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From: Christian Brauner <christian@brauner.io>
To: Jann Horn <jannh@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	kernel list <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Florian Weimer <fweimer@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Arnd Bergmann <arnd@arndb.de>, Shuah Khan <shuah@kernel.org>,
	David Howells <dhowells@redhat.com>,
	Todd Kjos <tkjos@android.com>,
	"Dmitry V. Levin" <ldv@altlinux.org>,
	Miklos Szeredi <miklos@szeredi.hu>,
	linux-alpha@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	linux-s390 <linux-s390@vger.kernel.org>,
	linux-sh@vger.kernel.org, sparclinux@vger.kernel.org,
	linux-xtensa@linux-xtensa.org,
	linux-arch <linux-arch@vger.kernel.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [PATCH v1 1/2] open: add close_range()
Date: Thu, 23 May 2019 16:37:26 +0200	[thread overview]
Message-ID: <20190523143725.y67czx4jxsy6yqrj@brauner.io> (raw)
In-Reply-To: <CAG48ez0Uq2GQnQsuPkNrDdJVku_6GPeZ_5F_-5J3iy2CULr0_Q@mail.gmail.com>

On Thu, May 23, 2019 at 04:32:14PM +0200, Jann Horn wrote:
> On Thu, May 23, 2019 at 1:51 PM Christian Brauner <christian@brauner.io> wrote:
> [...]
> > I kept it dumb and was about to reply that your solution introduces more
> > code when it seemed we wanted to keep this very simple for now.
> > But then I saw that find_next_opened_fd() already exists as
> > find_next_fd(). So it's actually not bad compared to what I sent in v1.
> > So - with some small tweaks (need to test it and all now) - how do we
> > feel about?:
> [...]
> > static int __close_next_open_fd(struct files_struct *files, unsigned *curfd, unsigned maxfd)
> > {
> >         struct file *file = NULL;
> >         unsigned fd;
> >         struct fdtable *fdt;
> >
> >         spin_lock(&files->file_lock);
> >         fdt = files_fdtable(files);
> >         fd = find_next_fd(fdt, *curfd);
> 
> find_next_fd() finds free fds, not used ones.
> 
> >         if (fd >= fdt->max_fds || fd > maxfd)
> >                 goto out_unlock;
> >
> >         file = fdt->fd[fd];
> >         rcu_assign_pointer(fdt->fd[fd], NULL);
> >         __put_unused_fd(files, fd);
> 
> You can't do __put_unused_fd() if the old pointer in fdt->fd[fd] was
> NULL - because that means that the fd has been reserved by another
> thread that is about to put a file pointer in there, and if you
> release the fd here, that messes up the refcounting (or hits the
> BUG_ON() in __fd_install()).
> 
> > out_unlock:
> >         spin_unlock(&files->file_lock);
> >
> >         if (!file)
> >                 return -EBADF;
> >
> >         *curfd = fd;
> >         filp_close(file, files);
> >         return 0;
> > }
> >
> > int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> > {
> >         if (fd > max_fd)
> >                 return -EINVAL;
> >
> >         while (fd <= max_fd) {
> 
> Note that with a pattern like this, you have to be careful about what
> happens if someone gives you max_fd==0xffffffff - then this condition
> is always true and the loop can not terminate this way.
> 
> >                 if (__close_next_fd(files, &fd, maxfd))
> >                         break;
> 
> (obviously it can still terminate this way)

Yup, this was only a quick draft.
I think the dumb simple thing that I did before was the best way to do
it for now.
I first thought that the find_next_open_fd() function already exists but
when I went to write a POC for testing realized it doesn't anyway.

  reply	other threads:[~2019-05-23 14:37 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-22 15:52 Christian Brauner
2019-05-22 15:52 ` [PATCH v1 2/2] tests: add close_range() tests Christian Brauner
2019-05-22 16:57 ` [PATCH v1 1/2] open: add close_range() Oleg Nesterov
2019-05-23 11:51   ` Christian Brauner
2019-05-23 14:14     ` Christian Brauner
2019-05-23 14:28       ` Christian Brauner
2019-05-23 16:21         ` Oleg Nesterov
2019-05-23 14:32     ` Jann Horn
2019-05-23 14:37       ` Christian Brauner [this message]
2019-05-23 16:22 ` Konstantin Khlebnikov
2019-05-23 16:29   ` David Laight
2019-05-23 16:33   ` Christian Brauner
2019-05-24  7:43     ` Arnd Bergmann
2019-05-24  9:57       ` Christian Brauner
2019-05-24 10:31   ` Christian Brauner

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