[PATCH] Cygwin: c32rtomb: add missing check for invalid UNICODE character

[Takashi Yano]

On Wed, 14 Jan 2026 23:31:06 +0100
Corinna Vinschen wrote:
> From: Corinna Vinschen <corinna@vinschen.de>
> 
> c32rtomb neglects to check the input character for being outside
> the valid UNICODE planes.  It happily converts the invalid character
> into a valid (but wrong) surrogate pair and carries on.
> 
> Add a check so characters beyond 0x10ffff are not converted anymore.
> Return -1 with errno set to EILSEQ instead.
> 
> Fixes: 4f258c55e87f ("Cygwin: Add ISO C11 functions c16rtomb, c32rtomb, mbrtoc16, mbrtoc32.")
> Signed-off-by: Corinna Vinschen <corinna@vinschen.de>
> ---
>  winsup/cygwin/release/3.6.7 | 5 +++++
>  winsup/cygwin/strfuncs.cc   | 7 +++++++
>  2 files changed, 12 insertions(+)
>  create mode 100644 winsup/cygwin/release/3.6.7
> 
> diff --git a/winsup/cygwin/release/3.6.7 b/winsup/cygwin/release/3.6.7
> new file mode 100644
> index 000000000000..defe55ffe75e
> --- /dev/null
> +++ b/winsup/cygwin/release/3.6.7
> @@ -0,0 +1,5 @@
> +Fixes:
> +------
> +
> +- Guard c32rtomb against invalid input characters.
> +  Addresses a testsuite error in current gawk git master.
> diff --git a/winsup/cygwin/strfuncs.cc b/winsup/cygwin/strfuncs.cc
> index eb6576051d90..0cf41cefc8a2 100644
> --- a/winsup/cygwin/strfuncs.cc
> +++ b/winsup/cygwin/strfuncs.cc
> @@ -146,6 +146,13 @@ c32rtomb (char *s, char32_t wc, mbstate_t *ps)
>      if (wc <= 0xffff || !s)
>        return wcrtomb (s, (wchar_t) wc, ps);
>  
> +    /* Check for character outside valid UNICODE planes */
> +    if (wc > 0x10ffff)
> +      {
> +	_REENT_ERRNO(_REENT) = EILSEQ;
> +	return (size_t)(-1);
> +      }
> +
>      wchar_t wc_arr[2];
>      const wchar_t *wcp = wc_arr;
>  
> -- 
> 2.52.0

LGTM. What does this change address for?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>