Fix mbsnrtowcs

mbsnrtowcs writes up to __wn wide characters into wchar_t *__d. The destination
capacity is __b / sizeof(wchar_t) wide characters, but the
else branch clamps __n (source byte limit) to __b (destination byte size).

__wn (the actual output count) is passed through unclamped. Example: __b=8
(dest holds 2 wchar_t), __n=100, __wn=25. The else branch applies (25 <=
100/4), clamps source to 8 bytes, but passes __wn=25 — the function can write
25 wchar_t (100 bytes) into an 8-byte buffer.

The first branch is also wrong: it divides __b (bytes) by sizeof(wchar_t) to
get wchar_t capacity, which is correct for the destination — but the condition
__wn > __n / sizeof(wchar_t) uses integer division that can produce incorrect
routing between branches.

The fix mirrors the already-correct mbsrtowcs pattern: clamp __wn (the output
wide-char count) to the destination's wchar_t capacity, and pass __n (source
byte limit) through unchanged.

3 files changed, 56 insertions, 0 deletions

diff --git a/tests/Makefile b/tests/Makefile
index 9fc8287..6904b2d 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -50,6 +50,8 @@ RUNTIME_TARGETS= \
         test_mempcpy_static_write     \
         test_memset_dynamic_write     \
         test_memset_static_write      \
+        test_mbsnrtowcs_dynamic       \
+        test_mbsnrtowcs_static        \
         test_poll_dynamic             \
         test_poll_static              \
         test_ppoll_dynamic            \
diff --git a/tests/test_mbsnrtowcs_dynamic.c b/tests/test_mbsnrtowcs_dynamic.c
new file mode 100644
index 0000000..77b9082
--- /dev/null
+++ b/tests/test_mbsnrtowcs_dynamic.c
@@ -0,0 +1,28 @@
+#include "common.h"
+
+#include <wchar.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+  wchar_t buffer[4] = {0};
+  const char *src = "ABCDEFGHIJ";
+  const char *srcp = src;
+  mbstate_t st;
+  memset(&st, 0, sizeof(st));
+
+  /* Safe: convert up to 2 source bytes into at most 2 wide chars */
+  srcp = src;
+  mbsnrtowcs(buffer, &srcp, 2, 2, &st);
+
+  /* Unsafe: ask to write argc (10) wide chars into 4-element buffer.
+   * Before the fix, the else branch clamped source bytes instead of
+   * the output wide-char count, allowing destination overflow. */
+  CHK_FAIL_START
+  srcp = src;
+  memset(&st, 0, sizeof(st));
+  mbsnrtowcs(buffer, &srcp, 10, argc, &st);
+  CHK_FAIL_END
+
+  printf("%ls\n", buffer);
+  return ret;
+}
diff --git a/tests/test_mbsnrtowcs_static.c b/tests/test_mbsnrtowcs_static.c
new file mode 100644
index 0000000..755d453
--- /dev/null
+++ b/tests/test_mbsnrtowcs_static.c
@@ -0,0 +1,26 @@
+#include "common.h"
+
+#include <wchar.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+  wchar_t buffer[4] = {0};
+  const char *src = "ABCDEFGHIJKLMNOP";
+  const char *srcp = src;
+  mbstate_t st;
+  memset(&st, 0, sizeof(st));
+
+  /* Safe: convert up to 4 source bytes into at most 2 wide chars */
+  srcp = src;
+  mbsnrtowcs(buffer, &srcp, 4, 2, &st);
+
+  /* Unsafe: ask to write 16 wide chars into 4-element buffer */
+  CHK_FAIL_START
+  srcp = src;
+  memset(&st, 0, sizeof(st));
+  mbsnrtowcs(buffer, &srcp, 16, 16, &st);
+  CHK_FAIL_END
+
+  printf("%ls\n", buffer);
+  return ret;
+}