From mboxrd@z Thu Jan  1 00:00:00 1970
From: Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH 1/2] open: add close_range()
Date: Tue, 21 May 2019 14:09:29 +0200
Message-ID: <87tvdoau12.fsf@oldenburg2.str.redhat.com>
References: <20190521113448.20654-1-christian@brauner.io>
Mime-Version: 1.0
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20190521113448.20654-1-christian@brauner.io> (Christian
        Brauner's message of "Tue, 21 May 2019 13:34:47 +0200")
Sender: linux-kernel-owner@vger.kernel.org
List-Archive: <https://lore.kernel.org/linux-api/>
List-Post: <mailto:linux-api@vger.kernel.org>
To: Christian Brauner <christian@brauner.io>
Cc: viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org, jannh@google.com, oleg@redhat.com, tglx@linutronix.de, torvalds@linux-foundation.org, arnd@arndb.de, shuah@kernel.org, dhowells@redhat.com, tkjos@android.com, ldv@altlinux.org, 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@vger.kernel.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org, linux-xtensa@linux-xtensa.org, linux-arch@vger.kernel.org, linux-kselftest@vger.kernel.org, x86@kernel.org
List-ID: <linux-s390.vger.kernel.org>

* Christian Brauner:

> +/**
> + * __close_range() - Close all file descriptors in a given range.
> + *
> + * @fd:     starting file descriptor to close
> + * @max_fd: last file descriptor to close
> + *
> + * This closes a range of file descriptors. All file descriptors
> + * from @fd up to and including @max_fd are closed.
> + */
> +int __close_range(struct files_struct *files, unsigned fd, unsigned max_fd)
> +{
> +	unsigned int cur_max;
> +
> +	if (fd > max_fd)
> +		return -EINVAL;
> +
> +	rcu_read_lock();
> +	cur_max = files_fdtable(files)->max_fds;
> +	rcu_read_unlock();
> +
> +	/* cap to last valid index into fdtable */
> +	if (max_fd >= cur_max)
> +		max_fd = cur_max - 1;
> +
> +	while (fd <= max_fd)
> +		__close_fd(files, fd++);
> +
> +	return 0;
> +}

This seems rather drastic.  How long does this block in kernel mode?
Maybe it's okay as long as the maximum possible value for cur_max stays
around 4 million or so.

Solaris has an fdwalk function:

  <https://docs.oracle.com/cd/E88353_01/html/E37843/closefrom-3c.html>

So a different way to implement this would expose a nextfd system call
to userspace, so that we can use that to implement both fdwalk and
closefrom.  But maybe fdwalk is just too obscure, given the existence of
/proc.

I'll happily implement closefrom on top of close_range in glibc (plus
fallback for older kernels based on /proc—with an abort in case that
doesn't work because the RLIMIT_NOFILE hack is unreliable
unfortunately).

Thanks,
Florian