mcheck Fix malloc_usable_size [BZ #22057]

Interpose malloc_usable_size to return the correct mcheck value for
malloc_usable_size.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>

malloc/Makefile

@@ -83,7 +83,7 @@ ifeq ($(have-GLIBC_2.23)$(build-shared),yesyes)
 # the tests expect specific internal behavior that is changed due to linking to
 # libmcheck.a.
 tests-exclude-mcheck = tst-mallocstate \
-	tst-safe-linking tst-malloc-usable \
+	tst-safe-linking \
 	tst-malloc-backtrace \
 	tst-malloc-fork-deadlock \
 	tst-malloc-stats-cancellation \
@@ -92,8 +92,6 @@ tests-exclude-mcheck = tst-mallocstate \
 	tst-malloc-thread-fail \
 	tst-malloc-usable-tunables \
 	tst-malloc_info \
-	tst-pvalloc-fortify \
-	tst-reallocarray \
 	tst-compathooks-off tst-compathooks-on
 
 tests-mcheck = $(filter-out $(tests-exclude-mcheck), $(tests))

malloc/malloc-debug.c

@@ -399,6 +399,8 @@ strong_alias (__debug_calloc, calloc)
 size_t
 malloc_usable_size (void *mem)
 {
+  if (__is_malloc_debug_enabled (MALLOC_MCHECK_HOOK))
+    return mcheck_usable_size (mem);
   if (__is_malloc_debug_enabled (MALLOC_CHECK_HOOK))
     return malloc_check_get_size (mem);
 

malloc/mcheck-impl.c

@@ -404,3 +404,9 @@ __mcheck_initialize (void (*func) (enum mcheck_status), bool in_pedantic)
   pedantic = in_pedantic;
   return 0;
 }
+
+static int
+mcheck_usable_size (struct hdr *h)
+{
+  return (h - 1)->size;
+}