Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier

From: Sargun Dhillon <sargun@sargun.me>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Linux API <linux-api@vger.kernel.org>,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Tycho Andersen <tycho@tycho.ws>,
	Kees Cook <keescook@chromium.org>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Jeffrey Vander Stoep <jeffv@google.com>,
	Jann Horn <jannh@google.com>, Robert Sesek <rsesek@google.com>,
	Chris Palmer <palmer@google.com>,
	Matt Denton <mpdenton@google.com>,
	Kees Cook <keescook@google.com>
Subject: Re: [PATCH 2/5] seccomp: Introduce addfd ioctl to seccomp user notifier
Date: Sun, 24 May 2020 17:27:58 -0700	[thread overview]
Message-ID: <CAMp4zn-F3b8Z4ZDjgnJM1Fbban2oVtCm_rZkhBhDKs6fw2_=rg@mail.gmail.com> (raw)
In-Reply-To: <20200525000537.GB23230@ZenIV.linux.org.uk>

On Sun, May 24, 2020 at 5:05 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Sun, May 24, 2020 at 04:39:39PM -0700, Sargun Dhillon wrote:
>
> Bad refcounting rules.  *IF* we go with anything of that sort (and I'm not
> convinced that the entire series makes sense), it's better to have more
> uniform rules re reference consumption/disposal.
>
> Make the destructor of addfd *ALWAYS* drop its reference.  And have this
> function go
Are you suggesting the in both the error, and non-error cases the ioctl
invoker side is responsible for fput'ing the final reference in both the
success and non-success cases? Would we take an extra reference
prior to fd_install?
>
>         if (addfd->fd >= 0) {
>                 ret = replace_fd(addfd->fd, addfd->file, addfd->flags);
>         } else {
>                 ret = get_unused_fd_flags(addfd->flags);
>                 if (ret >= 0)
>                         fd_install(ret, get_file(addfd->file));
>         }
>
Wouldn't this result in consumption of reference in one case (fd_install),
and the fd still having a reference in the replace_fd case?