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From: Jann Horn <jannh@google.com>
To: Christian Brauner <christian@brauner.io>
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 16:32:14 +0200	[thread overview]
Message-ID: <CAG48ez0Uq2GQnQsuPkNrDdJVku_6GPeZ_5F_-5J3iy2CULr0_Q@mail.gmail.com> (raw)
In-Reply-To: <20190523115118.pmscbd6kaqy37dym@brauner.io>

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)

>                 cond_resched();
>                 fd++;
>         }
>
>         return 0;
> }

  parent reply	other threads:[~2019-05-23 14:32 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 [this message]
2019-05-23 14:37       ` Christian Brauner
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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