Bug #22410: `IO#external_encoding` / `IO#internal_encoding` segfault on an uninitialized IO

• Status: Open
• Assignee: -
• Target version: -
• Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN

[[e-mail] (Yuancheng Jiang)]

The following code:

IO.allocate.external_encoding

(also IO.allocate.internal_encoding, or any IO/File subclass whose initialize does not call super, e.g.
class A < IO; def initialize(*) = nil; end; A.new.external_encoding; Marshal.dump of such an object reaches the same code via encoding_name.)

Resulted in this output:

io_min.rb:1: [BUG] Segmentation fault at 0x0000000000000068
ruby 4.1.0dev (2026-10-07) +PRISM [x86_64-linux]

-- Ruby level backtrace information ----------------------------------------
io_min.rb:1:in '<main>'
io_min.rb:1:in 'external_encoding'

-- C level backtrace information -------------------------------------------
(rb_bug_for_fatal_signal+0x3b8) error.c:1183
(sigsegv+0x97) signal.c:982
libc.so.6
(rb_io_external_encoding+0x8d) io.c:13775
(vm_call_cfunc_with_frame_+0x27b) ../vm_insnhelper.c:3906
(vm_call_method_each_type+0x24a) ../vm_insnhelper.c:4898
(vm_call_method+0x39f)
(vm_exec_core+0x113bc) ../vm_insnhelper.c:6285
(rb_vm_exec+0x242) vm.c:2909
(rb_ec_exec_node+0x56) eval.c:300
(ruby_run_node) eval.c:338

rb_io_external_encoding and rb_io_internal_encoding read RFILE(rb_io_taint_check(io))->fptr and dereference it
without rb_io_check_initialized/GetOpenFile, so fptr == NULL crashes. Other IO methods raise
IOError (uninitialized stream) here. Also reproduces on a plain release build (Ubuntu's ruby 3.2.3).

To reproduce:

ruby ./min.rb

Commit:

3296d5c99ce005e4698fb27a405e01d766ad2647 (2026-09-15); also reproduced on master 9ce0df671980d669cbab8afc48124c7453897533 (2026-10-07)

Build configuration:

../configure --disable-install-doc CC=clang-18 cflags="-fsanitize=address -fno-omit-frame-pointer -DUSE_MN_THREADS=0" cppflags="-DRUBY_DEBUG=1" optflags="-O1" debugflags="-g"

Operating System:

Ubuntu 24.04.4 LTS (x86_64), clang 18.1.3

This bug was found by fusion-fuzz