[newlib-cygwin/cygwin-3_6-branch] Cygwin: c32rtomb: add missing check for invalid UNICODE character

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=53cfd724332d0004a1b963d180308680a2b4b60d

commit 53cfd724332d0004a1b963d180308680a2b4b60d
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Wed Jan 14 20:31:18 2026 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Fri Jan 16 12:33:28 2026 +0100

    Cygwin: c32rtomb: add missing check for invalid UNICODE character
    
    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>
    (cherry picked from commit 7c9c9bcfcf59f43fd9776c6a809ba67eef76bcb4)

Diff:
---
 winsup/cygwin/release/3.6.7 | 5 +++++
 winsup/cygwin/strfuncs.cc   | 7 +++++++
 2 files changed, 12 insertions(+)

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;