Support running in root less containers (#23)

* initial

* fixes

src/LockCheck/Linux/NativeMethods.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Buffers;
 using System.IO;
+using System.Reflection.Metadata;
 using System.Runtime.InteropServices;
+using System.Text;
 
 #pragma warning disable IDE1006 // Naming Styles - off here, because we want to use native names
 
@@ -134,6 +137,62 @@ internal unsafe struct Passwd
         [LibraryImport(SystemNative, EntryPoint = "SystemNative_GetPwUidR", SetLastError = false)]
         internal static unsafe partial int GetPwUidR(uint uid, out Passwd pwd, byte* buf, int bufLen);
 
+        [LibraryImport(SystemNative, EntryPoint = "SystemNative_ReadLink", SetLastError = true)]
+        private static partial int ReadLink(ref byte path, ref byte buffer, int bufferSize);
+
+        internal static string ReadLink(ReadOnlySpan<char> path)
+        {
+            const int StackBufferSize = 256;
+
+            Span<byte> spanBuffer = stackalloc byte[StackBufferSize];
+            byte[] arrayBuffer = null;
+
+            // Convert path to UTF-8 bytes, zero terminated - CLR code used internal ValueUtf8Converter for this.
+            int maxSize = checked(Encoding.UTF8.GetMaxByteCount(path.Length) + 1);
+            Span<byte> pathBytes = maxSize <= StackBufferSize ? stackalloc byte[maxSize] : new byte[maxSize];
+            int pathBytesCount = Encoding.UTF8.GetBytes(path, pathBytes);
+            pathBytes[pathBytesCount] = 0;
+            pathBytes = pathBytes.Slice(0, pathBytesCount + 1);
+            ref byte pathReference = ref MemoryMarshal.GetReference(pathBytes);
+
+            while (true)
+            {
+                int error = 0;
+                try
+                {
+                    int resultLength = ReadLink(ref pathReference, ref MemoryMarshal.GetReference(spanBuffer), spanBuffer.Length);
+
+                    if (resultLength < 0)
+                    {
+                        // error
+                        error = Marshal.GetLastPInvokeError();
+                        return null;
+                    }
+                    else if (resultLength < spanBuffer.Length)
+                    {
+                        // success
+                        return Encoding.UTF8.GetString(spanBuffer.Slice(0, resultLength));
+                    }
+                }
+                finally
+                {
+                    if (arrayBuffer != null)
+                    {
+                        ArrayPool<byte>.Shared.Return(arrayBuffer);
+                    }
+
+                    if (error > 0)
+                    {
+                        Marshal.SetLastPInvokeError(error);
+                    }
+                }
+
+                // Output buffer was too small, loop around again and try with a larger buffer.
+                arrayBuffer = ArrayPool<byte>.Shared.Rent(spanBuffer.Length * 2);
+                spanBuffer = arrayBuffer;
+            }
+        }
+
         // ---END-----------------------------------------------------------------------------------
     }
 }