Index: src/pkg/archive/tar/writer.go
===================================================================
--- a/src/pkg/archive/tar/writer.go
+++ b/src/pkg/archive/tar/writer.go
@@ -263,8 +263,11 @@
 		length--
 	}
 	i := strings.LastIndex(name[:length], "/")
-	nlen := length - i - 1
-	if i <= 0 || nlen > fileNameSize || nlen == 0 {
+	// nlen contains the resulting length in the name field.
+	// plen contains the resulting length in the prefix field.
+	nlen := len(name) - i - 1
+	plen := i
+	if i <= 0 || nlen > fileNameSize || nlen == 0 || plen > fileNamePrefixSize {
 		err = errNameTooLong
 		return
 	}
Index: src/pkg/archive/tar/writer_test.go
===================================================================
--- a/src/pkg/archive/tar/writer_test.go
+++ b/src/pkg/archive/tar/writer_test.go
@@ -355,3 +355,39 @@
 		}
 	}
 }
+
+func TestUSTARLongName(t *testing.T) {
+	// Create an archive with a path that failed to split with USTAR extension in previous versions.
+	fileinfo, err := os.Stat("testdata/small.txt")
+	if err != nil {
+		t.Fatal(err)
+	}
+	hdr, err := FileInfoHeader(fileinfo, "")
+	hdr.Typeflag = TypeDir
+	if err != nil {
+		t.Fatalf("os.Stat:1 %v", err)
+	}
+	// Force a PAX long name to be written. The name was taken from a practical example
+	// that fails and replaced ever char through numbers to anonymize the sample.
+	longName := "/0000_0000000/00000-000000000/0000_0000000/00000-0000000000000/0000_0000000/00000-0000000-00000000/0000_0000000/00000000/0000_0000000/000/0000_0000000/00000000v00/0000_0000000/000000/0000_0000000/0000000/0000_0000000/00000y-00/0000/0000/00000000/0x000000/"
+	hdr.Name = longName
+
+	hdr.Size = 0
+	var buf bytes.Buffer
+	writer := NewWriter(&buf)
+	if err := writer.WriteHeader(hdr); err != nil {
+		t.Fatal(err)
+	}
+	if err := writer.Close(); err != nil {
+		t.Fatal(err)
+	}
+	// Test that we can get a long name back out of the archive.
+	reader := NewReader(&buf)
+	hdr, err = reader.Next()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if hdr.Name != longName {
+		t.Fatal("Couldn't recover long name")
+	}
+}