ungetc: Fix backup buffer leak on program exit [BZ #27821]

If a file descriptor is left unclosed and is cleaned up by _IO_cleanup
on exit, its backup buffer remains unfreed, registering as a leak in
valgrind.  This is not strictly an issue since (1) the program should
ideally be closing the stream once it's not in use and (2) the program
is about to exit anyway, so keeping the backup buffer around a wee bit
longer isn't a real problem.  Free it anyway to keep valgrind happy
when the streams in question are the standard ones, i.e. stdout, stdin
or stderr.

Also, the _IO_have_backup macro checks for _IO_save_base,
which is a roundabout way to check for a backup buffer instead of
directly looking for _IO_backup_base.  The roundabout check breaks when
the main get area has not been used and user pushes a char into the
backup buffer with ungetc.  Fix this to use the _IO_backup_base
directly.

Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
(cherry picked from commit 3e1d8d1d1d)

libio/genops.c

@@ -789,6 +789,12 @@ _IO_unbuffer_all (void)
 	legacy = 1;
 #endif
 
+      /* Free up the backup area if it was ever allocated.  */
+      if (_IO_have_backup (fp))
+	_IO_free_backup_area (fp);
+      if (fp->_mode > 0 && _IO_have_wbackup (fp))
+	_IO_free_wbackup_area (fp);
+
       if (! (fp->_flags & _IO_UNBUFFERED)
 	  /* Iff stream is un-orientated, it wasn't used. */
 	  && (legacy || fp->_mode != 0))

libio/libioP.h

@@ -577,8 +577,8 @@ extern void _IO_old_init (FILE *fp, int flags) __THROW;
        ((__fp)->_wide_data->_IO_write_base \
 	= (__fp)->_wide_data->_IO_write_ptr = __p, \
 	(__fp)->_wide_data->_IO_write_end = (__ep))
-#define _IO_have_backup(fp) ((fp)->_IO_save_base != NULL)
-#define _IO_have_wbackup(fp) ((fp)->_wide_data->_IO_save_base != NULL)
+#define _IO_have_backup(fp) ((fp)->_IO_backup_base != NULL)
+#define _IO_have_wbackup(fp) ((fp)->_wide_data->_IO_backup_base != NULL)
 #define _IO_in_backup(fp) ((fp)->_flags & _IO_IN_BACKUP)
 #define _IO_have_markers(fp) ((fp)->_markers != NULL)
 #define _IO_blen(fp) ((fp)->_IO_buf_end - (fp)->_IO_buf_base)

stdio-common/Makefile

@@ -256,6 +256,7 @@ tests := \
   tst-swscanf \
   tst-tmpnam \
   tst-ungetc \
+  tst-ungetc-leak \
   tst-unlockedio \
   tst-vfprintf-mbs-prec \
   tst-vfprintf-user-type \
@@ -300,6 +301,7 @@ tests-special += \
   $(objpfx)tst-printfsz-islongdouble.out \
   $(objpfx)tst-setvbuf1-cmp.out \
   $(objpfx)tst-unbputc.out \
+  $(objpfx)tst-ungetc-leak-mem.out \
   $(objpfx)tst-vfprintf-width-prec-mem.out \
   # tests-special
 
@@ -314,6 +316,8 @@ generated += \
   tst-printf-fp-leak-mem.out \
   tst-printf-fp-leak.mtrace \
   tst-scanf-bz27650.mtrace \
+  tst-ungetc-leak-mem.out \
+  tst-ungetc-leak.mtrace \
   tst-vfprintf-width-prec-mem.out \
   tst-vfprintf-width-prec.mtrace \
   # generated
@@ -403,6 +407,9 @@ tst-printf-fp-leak-ENV = \
 tst-scanf-bz27650-ENV = \
   MALLOC_TRACE=$(objpfx)tst-scanf-bz27650.mtrace \
   LD_PRELOAD=$(common-objpfx)malloc/libc_malloc_debug.so
+tst-ungetc-leak-ENV = \
+  MALLOC_TRACE=$(objpfx)tst-ungetc-leak.mtrace \
+  LD_PRELOAD=$(common-objpfx)malloc/libc_malloc_debug.so
 
 $(objpfx)tst-unbputc.out: tst-unbputc.sh $(objpfx)tst-unbputc
 	$(SHELL) $< $(common-objpfx) '$(test-program-prefix)'; \

stdio-common/tst-ungetc-leak.c

@@ -0,0 +1,32 @@
+/* Test for memory leak with ungetc when stream is unused.
+   Copyright The GNU Toolchain Authors.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <stdio.h>
+#include <mcheck.h>
+#include <support/check.h>
+#include <support/support.h>
+
+static int
+do_test (void)
+{
+  mtrace ();
+  TEST_COMPARE (ungetc('y', stdin), 'y');
+  return 0;
+}
+
+#include <support/test-driver.c>