Fix creating subdirectories under directories with trailing separators

src/System.IO.FileSystem/src/System/IO/DirectoryInfo.cs

@@ -61,17 +61,22 @@ public DirectoryInfo CreateSubdirectory(string path)
             if (Path.IsPathRooted(path))
                 throw new ArgumentException(SR.Arg_Path2IsRooted, nameof(path));
 
-            string fullPath = Path.GetFullPath(Path.Combine(FullPath, path));
+            string newPath = Path.GetFullPath(Path.Combine(FullPath, path));
 
-            if (fullPath.Length < FullPath.Length 
-                || (fullPath.Length > FullPath.Length && !PathInternal.IsDirectorySeparator(fullPath[FullPath.Length]))
-                || string.Compare(FullPath, 0, fullPath, 0, FullPath.Length, PathInternal.StringComparison) != 0)
+            ReadOnlySpan<char> trimmedNewPath = PathInternal.TrimEndingDirectorySeparator(newPath.AsSpan());
+            ReadOnlySpan<char> trimmedCurrentPath = PathInternal.TrimEndingDirectorySeparator(FullPath.AsSpan());
+
+            // We want to make sure the requested directory is actually under the subdirectory.
+            if (trimmedNewPath.StartsWith(trimmedCurrentPath, PathInternal.StringComparison)
+                // Allow the exact same path, but prevent allowing "..\FooBar" through when the directory is "Foo"
+                && ((trimmedNewPath.Length == trimmedCurrentPath.Length) || PathInternal.IsDirectorySeparator(newPath[trimmedCurrentPath.Length])))
             {
-                throw new ArgumentException(SR.Format(SR.Argument_InvalidSubPath, path, FullPath), nameof(path));
+                FileSystem.CreateDirectory(newPath);
+                return new DirectoryInfo(newPath);
             }
 
-            FileSystem.CreateDirectory(fullPath);
-            return new DirectoryInfo(fullPath);
+            // We weren't nested
+            throw new ArgumentException(SR.Format(SR.Argument_InvalidSubPath, path, FullPath), nameof(path));
         }
 
         public void Create() => FileSystem.CreateDirectory(FullPath);

src/System.IO.FileSystem/tests/DirectoryInfo/CreateSubdirectory.cs

@@ -89,17 +89,41 @@ public void SubDirectoryIsParentDirectory_ThrowsArgumentException()
             Assert.Throws<ArgumentException>(() => new DirectoryInfo(TestDirectory + "/path").CreateSubdirectory("../../path2"));
         }
 
+        [Fact]
+        [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework)]
+        public void SubdirectoryOverlappingName_ThrowsArgumentException()
+        {
+            // What we're looking for here is trying to create C:\FooBar under C:\Foo by passing "..\FooBar"
+            DirectoryInfo info = Directory.CreateDirectory(GetTestFilePath());
+
+            string overlappingName = ".." + Path.DirectorySeparatorChar + info.Name + "overlap";
+
+            Assert.Throws<ArgumentException>(() => info.CreateSubdirectory(overlappingName));
+
+            // Now try with an info with a trailing separator
+            info = new DirectoryInfo(info.FullName + Path.DirectorySeparatorChar);
+            Assert.Throws<ArgumentException>(() => info.CreateSubdirectory(overlappingName));
+        }
+
         [Theory,
             MemberData(nameof(ValidPathComponentNames))]
         public void ValidPathWithTrailingSlash(string component)
         {
             DirectoryInfo testDir = Directory.CreateDirectory(GetTestFilePath());
 
-            string path = IOServices.AddTrailingSlashIfNeeded(component);
+            string path = component + Path.DirectorySeparatorChar;
             DirectoryInfo result = new DirectoryInfo(testDir.FullName).CreateSubdirectory(path);
 
             Assert.Equal(Path.Combine(testDir.FullName, path), result.FullName);
             Assert.True(Directory.Exists(result.FullName));
+
+            // Now try creating subdirectories when the directory info itself has a slash
+            testDir = Directory.CreateDirectory(GetTestFilePath() + Path.DirectorySeparatorChar);
+
+            result = new DirectoryInfo(testDir.FullName).CreateSubdirectory(path);
+
+            Assert.Equal(Path.Combine(testDir.FullName, path), result.FullName);
+            Assert.True(Directory.Exists(result.FullName));
         }
 
         [Theory,
@@ -114,6 +138,13 @@ public void ValidPathWithoutTrailingSlash(string component)
             Assert.Equal(Path.Combine(testDir.FullName, path), result.FullName);
             Assert.True(Directory.Exists(result.FullName));
 
+            // Now try creating subdirectories when the directory info itself has a slash
+            testDir = Directory.CreateDirectory(GetTestFilePath() + Path.DirectorySeparatorChar);
+
+            result = new DirectoryInfo(testDir.FullName).CreateSubdirectory(path);
+
+            Assert.Equal(Path.Combine(testDir.FullName, path), result.FullName);
+            Assert.True(Directory.Exists(result.FullName));
         }
 
         [Fact]