[PATCH v2] libio: Fix wide stream backup buffer leak on fclose [BZ #33999]

[Gao Xiang <gaoxiang@kylinos.cn>]

From: Xiang Gao <gaoxiang@kylinos.cn>

This patch fixes a memory leak when ungetwc is used on a wide oriented stream,
The backup buffer was never freed on fclose, causing a memory leak per
ungetwc/fclose call.

The leak has two causes:

In iofclose.c, for wide streams (fp->mode > 0), _IO_new_fclose never calls
_IO_free_wbackup_area. Fixed by adding the missing call.

In wgenops.c, _IO_wdefault_finish checks fp->_IO_save_base (the narrow field,
always NULL for wide streams) instead of fp->_wide_data->_IO_save_base,
and uses a bare free() that leaves _IO_save_end and _IO_backup_base dangling.
Replace the hand-rolled cleanup with _IO_hava_wbackup/_IO_free_wbackup_area,
which handles backup-mode switching and clears all three pointers.

This was independently reported by Rocket Ma [1], whose patch corrects the condition
but still uses the manual free path.

Apply the same _IO_hava_backup condition in genops.c for consistency.

Tested by:
make test t=libio/tst-wbackup-leak

[1] https://patchwork.sourceware.org/project/glibc/patch/20260323171742.1039768-1-marocketbd@gmail.com/

Signed-off-by: Xiang Gao <gaoxiang@kylinos.cn>
---
Thank you for the review.

Changes since v1:
  - Use _IO_have_backup / _IO_have_wbackup as conditions.
  - Keeps wmemstream in the test:  Agreed that ungetwc on a wmemstream is not
    a valid application use.However I was unable to trigger the backup buffer
    allocation through a tmpfile-based read/ungetwc sequence.
    Using wmemstream route is the most direct way to reach the leaking code in
    _IO_wdefault_finish. Open to suggestions if you know a cleaner way to trigger it.
---
 libio/Makefile           |  1 +
 libio/genops.c           |  7 ++-----
 libio/iofclose.c         |  7 +++++--
 libio/tst-wbackup-leak.c | 45 ++++++++++++++++++++++++++++++++++++++++
 libio/wgenops.c          |  7 ++-----
 5 files changed, 55 insertions(+), 12 deletions(-)
 create mode 100644 libio/tst-wbackup-leak.c

diff --git a/libio/Makefile b/libio/Makefile
index 08e1e0ec25..93656466df 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -135,6 +135,7 @@ tests = \
   tst-swscanf \
   tst-ungetwc1 \
   tst-ungetwc2 \
+  tst-wbackup-leak \
   tst-wfile-sync \
   tst-wfiledoallocate-static \
   tst-widetext \
diff --git a/libio/genops.c b/libio/genops.c
index cc1684e00a..90e08e6571 100644
--- a/libio/genops.c
+++ b/libio/genops.c
@@ -636,11 +636,8 @@ _IO_default_finish (FILE *fp, int dummy)
   for (mark = fp->_markers; mark != NULL; mark = mark->_next)
     mark->_sbuf = NULL;
 
-  if (fp->_IO_save_base)
-    {
-      _IO_free_backup_buf (fp, fp->_IO_save_base);
-      fp->_IO_save_base = NULL;
-    }
+  if (_IO_have_backup (fp))
+    _IO_free_backup_area (fp);
 
   _IO_un_link ((struct _IO_FILE_plus *) fp);
 
diff --git a/libio/iofclose.c b/libio/iofclose.c
index 89782e99d7..3d1ee3e3cb 100644
--- a/libio/iofclose.c
+++ b/libio/iofclose.c
@@ -67,8 +67,11 @@ _IO_new_fclose (FILE *fp)
   _IO_FINISH (fp);
   if (fp->_mode > 0)
     {
+      if (fp->_wide_data->_IO_save_base)
+        _IO_free_wbackup_area (fp);
+
       /* This stream has a wide orientation.  This means we have to free
-	 the conversion functions.  */
+        the conversion functions.  */
       struct _IO_codecvt *cc = fp->_codecvt;
 
       __libc_lock_lock (__gconv_lock);
@@ -79,7 +82,7 @@ _IO_new_fclose (FILE *fp)
   else
     {
       if (_IO_have_backup (fp))
-	_IO_free_backup_area (fp);
+        _IO_free_backup_area (fp);
     }
   _IO_deallocate_file (fp);
   return status;
diff --git a/libio/tst-wbackup-leak.c b/libio/tst-wbackup-leak.c
new file mode 100644
index 0000000000..c5bf128237
--- /dev/null
+++ b/libio/tst-wbackup-leak.c
@@ -0,0 +1,45 @@
+/* Test _IO_wdefault_finish frees wide backup buffer [BZ #33999].  */
+
+#include <malloc.h>
+#include <stdio.h>
+#include <wchar.h>
+#include <support/check.h>
+
+static void
+one_round (void)
+{
+  wchar_t *buf = NULL;
+  size_t size = 0;
+
+  FILE *fp = open_wmemstream (&buf, &size);
+  TEST_VERIFY_EXIT (fp != NULL);
+  fputwc (L'A', fp);
+  fflush (fp);
+  /*·Push·back·without·prior·read.··read_ptr·==·read_base,·so
+···*··_IO_wdefault_pbackfail·skips·the·buggy·narrow·read_ptr·access
+···*··(BZ·#33998)·and·goes·straight·to·allocating·a·wide·backup
+  ·*··buffer·at·fp->_wide_data->_IO_save_base.··*/
+  ungetwc (L'Z', fp);
+  fclose (fp);
+  free (buf);
+}
+
+static int
+do_test (void)
+{
+  /* Warm up to stabilize allocator state.*/
+  one_round ();
+
+  struct mallinfo2 before = mallinfo2 ();
+  for (int i = 0; i < 1000; i++)
+    one_round ();
+  struct mallinfo2 after = mallinfo2 ();
+
+  /* Each leak is 128 * sizeof(wchar_t) = 512 bytes.
+   * 1000 iterations would leak ~512 KB.  Allow 4 KB noise. */
+  TEST_VERIFY (after.uordblks - before.uordblks < 4096);
+
+  return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/wgenops.c b/libio/wgenops.c
index 064d71266d..6829477e0c 100644
--- a/libio/wgenops.c
+++ b/libio/wgenops.c
@@ -181,11 +181,8 @@ _IO_wdefault_finish (FILE *fp, int dummy)
   for (mark = fp->_markers; mark != NULL; mark = mark->_next)
     mark->_sbuf = NULL;
 
-  if (fp->_IO_save_base)
-    {
-      free (fp->_wide_data->_IO_save_base);
-      fp->_IO_save_base = NULL;
-    }
+  if (_IO_have_wbackup (fp))
+    _IO_free_wbackup_area (fp);
 
 #ifdef _IO_MTSAFE_IO
   if (fp->_lock != NULL)
-- 
2.53.0