[ruby-core:125497] [Ruby Bug#22070] `Thread.each_caller_location(1, 1)` segfaults when called from a cfunc

13 May 2026

      Issue #22070 has been reported by AMomchilov (Alexander Momchilov).

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Bug #22070: `Thread.each_caller_location(1, 1)` segfaults when called from a cfunc
https://bugs.ruby-lang.org/issues/22070

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* ruby -v: ruby 4.0.2 (2026-03-17 revision d3da9fec82) +PRISM [arm64-darwin24]
* Backport: 3.3: UNKNOWN, 3.4: UNKNOWN, 4.0: UNKNOWN
----------------------------------------
Reading the `label` of a `Thread::Backtrace::Location` (directly or indirectly via e.g. `to_s`) segfauls if called from within `Thread.each_caller_location(1, 1) { it.label }`.

Reading the `lineno` or `path` seems to not cause any problems, perhaps by coincidence.

## Minimal repro

```sh
ruby -e 'tap { Thread.each_caller_location(1, 1) { it.label } }' # 🧨 Boom
```

## More cases

<details><summary><code>repro.rb</code></summary>

```ruby
# All four conditions must hold; varying any one makes the crash go away:
#
# 1. The yielded location is inspected via a `cme`-reading method
#    (`label`, `base_label`, `to_s`, `inspect`).
#    * The `iseq`-based methods (`path`, `absolute_path`, `lineno`) seem to survive by accident.
# 2. `start` is exactly 1.
# 3. `length` is exactly 1.
#    * Ranges that  compute to length=1 (`1..1`, `1...2`) crash
#    * e.g. `1..3` (length=3) does not.
# 4. The caller is inside a C-method frame (`tap`, `Array#each`, `instance_exec`, `eval`, ...).
#    * Top-level and plain Ruby methods are safe.

$ruby = begin
  require "rbconfig"
  RbConfig.ruby
rescue LoadError
  ENV["_"] || "ruby"
end

def check(label, code)
  out = IO.popen([$ruby, "-e", code, err: [:child, :out]], &:read)
  crashed = !$?.success? || out.include?("[BUG]")
  printf "  %-7s %s\n", (crashed ? "CRASH" : "OK"), label
end

puts "== Which Location method is accessed (start=1, length=1, from tap{}) =="
check("it.path",          "tap { Thread.each_caller_location(1, 1) { it.path } }")
check("it.absolute_path", "tap { Thread.each_caller_location(1, 1) { it.absolute_path } }")
check("it.lineno",        "tap { Thread.each_caller_location(1, 1) { it.lineno } }")
check("it.label",         "tap { Thread.each_caller_location(1, 1) { it.label } }")
check("it.base_label",    "tap { Thread.each_caller_location(1, 1) { it.base_label } }")
check("it.to_s",          "tap { Thread.each_caller_location(1, 1) { it.to_s } }")
check("it.inspect",       "tap { Thread.each_caller_location(1, 1) { it.inspect } }")

puts "== Vary `start` (length=1, .label, from tap{}) =="
check("start=0", "tap { Thread.each_caller_location(0, 1) { it.label } }")
check("start=1", "tap { Thread.each_caller_location(1, 1) { it.label } }")
check("start=2", "tap { Thread.each_caller_location(2, 1) { it.label } }")
check("start=3", "tap { Thread.each_caller_location(3, 1) { it.label } }")

puts "== Vary `length` (start=1, .label, from tap{}) =="
check("length=1", "tap { Thread.each_caller_location(1, 1) { it.label } }")
check("length=2", "tap { Thread.each_caller_location(1, 2) { it.label } }")
check("length=3", "tap { Thread.each_caller_location(1, 3) { it.label } }")
check("length=4", "tap { Thread.each_caller_location(1, 4) { it.label } }")

puts "== Range forms equivalent to (start=1, length=1) =="
check("(1, 1)",  "tap { Thread.each_caller_location(1, 1) { it.label } }")
check("(1..1)",  "tap { Thread.each_caller_location(1..1) { it.label } }")
check("(1...2)", "tap { Thread.each_caller_location(1...2) { it.label } }")
check("(1..2)",  "tap { Thread.each_caller_location(1..2) { it.label } }")

puts "== Caller context (start=1, length=1, .label) =="
check("top-level",          'Thread.each_caller_location(1, 1) { it.label }')
check("tap { ... }",        'tap { Thread.each_caller_location(1, 1) { it.label } }')
check("[1].each { ... }",   '[1].each { Thread.each_caller_location(1, 1) { it.label } }')
check("instance_exec",      'instance_exec { Thread.each_caller_location(1, 1) { it.label } }')
check("eval '...'",         %{eval 'Thread.each_caller_location(1, 1) { it.label }'})
check("def m; ...; end; m", 'def m; Thread.each_caller_location(1, 1) { it.label }; end; m')
check("tap { m }",          'def m; Thread.each_caller_location(1, 1) { it.label }; end; tap { m }')

puts
puts "Ruby: #{RUBY_DESCRIPTION}"
```

</details>

-- 
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