linux-kernel - Re: [RFC][PATCH] vfs: add closefrom(2) syscall

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Date:	Tue, 8 Apr 2014 10:21:37 +0200
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Zheng Liu <gnehzuil.liu@...il.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Zheng Liu <wenqing.lz@...bao.com>
Subject: Re: [RFC][PATCH] vfs: add closefrom(2) syscall

On Tue, Apr 08, 2014 at 03:12:22PM +0800, Zheng Liu wrote:
>  
> +int __close_fds(struct files_struct *files, int lowfd)
> +{
> +	struct file *file;
> +	struct fdtable *fdt;
> +	int fd;
> +
> +	if (lowfd < 0)
> +		lowfd = 0;
> +	spin_lock(&files->file_lock);
> +	fdt = files_fdtable(files);
> +	if (lowfd >= fdt->max_fds)
> +		goto out_unlock;
> +	for (fd = lowfd; fd < fdt->max_fds; fd++) {
> +		file = fdt->fd[fd];
> +		if (!file)
> +			continue;
> +
> +		rcu_assign_pointer(fdt->fd[fd], NULL);
> +		__clear_close_on_exec(fd, fdt);
> +		__put_unused_fd(files, fd);
> +		spin_unlock(&files->file_lock);
> +		filp_close(file, files);
> +		spin_lock(&files->file_lock);
> +	}
> +
> +out_unlock:
> +	spin_unlock(&files->file_lock);
> +	return 0;
> +}
> +

Can't comment on the usefulness of the patch, but I would like to note:

1. fdt could be freed after you drop the lock, but you never reload the
pointer, thus this looks like use-after-free
2. most of this looks like __close_fd, maybe some parts could be moved
to an inline function so that code duplication is reduced?

-- 
Mateusz Guzik
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