[PATCH] Test fclose on an unopened file.

[Florian Weimer]

* Aaron Merey:

> Add new file libio/tst-fclosed-unopened.c that tests whether fclose on
> an unopened file returns EOF.
>
> fclose returning EOF for unopened files is not part of the external
> contract but there are dependancies on this behaviour.  For example,
> gnulib's close_stdout in lib/closeout.c.
>
> Tested for x86_64.
>
> Signed-off-by: Aaron Merey <amerey@redhat.com>
> ---
>  libio/Makefile              |  1 +
>  libio/tst-fclose-unopened.c | 40 +++++++++++++++++++++++++++++++++++++
>  2 files changed, 41 insertions(+)
>  create mode 100644 libio/tst-fclose-unopened.c
>
> diff --git a/libio/Makefile b/libio/Makefile
> index 6a507b67ea..59f3ee0b7c 100644
> --- a/libio/Makefile
> +++ b/libio/Makefile
> @@ -95,6 +95,7 @@ tests = \
>    tst-eof \
>    tst-ext \
>    tst-ext2 \
> +  tst-fclose-unopened \
>    tst-fdopen-seek-failure \
>    tst-fgetc-after-eof \
>    tst-fgetwc \
> diff --git a/libio/tst-fclose-unopened.c b/libio/tst-fclose-unopened.c
> new file mode 100644
> index 0000000000..2c7cea0b52
> --- /dev/null
> +++ b/libio/tst-fclose-unopened.c
> @@ -0,0 +1,40 @@
> +/* Test using fclose on an unopened file.
> +   Copyright (C) 2024 Free Software Foundation, Inc.
> +   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 <support/check.h>
> +
> +/* Verify that fclose on an unopened file returns EOF.  This is not part
> +   of the fclose external contract but there are dependancies on this
> +   behaviour.  */
> +
> +static int
> +do_test (void)
> +{
> +  TEST_VERIFY (fclose (stdin) == 0);
> +
> +  /* Attempt to close the unopened file and verify that EOF is returned.
> +     Calling fclose on a file twice normally causes a use-after-free bug,
> +     however the standard streams are an exception since they are not
> +     deallocated by fclose.  */
> +  TEST_VERIFY (fclose (stdin) == EOF);

Sorry, missed that you can use

  TEST_COMPARE (fclose (stdin), 0);
  TEST_COMPARE (fclose (stdin), EOF);

It will provide additional diagnostics on failure.

We should also add a second test with an
libio/tst-fclose-unopened2.input file: read something from it, and then
do the same double fclose.  This will cover the case where a buffer has
been allocated.  We can do this in a separate patch if you prefer.

Do you have commit access to the glibc project, or should I push the
revised version (with TEST_COMPARE) on your behalf?

Thanks,
Florian