libc: locale: fix EUC shift check

wchar_t is unsigned on ARM platforms, and signed pretty much everywhere
else.  On signed platforms, `nm` ends up with bogus upper bits set if we
did in-fact have a valid CS2 or CS3 (MSB set).  Mask just the low byte
to avoid sign bit garbage.

Bare basic test of converting a CS2 widechar in eucCN, which would
previously kick back an EILSEQ.

Reviewed by:	bapt, rew
Sponsored by:	Klara, Inc.
Differential Revision:	https://reviews.freebsd.org/D43262

lib/libc/locale/euc.c

@@ -426,7 +426,7 @@ _EUC_wcrtomb_impl(char * __restrict s, wchar_t wc,
 	/* This first check excludes CS1, which is implicitly valid. */
 	if ((wc < 0xa100) || (wc > 0xffff)) {
 		/* Check for valid CS2 or CS3 */
-		nm = (wc >> ((len - 1) * 8));
+		nm = (wc >> ((len - 1) * 8)) & 0xff;
 		if (nm == cs2) {
 			if (len != cs2width) {
 				errno = EILSEQ;

lib/libc/tests/locale/wctomb_test.c

@@ -41,6 +41,18 @@
 
 #include <atf-c.h>
 
+ATF_TC_WITHOUT_HEAD(euccs1_test);
+ATF_TC_BODY(euccs1_test, tc)
+{
+	wchar_t wc = 0x8e000000;
+	char buf[MB_LEN_MAX];
+
+	ATF_REQUIRE(strcmp(setlocale(LC_CTYPE, "zh_CN.eucCN"),
+	    "zh_CN.eucCN") == 0);
+
+	ATF_REQUIRE(wctomb(&buf[0], wc) == 4);
+}
+
 ATF_TC_WITHOUT_HEAD(wctomb_test);
 ATF_TC_BODY(wctomb_test, tc)
 {
@@ -104,6 +116,7 @@ ATF_TC_BODY(wctomb_test, tc)
 ATF_TP_ADD_TCS(tp)
 {
 
+	ATF_TP_ADD_TC(tp, euccs1_test);
 	ATF_TP_ADD_TC(tp, wctomb_test);
 
 	return (atf_no_error());