From 23c0113932407abccddbc6ee5b297d38d2d2bb9c Mon Sep 17 00:00:00 2001
From: koh-sh <34917718+koh-sh@users.noreply.github.com>
Date: Sat, 9 Aug 2025 18:30:17 +0900
Subject: [PATCH] [ruby/stringio] fix: prevent segfault in StringIO#seek with
 SEEK_END on null device (https://github.com/ruby/stringio/pull/137)

Fixes segmentation fault when calling `seek` with `SEEK_END` on null
device StringIO created by
  `StringIO.new(nil)`.

```bash
ruby -e "require 'stringio'; StringIO.new(nil).seek(0, IO::SEEK_END)"
```

I tested with below versions.

```bash
[koh@Kohs-MacBook-Pro] ~
% ruby -v;gem info stringio;sw_vers
ruby 3.4.5 (2025-07-16 revision https://github.com/ruby/stringio/commit/20cda200d3) +PRISM [arm64-darwin24]

*** LOCAL GEMS ***

stringio (3.1.2)
    Authors: Nobu Nakada, Charles Oliver Nutter
    Homepage: https://github.com/ruby/stringio
    Licenses: Ruby, BSD-2-Clause
    Installed at (default): /Users/koh/.local/share/mise/installs/ruby/3.4.5/lib/ruby/gems/3.4.0

    Pseudo IO on String
ProductName:            macOS
ProductVersion:         15.5
BuildVersion:           24F74
[koh@Kohs-MacBook-Pro] ~
%
```

https://github.com/ruby/stringio/commit/9399747bf9
---
 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 3003939e10..d9beb25434 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 002b946b6f..4c9cf37425 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -70,6 +70,16 @@ class TestStringIO < Test::Unit::TestCase
     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"