realpath: guard slow/trap path with PATH_MAX

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

char buf[PATH_MAX];
r = realpath(path, buf);

The change keeps the "unknown object size" case handling intact.

1 files changed, 3 insertions, 5 deletions

diff --git a/include/stdlib.h b/include/stdlib.h
index ef70995..11155cf 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -39,12 +39,10 @@ extern "C" {
 #undef realpath
 _FORTIFY_FN(realpath) char *realpath(const char *__p, char *__r)
 {
-        size_t __b = __builtin_object_size(__r, 0);
-
-        if (__r) {
 #ifndef PATH_MAX
 #error PATH_MAX unset. A fortified realpath will not work.
 #else
+        if (__r && PATH_MAX > __builtin_object_size(__r, 2)) {
                 char __buf[PATH_MAX], *__ret;
                 size_t __l;
 
@@ -52,13 +50,13 @@ _FORTIFY_FN(realpath) char *realpath(const char *__p, char *__r)
                 if (!__ret)
                         return NULL;
                 __l = __builtin_strlen(__ret) + 1;
-                if (__l > __b)
+                if (__l > __builtin_object_size(__r, 0))
                         __builtin_trap();
                 __builtin_memcpy(__r, __ret, __l);
                 return __r;
-#endif
         }
         return __orig_realpath(__p, __r);
+#endif
 }
 #endif