stdlib: Remove attr_write from mbstows if dst is NULL [BZ: 29265]

mbstows is defined if dst is NULL and is defined to special cased if
dst is NULL so the fortify objsize check if incorrect in that case.

Tested on x86-64 linux.
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

stdlib/Makefile

@@ -373,6 +373,9 @@ CFLAGS-tst-qsort.c += $(stack-align-test-flags)
 CFLAGS-tst-makecontext.c += -funwind-tables
 CFLAGS-tst-makecontext2.c += $(stack-align-test-flags)
 
+CFLAGS-testmb.c += -D_FORTIFY_SOURCE=2 -Wall -Werror
+
+
 # Run a test on the header files we use.
 tests-special += $(objpfx)isomac.out
 

stdlib/bits/stdlib.h

@@ -96,6 +96,11 @@ extern size_t __mbstowcs_chk (wchar_t *__restrict __dst,
 			      const char *__restrict __src,
 			      size_t __len, size_t __dstlen) __THROW
     __attr_access ((__write_only__, 1, 3)) __attr_access ((__read_only__, 2));
+extern size_t __REDIRECT_NTH (__mbstowcs_chk_nulldst,
+			      (wchar_t *__restrict __dst,
+			       const char *__restrict __src,
+			       size_t __len), mbstowcs_chk)
+    __attr_access ((__read_only__, 2));
 extern size_t __REDIRECT_NTH (__mbstowcs_alias,
 			      (wchar_t *__restrict __dst,
 			       const char *__restrict __src,
@@ -108,16 +113,17 @@ extern size_t __REDIRECT_NTH (__mbstowcs_chk_warn,
      __warnattr ("mbstowcs called with dst buffer smaller than len "
 		 "* sizeof (wchar_t)");
 
-__fortify_function size_t
+__always_inline __fortify_function size_t
 __NTH (mbstowcs (wchar_t *__restrict __dst, const char *__restrict __src,
 		 size_t __len))
 {
-  return __glibc_fortify_n (mbstowcs, __len, sizeof (wchar_t),
-			    __glibc_objsize (__dst),
-			    __dst, __src, __len);
+  if (__builtin_constant_p (__dst == NULL) && __dst == NULL)
+    return __mbstowcs_chk_nulldst (__dst, __src, __len);
+  else
+    return __glibc_fortify_n (mbstowcs, __len, sizeof (wchar_t),
+			      __glibc_objsize (__dst), __dst, __src, __len);
 }
 
-
 extern size_t __wcstombs_chk (char *__restrict __dst,
 			      const wchar_t *__restrict __src,
 			      size_t __len, size_t __dstlen) __THROW

stdlib/testmb.c

@@ -16,6 +16,13 @@ main (int argc, char *argv[])
       lose = 1;
     }
 
+  i = mbstowcs (NULL, "bar", 4);
+  if (!(i == 3 && w[1] == 'a'))
+    {
+      puts ("mbstowcs FAILED2!");
+      lose = 1;
+    }
+
   mbstowcs (w, "blah", 5);
   i = wcstombs (c, w, 10);
   if (i != 4)