Fix strncat/wcsncat

Previously, no checks were done when __n <= __b, but strncat _appends_ after
existing content, making this a overly broad check check. For example, with an
8-byte buffer containing "12345\0", strncat(buf, "ABCD", 4) would have the
check skipped, but the result "12345ABCD\0" is 10 bytes, resulting in an
overflow.

This commit fixes this oversight, and adds a bunch of tests.

1 files changed, 1 insertions, 3 deletions

diff --git a/tests/test_mbsnrtowcs_dynamic.c b/tests/test_mbsnrtowcs_dynamic.c
index 77b9082..58575d3 100644
--- a/tests/test_mbsnrtowcs_dynamic.c
+++ b/tests/test_mbsnrtowcs_dynamic.c
@@ -14,9 +14,7 @@ int main(int argc, char** argv) {
   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. */
+  /* Unsafe: ask to write argc (10) wide chars into 4-element buffer. */
   CHK_FAIL_START
   srcp = src;
   memset(&st, 0, sizeof(st));