[CRIU] [PATCH 09/17] file: Implement fnext_task

[Cyrill Gorcunov]

On Mon, Aug 17, 2020 at 05:04:17PM -0500, Eric W. Biederman wrote:
> As a companion to fget_task and fcheck_task implement fnext_task that
> will return the struct file for the first file descriptor show number
> is equal or greater than the fd argument value, or NULL if there is
> no such struct file.
> 
> This allows file descriptors of foreign processes to be iterated through
> safely, without needed to increment the count on files_struct.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>
> ---
>  fs/file.c               | 21 +++++++++++++++++++++
>  include/linux/fdtable.h |  1 +
>  2 files changed, 22 insertions(+)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 8d4b385055e9..88f9f78869f8 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -876,6 +876,27 @@ struct file *fcheck_task(struct task_struct *task, unsigned int fd)
>  	return file;
>  }
>  
> +struct file *fnext_task(struct task_struct *task, unsigned int *ret_fd)
> +{
> +	/* Must be called with rcu_read_lock held */
> +	struct files_struct *files;
> +	unsigned int fd = *ret_fd;
> +	struct file *file = NULL;
> +
> +	task_lock(task);
> +	files = task->files;
> +	if (files) {
> +		for (; fd < files_fdtable(files)->max_fds; fd++) {
> +			file = fcheck_files(files, fd);
> +			if (file)
> +				break;
> +		}
> +	}
> +	task_unlock(task);
> +	*ret_fd = fd;
> +	return file;
> +}

Eric, if only I'm not missing something obvious you could
escape @fd/@ret_fd operations in case if task->files = NULL,
iow

	if (files) {
		unsigned int fd = *ret_fd;
		for (; fd < files_fdtable(files)->max_fds; fd++) {
			file = fcheck_files(files, fd);
			if (file)
				break;
		}
		*ret_fd = fd;
	}