFormatException("Zero length BigInteger");
         }
         Objects.checkFromIndexSize(off, len, val.length);
+        if (len == 0) {
+            mag = ZERO.mag;
+            signum = ZERO.signum;
+            return;
+        }
 
-        if (val[off] < 0) {
-            mag = makePositive(val, off, len);
+        int b = val[off];
+        if (b < 0) {
+            mag = makePositive(b, val, off, len);
             signum = -1;
         } else {
-            mag = stripLeadingZeroBytes(val, off, len);
+            mag = stripLeadingZeroBytes(b, val, off, len);
             signum = (mag.length == 0 ? 0 : 1);
         }
         if (mag.length >= MAX_MAG_LENGTH) {
@@ -4604,77 +4610,167 @@ private static int[] trustedStripLeadingZeroInts(int[] val) {
         return keep == 0 ? val : java.util.Arrays.copyOfRange(val, keep, vlen);
     }
 
-    /**
+    private static int[] stripLeadingZeroBytes(byte[] a, int from, int len) {
+        return stripLeadingZeroBytes(Integer.MIN_VALUE, a, from, len);
+    }
+
+    /*
      * Returns a copy of the input array stripped of any leading zero bytes.
+     * The returned array is either empty, or its 0-th element is non-zero,
+     * meeting the "minimal" requirement for field mag (see comment on mag).
+     *
+     * The range [from, from + len) must be well-formed w.r.t. array a.
+     *
+     * b < -128 means that a[from] has not yet been read.
+     * Otherwise, b must be a[from], have been read only once before invoking
+     * this method, and len > 0 must hold.
      */
-    private static int[] stripLeadingZeroBytes(byte[] a, int off, int len) {
-        int indexBound = off + len;
-        int keep;
-
-        // Find first nonzero byte
-        for (keep = off; keep < indexBound && a[keep] == 0; keep++)
-            ;
-
-        // Allocate new array and copy relevant part of input array
-        int intLength = ((indexBound - keep) + 3) >>> 2;
-        int[] result = new int[intLength];
-        int b = indexBound - 1;
-        for (int i = intLength-1; i >= 0; i--) {
-            result[i] = a[b--] & 0xff;
-            int bytesRemaining = b - keep + 1;
-            int bytesToTransfer = Math.min(3, bytesRemaining);
-            for (int j=8; j <= (bytesToTransfer << 3); j += 8)
-                result[i] |= ((a[b--] & 0xff) << j);
+    private static int[] stripLeadingZeroBytes(int b, byte[] a, int from, int len) {
+        /*
+         * Except for the first byte, each read access to the input array a
+         * is of the form a[from++].
+         * The index from is never otherwise altered, except right below,
+         * and only increases in steps of 1, always up to index to.
+         * Hence, each byte in the array is read exactly once, from lower to
+         * higher indices (from most to least significant byte).
+         */
+        if (len == 0) {
+            return ZERO.mag;
         }
-        return result;
+        int to = from + len;
+        if (b < -128) {
+            b = a[from];
+        }
+        /* Either way, a[from] has now been read exactly once, skip to next. */
+        ++from;
+        /*
+         * Set up the shortest int[] for the sequence of the bytes
+         *      b, a[from+1], ..., a[to-1]    (len > 0)
+         * Shortest means first skipping leading zeros.
+         */
+        for (; b == 0 && from < to; b = a[from++])
+            ;  //empty body
+        if (b == 0) {
+            /* Here, from == to as well. All bytes are zeros. */
+            return ZERO.mag;
+        }
+        /*
+         * Allocate just enough ints to hold (to - from + 1) bytes, that is
+         *      ((to - from + 1) + 3) / 4 = (to - from) / 4 + 1
+         */
+        int[] res = new int[((to - from) >> 2) + 1];
+        /*
+         * A "digit" is a group of 4 adjacent bytes aligned w.r.t. index to.
+         * (Implied 0 bytes are prepended as needed.)
+         * b is the most significant byte not 0.
+         * Digit d0 spans the range of indices that includes current (from - 1).
+         */
+        int d0 = b & 0xFF;
+        while (((to - from) & 0x3) != 0) {
+            d0 = d0 << 8 | a[from++] & 0xFF;
+        }
+        res[0] = d0;
+        /*
+         * Prepare the remaining digits.
+         * (to - from) is a multiple of 4, so prepare an int for every 4 bytes.
+         * This is a candidate for Unsafe.copy[Swap]Memory().
+         */
+        int i = 1;
+        while (from < to) {
+            res[i++] = a[from++] << 24 | (a[from++] & 0xFF) << 16
+                    | (a[from++] & 0xFF) << 8 | (a[from++] & 0xFF);
+        }
+        return res;
     }
 
-    /**
+    /*
      * Takes an array a representing a negative 2's-complement number and
      * returns the minimal (no leading zero bytes) unsigned whose value is -a.
+     *
+     * len > 0 must hold.
+     * The range [from, from + len) must be well-formed w.r.t. array a.
+     * b is assumed to be the result of reading a[from] and to meet b < 0.
      */
-    private static int[] makePositive(byte[] a, int off, int len) {
-        int keep, k;
-        int indexBound = off + len;
-
-        // Find first non-sign (0xff) byte of input
-        for (keep=off; keep < indexBound && a[keep] == -1; keep++)
-            ;
-
-
-        /* Allocate output array.  If all non-sign bytes are 0x00, we must
-         * allocate space for one extra output byte. */
-        for (k=keep; k < indexBound && a[k] == 0; k++)
-            ;
-
-        int extraByte = (k == indexBound) ? 1 : 0;
-        int intLength = ((indexBound - keep + extraByte) + 3) >>> 2;
-        int result[] = new int[intLength];
-
-        /* Copy one's complement of input into output, leaving extra
-         * byte (if it exists) == 0x00 */
-        int b = indexBound - 1;
-        for (int i = intLength-1; i >= 0; i--) {
-            result[i] = a[b--] & 0xff;
-            int numBytesToTransfer = Math.min(3, b-keep+1);
-            if (numBytesToTransfer < 0)
-                numBytesToTransfer = 0;
-            for (int j=8; j <= 8*numBytesToTransfer; j += 8)
-                result[i] |= ((a[b--] & 0xff) << j);
-
-            // Mask indicates which bits must be complemented
-            int mask = -1 >>> (8*(3-numBytesToTransfer));
-            result[i] = ~result[i] & mask;
+    private static int[] makePositive(int b, byte[] a, int from, int len) {
+        /*
+         * By assumption, b == a[from] < 0 and len > 0.
+         *
+         * First collect the bytes into the resulting array res.
+         * Then convert res to two's complement.
+         *
+         * Except for b == a[from], each read access to the input array a
+         * is of the form a[from++].
+         * The index from is never otherwise altered, except right below,
+         * and only increases in steps of 1, always up to index to.
+         * Hence, each byte in the array is read exactly once, from lower to
+         * higher indices (from most to least significant byte).
+         */
+        int to = from + len;
+        /* b == a[from] has been read exactly once, skip to next index. */
+        ++from;
+        /* Skip leading -1 bytes. */
+        for (; b == -1 && from < to; b = a[from++])
+            ;  //empty body
+        /*
+         * A "digit" is a group of 4 adjacent bytes aligned w.r.t. index to.
+         * b is the most significant byte not -1, or -1 only if from == to.
+         * Digit d0 spans the range of indices that includes current (from - 1).
+         * (Implied -1 bytes are prepended to array a as needed.)
+         * It usually corresponds to res[0], except for the special case below.
+         */
+        int d0 = -1 << 8 | b & 0xFF;
+        while (((to - from) & 0x3) != 0) {
+            d0 = d0 << 8 | (b = a[from++]) & 0xFF;
+        }
+        int f = from;  // keeps the current from for sizing purposes later
+        /* Skip zeros adjacent to d0, if at all. */
+        for (; b == 0 && from < to; b = a[from++])
+            ;  //empty body
+        /*
+         * b is the most significant non-zero byte at or after (f - 1),
+         * or 0 only if from == to.
+         * Digit d spans the range of indices that includes (f - 1).
+         */
+        int d = b & 0xFF;
+        while (((to - from) & 0x3) != 0) {
+            d = d << 8 | a[from++] & 0xFF;
         }
-
-        // Add one to one's complement to generate two's complement
-        for (int i=result.length-1; i >= 0; i--) {
-            result[i] = (int)((result[i] & LONG_MASK) + 1);
-            if (result[i] != 0)
-                break;
+        /*
+         * If the situation here is like this:
+         * index:                      f                        to == from
+         *      ..., -1,-1,  0,0,0,0,  0,0,0,0,  ...,  0,0,0,0
+         * digit:               d0                        d
+         * then, as shown, the number of zeros is a positive multiple of 4.
+         * The array res needs a minimal length of (1 + 1 + (to - f) / 4)
+         * to accommodate the two's complement, including a leading 1.
+         * In any other case, there is at least one byte that is non-zero.
+         * The array for the two's complement has length (0 + 1 + (to - f) / 4).
+         * c is 1, resp., 0 for the two situations.
+         */
+        int c = (to - from | d0 | d) == 0 ? 1 : 0;
+        int[] res = new int[c + 1 + ((to - f) >> 2)];
+        res[0] = c == 0 ? d0 : -1;
+        int i = res.length - ((to - from) >> 2);
+        if (i > 1) {
+            res[i - 1] = d;
         }
-
-        return result;
+        /*
+         * Prepare the remaining digits.
+         * (to - from) is a multiple of 4, so prepare an int for every 4 bytes.
+         * This is a candidate for Unsafe.copy[Swap]Memory().
+         */
+        while (from < to) {
+            res[i++] = a[from++] << 24 | (a[from++] & 0xFF) << 16
+                    | (a[from++] & 0xFF) << 8 | (a[from++] & 0xFF);
+        }
+        /* Convert to two's complement. Here, i == res.length */
+        while (--i >= 0 && res[i] == 0)
+            ;  // empty body
+        res[i] = -res[i];
+        while (--i >= 0) {
+            res[i] = ~res[i];
+        }
+        return res;
     }
 
     /**

test/jdk/java/math/BigInteger/ByteArrayConstructorTest.java

@@ -0,0 +1,148 @@
+/*
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import jdk.test.lib.RandomFactory;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.Test;
+
+import java.math.Accessor;
+import java.math.BigInteger;
+import java.util.Random;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * @test
+ * @bug 8319174
+ * @summary Exercises minimality of BigInteger.mag field (use -Dseed=X to set PRANDOM seed)
+ * @library /test/lib
+ * @build jdk.test.lib.RandomFactory
+ * @build java.base/java.math.Accessor
+ * @key randomness
+ * @run junit/othervm -DmaxDurationMillis=3000 ByteArrayConstructorTest
+ */
+public class ByteArrayConstructorTest {
+
+    private static final int DEFAULT_MAX_DURATION_MILLIS = 3_000;
+
+    public static final int N = 1_024;
+
+    private static int maxDurationMillis;
+    private static final Random rnd = RandomFactory.getRandom();
+    private volatile boolean stop = false;
+
+    @BeforeAll
+    static void setMaxDurationMillis() {
+        maxDurationMillis = Math.max(maxDurationMillis(), 0);
+    }
+
+    @Test
+    public void testNonNegative() throws InterruptedException {
+        byte[] ba = nonNegativeBytes();
+        doBigIntegers(ba, ba[0]);  // a mask to flip to 0 and back to ba[0]
+    }
+
+    @Test
+    public void testNegative() throws InterruptedException {
+        byte[] ba = negativeBytes();
+        doBigIntegers(ba, (byte) ~ba[0]);  // a mask to flip to -1 and back to ba[0]
+    }
+
+    /*
+     * Starts a thread th that keeps flipping the "sign" byte in the array ba
+     * from the original value to 0 or -1 and back, depending on ba[0] being
+     * non-negative or negative, resp.
+     * (ba is "big endian", the least significant byte is the one with the
+     * highest index.)
+     *
+     * In the meantime, the current thread keeps creating BigInteger instances
+     * with ba and checks that the internal invariant holds, despite the
+     * attempts by thread th to racily modify ba.
+     * It does so at least as indicated by maxDurationMillis.
+     *
+     * Finally, this thread requests th to stop and joins with it, either
+     * because maxDurationMillis has expired, or because of an invalid invariant.
+     */
+    private void doBigIntegers(byte[] ba, byte mask) throws InterruptedException {
+        Thread th = new Thread(() -> {
+            while (!stop) {
+                ba[0] ^= mask;
+            }
+        });
+        th.start();
+
+        try {
+            createBigIntegers(maxDurationMillis, ba);
+        } finally {
+            stop = true;
+            th.join(1_000);
+        }
+    }
+
+    private void createBigIntegers(int maxDurationMillis, byte[] ba) {
+        long start = System.currentTimeMillis();
+        while (System.currentTimeMillis() - start < maxDurationMillis) {
+            BigInteger bi = new BigInteger(ba);
+            int[] mag = Accessor.mag(bi);
+            assertTrue(mag.length == 0 || mag[0] != 0,
+                    String.format("inconsistent BigInteger: mag.length=%d, mag[0]=%d",
+                        mag.length, mag[0]));
+        }
+    }
+
+    private byte[] nonNegativeBytes() {
+        byte[] ba = new byte[1 + N];
+        byte b0;
+        while ((b0 = (byte) rnd.nextInt()) < 0);  // empty body
+        rnd.nextBytes(ba);
+        ba[0] = b0;
+        /* Except for ba[0], fill most significant half with zeros. */
+        for (int i = 1; i <= N / 2; ++i) {
+            ba[i] = 0;
+        }
+        return ba;
+    }
+
+    private byte[] negativeBytes() {
+        byte[] ba = new byte[1 + N];
+        byte b0;
+        while ((b0 = (byte) rnd.nextInt()) >= 0);  // empty body
+        rnd.nextBytes(ba);
+        ba[0] = b0;
+        /* Except for ba[0], fill most significant half with -1 bytes. */
+        for (int i = 1; i <= N / 2; ++i) {
+            ba[i] = -1;
+        }
+        return ba;
+    }
+
+    private static int maxDurationMillis() {
+        try {
+            return Integer.parseInt(System.getProperty("maxDurationMillis",
+                    Integer.toString(DEFAULT_MAX_DURATION_MILLIS)));
+        } catch (NumberFormatException ignore) {
+        }
+        return DEFAULT_MAX_DURATION_MILLIS;
+    }
+
+}