Tar: Fix buffer is too small error when writing Prefix on large filenames

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarHeader.Write.cs

@@ -374,12 +374,9 @@ private int WritePosixName(Span<byte> buffer)
 
             if (_name.Length > FieldLengths.Name)
             {
-                int prefixBytesLength = Math.Min(_name.Length - FieldLengths.Name, FieldLengths.Name);
-                Span<byte> remaining = prefixBytesLength <= 256 ?
-                    stackalloc byte[prefixBytesLength] :
-                    new byte[prefixBytesLength];
-
-                int encoded = Encoding.ASCII.GetBytes(_name.AsSpan(FieldLengths.Name), remaining);
+                int prefixBytesLength = Math.Min(_name.Length - FieldLengths.Name, FieldLengths.Prefix);
+                Span<byte> remaining = stackalloc byte[prefixBytesLength];
+                int encoded = Encoding.ASCII.GetBytes(_name.AsSpan(FieldLengths.Name, prefixBytesLength), remaining);
                 Debug.Assert(encoded == remaining.Length);
 
                 checksum += WriteLeftAlignedBytesAndGetChecksum(remaining, buffer.Slice(FieldLocations.Prefix, FieldLengths.Prefix));

src/libraries/System.Formats.Tar/tests/TarFile/TarFile.CreateFromDirectory.File.Tests.cs

@@ -3,7 +3,9 @@
 
 using System.Collections.Generic;
 using System.IO;
+using System.IO.Enumeration;
 using System.Linq;
+using Microsoft.DotNet.XUnitExtensions;
 using Xunit;
 
 namespace System.Formats.Tar.Tests
@@ -253,5 +255,56 @@ public void SkipRecursionIntoBaseDirectorySymlink()
 
             Assert.Null(reader.GetNextEntry());
         }
+
+        [ConditionalTheory(nameof(CanExtractWithTarTool))]
+        [MemberData(nameof(GetPaxAndGnuTestCaseNames))]
+        public void ResultingArchive_CanBeExtractedByOtherTools(string testCaseName)
+        {
+            if (testCaseName == "specialfiles" && !PlatformDetection.IsSuperUser)
+            {
+                throw new SkipTestException("specialfiles is only supported on Unix and it needs sudo permissions for extraction");
+            }
+
+            using TempDirectory root = new TempDirectory();
+
+            string archivePath = GetTarFilePath(CompressionMethod.Uncompressed, TestTarFormat.pax.ToString(), testCaseName);
+            string tarExtractedPath = Path.Join(root.Path, "tarExtracted");
+            Directory.CreateDirectory(tarExtractedPath);
+
+            // extract with /usr/bin/tar
+            ExtractWithTarTool(archivePath, tarExtractedPath);
+
+            // create archive with TarFile
+            string dotnetTarArchive = Path.Join(root.Path, "dotnetTarArchive.tar");
+            TarFile.CreateFromDirectory(tarExtractedPath, dotnetTarArchive, includeBaseDirectory: false);
+
+            // extract TarFile's archive with /usr/bin/tar
+            string dotnetTarExtractedPath = Path.Join(root.Path, "dotnetTarExtracted");
+            Directory.CreateDirectory(dotnetTarExtractedPath);
+            ExtractWithTarTool(dotnetTarArchive, dotnetTarExtractedPath);
+
+            // compare results
+            Dictionary<string, FileSystemInfo> expectedEntries = GetFileSystemInfosRecursive(tarExtractedPath).ToDictionary(fsi => fsi.FullName.Substring(tarExtractedPath.Length));
+
+            foreach (var actualEntry in GetFileSystemInfosRecursive(dotnetTarExtractedPath))
+            {
+                string keyPath = actualEntry.FullName.Substring(dotnetTarExtractedPath.Length);
+
+                Assert.True(expectedEntries.TryGetValue(keyPath, out FileSystemInfo expectedEntry), "Entry not expected.");
+                Assert.Equal(expectedEntry.Attributes, actualEntry.Attributes);
+                Assert.Equal(expectedEntry.LinkTarget, actualEntry.LinkTarget);
+
+                expectedEntries.Remove(keyPath);
+            }
+
+            // All expected entries should've been found and removed.
+            Assert.Equal(0, expectedEntries.Count);
+        }
+
+        private static FileSystemEnumerable<FileSystemInfo> GetFileSystemInfosRecursive(string path)
+        {
+            var enumerationOptions = new EnumerationOptions { RecurseSubdirectories = true };
+            return new FileSystemEnumerable<FileSystemInfo>(path, (ref FileSystemEntry e) => e.ToFileSystemInfo(), enumerationOptions);
+        }
     }
 }

src/libraries/System.Formats.Tar/tests/TarTestsBase.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.IO;
 using System.Linq;
 using System.Runtime.CompilerServices;
@@ -611,5 +612,37 @@ public static IEnumerable<object[]> GetPaxAndGnuTestCaseNames()
                 yield return new object[] { name };
             }
         }
+
+        public static bool CanExtractWithTarTool = CanExtractWithTarTool_Method();
+
+        private static bool CanExtractWithTarTool_Method()
+        {
+            using Process tarToolProcess = new Process();
+            tarToolProcess.StartInfo.FileName = "tar"; // location varies: find it on the PATH.
+            tarToolProcess.StartInfo.Arguments = "--help";
+
+            tarToolProcess.StartInfo.RedirectStandardOutput = true;
+            tarToolProcess.Start();
+
+            tarToolProcess.StandardOutput.ReadToEnd();
+            tarToolProcess.WaitForExit();
+
+            return (tarToolProcess.ExitCode == 0);
+        }
+
+        public static void ExtractWithTarTool(string source, string destination)
+        {
+            using Process tarToolProcess = new Process();
+            tarToolProcess.StartInfo.FileName = "tar"; // location varies: find it on the PATH.
+            tarToolProcess.StartInfo.Arguments = $"-xf {source} -C {destination}";
+
+            tarToolProcess.StartInfo.RedirectStandardOutput = true;
+            tarToolProcess.Start();
+
+            tarToolProcess.StandardOutput.ReadToEnd();
+            tarToolProcess.WaitForExit();
+
+            Assert.Equal(0, tarToolProcess.ExitCode);
+        }
     }
 }