Add BoundedMemory tests for String ctor (dotnet/corefx#36162)

Use the new BoundedMemory feature to put additional correctness checks around the pointer-based String ctors

Commit migrated from https://github.com/dotnet/corefx/commit/5cf8f3b1785b44fcf72492847f12fd894d0da74b

diff --git a/src/libraries/System.Runtime/tests/System/StringTests.netcoreapp.cs b/src/libraries/System.Runtime/tests/System/StringTests.netcoreapp.cs
index d093c58..754484f 100644 (file)
--- a/src/libraries/System.Runtime/tests/System/StringTests.netcoreapp.cs
+++ b/src/libraries/System.Runtime/tests/System/StringTests.netcoreapp.cs
@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Buffers;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
@@ -42,6 +43,52 @@ namespace System.Tests
         }
 
         [Fact]
+        public static unsafe void Ctor_CharPtr_DoesNotAccessInvalidPage()
+        {
+            // Allocates a buffer of all 'x' followed by a null terminator,
+            // then attempts to create a string instance from this at various offsets.
+
+            const int MaxCharCount = 128;
+            using BoundedMemory<char> boundedMemory = BoundedMemory.Allocate<char>(MaxCharCount);
+            boundedMemory.Span.Fill('x');
+            boundedMemory.Span[MaxCharCount - 1] = '\0';
+            boundedMemory.MakeReadonly();
+
+            using MemoryHandle memoryHandle = boundedMemory.Memory.Pin();
+
+            for (int i = 0; i < MaxCharCount; i++)
+            {
+                string expectedString = new string('x', MaxCharCount - i - 1);
+                string actualString = new string((char*)memoryHandle.Pointer + i);
+                Assert.Equal(expectedString, actualString);
+            }
+        }
+
+        [ConditionalFact(nameof(IsSimpleActiveCodePage))]
+        public static unsafe void Ctor_SBytePtr_DoesNotAccessInvalidPage()
+        {
+            // Allocates a buffer of all ' ' followed by a null terminator,
+            // then attempts to create a string instance from this at various offsets.
+            // We use U+0020 SPACE instead of any other character because it lives
+            // at offset 0x20 across every supported code page.
+
+            const int MaxByteCount = 128;
+            using BoundedMemory<sbyte> boundedMemory = BoundedMemory.Allocate<sbyte>(MaxByteCount);
+            boundedMemory.Span.Fill((sbyte)' ');
+            boundedMemory.Span[MaxByteCount - 1] = (sbyte)'\0';
+            boundedMemory.MakeReadonly();
+
+            using MemoryHandle memoryHandle = boundedMemory.Memory.Pin();
+
+            for (int i = 0; i < MaxByteCount; i++)
+            {
+                string expectedString = new string(' ', MaxByteCount - i - 1);
+                string actualString = new string((sbyte*)memoryHandle.Pointer + i);
+                Assert.Equal(expectedString, actualString);
+            }
+        }
+
+        [Fact]
         public static void Create_InvalidArguments_Throw()
         {
             AssertExtensions.Throws<ArgumentNullException>("action", () => string.Create(-1, 0, null));
@@ -1109,5 +1156,28 @@ namespace System.Tests
             range = new Range(Index.FromEnd(s.Length + 1), Index.FromEnd(0));
             Assert.Throws<ArgumentOutOfRangeException>(() => s1 = s.Substring(range));
         }
+
+        /// <summary>
+        /// Returns true only if U+0020 SPACE is represented as the single byte 0x20 in the active code page.
+        /// </summary>
+        public unsafe static bool IsSimpleActiveCodePage
+        {
+            get
+            {
+                IntPtr pAnsiStr = IntPtr.Zero;
+                try
+                {
+                    pAnsiStr = Marshal.StringToHGlobalAnsi(" ");
+                    return ((byte*)pAnsiStr)[0] == (byte)' ' && ((byte*)pAnsiStr)[1] == (byte)'\0';
+                }
+                finally
+                {
+                    if (pAnsiStr != IntPtr.Zero)
+                    {
+                        Marshal.FreeHGlobal(pAnsiStr);
+                    }
+                }
+            }
+        }
     }
 }