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

[Eric W. Biederman]

Cyrill Gorcunov <gorcunov at gmail.com> writes:

> 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;
> 	}

You aren't missing anything.  I just don't see what would be gained
by skipping those steps in an uncommon case.

As it stands it is easy to verify that *ret_fd is always read
and always set, and that task_lock is not needed to read or
write to ret_fd.

Eric