wctomb, wcrtomb: guard slow/trap path with MB_LEN_MAX

This allows the compiler to optimize out the slow/trap path at all
for the typical correct code:

char buf[MB_LEN_MAX];
r = wctomb(buf, c);

The change tries to keep the "unknown object size" case handling in
wcrtomb() as is even if it seems redundant and not helping (we copy
__buf to possibly undersized __s in any case) and inconsistent with
wctomb() (where we let the original library method itself overwrite
the possibly undersized __s).

1 files changed, 5 insertions, 6 deletions

diff --git a/include/wchar.h b/include/wchar.h
index 09ffa97..c5d0e5d 100644
--- a/include/wchar.h
+++ b/include/wchar.h
@@ -111,15 +111,14 @@ _FORTIFY_FN(mbstowcs) size_t mbstowcs(wchar_t *__ws, const char *__s, size_t __w
 
 _FORTIFY_FN(wcrtomb) size_t wcrtomb(char *__s, wchar_t __w, mbstate_t *__st)
 {
-        char __buf[MB_LEN_MAX];
-        size_t __b = __builtin_object_size(__s, 0);
-        size_t __r;
+        if (__s && MB_LEN_MAX > __builtin_object_size(__s, 2)) {
+                char __buf[MB_LEN_MAX];
+                size_t __r;
 
-        if (__s) {
                 __r = __orig_wcrtomb(__buf, __w, __st);
                 if (__r == (size_t)-1)
                         return __r;
-                if (__r > __b)
+                if (__r > __builtin_object_size(__s, 0))
                         __builtin_trap();
                 __builtin_memcpy(__s, __buf, __r);
                 return __r;
@@ -218,7 +217,7 @@ _FORTIFY_FN(wctomb) int wctomb(char *__s, wchar_t __w)
 {
         size_t __b = __builtin_object_size(__s, 0);
 
-        if (__s && MB_CUR_MAX > __b)
+        if (__s && MB_LEN_MAX > __b && MB_CUR_MAX > __b)
                 __builtin_trap();
         return __orig_wctomb(__s, __w);
 }