From 7a953f4a0ab7a5147cd817f1d2730447769b5c72 Mon Sep 17 00:00:00 2001
From: koh-sh <34917718+koh-sh@users.noreply.github.com>
Date: Sat, 9 Aug 2025 12:13:52 +0900
Subject: [PATCH 1/2] fix: prevent segfault in StringIO#seek with SEEK_END on
 null device

---
 ext/stringio/stringio.c        |  6 +++++-
 test/stringio/test_stringio.rb | 10 ++++++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 45899bd9..17ef3fc6 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -837,7 +837,11 @@ strio_seek(int argc, VALUE *argv, VALUE self)
 	offset = ptr->pos;
 	break;
       case 2:
-	offset = RSTRING_LEN(ptr->string);
+	if (NIL_P(ptr->string)) {
+	    offset = 0;
+	} else {
+	    offset = RSTRING_LEN(ptr->string);
+	}
 	break;
       default:
 	error_inval("invalid whence");
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index 002b946b..4c9cf374 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -70,6 +70,16 @@ def test_null
     assert_nil io.getc
   end
 
+  def test_seek_null
+    io = StringIO.new(nil)
+    assert_equal(0, io.seek(0, IO::SEEK_SET))
+    assert_equal(0, io.pos)
+    assert_equal(0, io.seek(0, IO::SEEK_CUR))
+    assert_equal(0, io.pos)
+    assert_equal(0, io.seek(0, IO::SEEK_END))  # This should not segfault
+    assert_equal(0, io.pos)
+  end
+
   def test_truncate
     io = StringIO.new("")
     io.puts "abc"

From ac0ad2321e2cc0e6c8a553cc6d664c29f57f2a87 Mon Sep 17 00:00:00 2001
From: koh-sh <34917718+koh-sh@users.noreply.github.com>
Date: Sat, 9 Aug 2025 17:51:44 +0900
Subject: [PATCH 2/2] fix: prevent NullPointerException in StringIO#seek with
 SEEK_END on null device (JRuby)

---
 ext/java/org/jruby/ext/stringio/StringIO.java | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/ext/java/org/jruby/ext/stringio/StringIO.java b/ext/java/org/jruby/ext/stringio/StringIO.java
index 2002e2bb..83e331ec 100644
--- a/ext/java/org/jruby/ext/stringio/StringIO.java
+++ b/ext/java/org/jruby/ext/stringio/StringIO.java
@@ -1378,7 +1378,11 @@ private RubyFixnum seekCommon(ThreadContext context, int argc, IRubyObject arg0,
                     offset += ptr.pos;
                     break;
                 case 2:
-                    offset += ptr.string.size();
+                    if (ptr.string == null) {
+                        offset += 0;
+                    } else {
+                        offset += ptr.string.size();
+                    }
                     break;
                 default:
                     throw runtime.newErrnoEINVALError("invalid whence");