Enable support for GCC 11 -Wmismatched-dealloc.

To help detect common kinds of memory (and other resource) management
bugs, GCC 11 adds support for the detection of mismatched calls to
allocation and deallocation functions.  At each call site to a known
deallocation function GCC checks the set of allocation functions
the former can be paired with and, if the two don't match, issues
a -Wmismatched-dealloc warning (something similar happens in C++
for mismatched calls to new and delete).  GCC also uses the same
mechanism to detect attempts to deallocate objects not allocated
by any allocation function (or pointers past the first byte into
allocated objects) by -Wfree-nonheap-object.

This support is enabled for built-in functions like malloc and free.
To extend it beyond those, GCC extends attribute malloc to designate
a deallocation function to which pointers returned from the allocation
function may be passed to deallocate the allocated objects.  Another,
optional argument designates the positional argument to which
the pointer must be passed.

This change is the first step in enabling this extended support for
Glibc.

1 files changed, 37 insertions, 0 deletions

diff --git a/libio/tst-freopen.c b/libio/tst-freopen.c
index 660882a28a..c8bc0a3d07 100644
--- a/libio/tst-freopen.c
+++ b/libio/tst-freopen.c
@@ -81,6 +81,42 @@ do_test_basic (void)
   fclose (f);
 }
 
+#if defined __GNUC__ && __GNUC__ >= 11
+/* Force an error to detect incorrectly making freopen a deallocator
+   for its last argument via attribute malloc.  The function closes
+   the stream without deallocating it so either the argument or
+   the pointer returned from the function (but not both) can be passed
+   to fclose.  */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic error "-Wmismatched-dealloc"
+#endif
+
+/* Verify that freopen returns stream.  */
+static void
+do_test_return_stream (void)
+{
+  FILE *f1 = fopen (name, "r");
+  if (f1 == NULL)
+    FAIL_EXIT1 ("fopen: %m");
+
+  FILE *f2 = freopen (name, "r+", f1);
+  if (f2 == NULL)
+    FAIL_EXIT1 ("freopen: %m");
+
+  /* Verify that freopen isn't declared with the no-argument attribute
+     malloc (which could let GCC fold the inequality to false).  */
+  if (f1 != f2)
+    FAIL_EXIT1 ("freopen returned a different stream");
+
+  /* This shouldn't trigger -Wmismatched-dealloc.  */
+  fclose (f1);
+}
+
+#if defined __GNUC__ && __GNUC__ >= 11
+/* Pop -Wmismatched-dealloc set to error above.  */
+# pragma GCC diagnostic pop
+#endif
+
 /* Test for BZ#21398, where it tries to freopen stdio after the close
    of its file descriptor.  */
 static void
@@ -105,6 +141,7 @@ do_test (void)
 {
   do_test_basic ();
   do_test_bz21398 ();
+  do_test_return_stream ();
 
   return 0;
 }