Re: [PATCH 1/2] fs, close_range: add flag CLOSE_RANGE_CLOEXEC

[Giuseppe Scrivano <gscrivan@redhat.com>]

Christian Brauner <christian.brauner@ubuntu.com> writes:

> On Tue, Oct 13, 2020 at 04:06:08PM +0200, Giuseppe Scrivano wrote:
>
> Hey Guiseppe,
>
> Thanks for the patch!
>
>> When the flag CLOSE_RANGE_CLOEXEC is set, close_range doesn't
>> immediately close the files but it sets the close-on-exec bit.
>
> Hm, please expand on the use-cases a little here so people know where
> and how this is useful. Keeping the rationale for a change in the commit
> log is really important.
>
>> 
>> Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
>> ---
>
>>  fs/file.c                        | 56 ++++++++++++++++++++++----------
>>  include/uapi/linux/close_range.h |  3 ++
>>  2 files changed, 42 insertions(+), 17 deletions(-)
>> 
>> diff --git a/fs/file.c b/fs/file.c
>> index 21c0893f2f1d..ad4ebee41e09 100644
>> --- a/fs/file.c
>> +++ b/fs/file.c
>> @@ -672,6 +672,17 @@ int __close_fd(struct files_struct *files, unsigned fd)
>>  }
>>  EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
>>  
>> +static unsigned int __get_max_fds(struct files_struct *cur_fds)
>> +{
>> +	unsigned int max_fds;
>> +
>> +	rcu_read_lock();
>> +	/* cap to last valid index into fdtable */
>> +	max_fds = files_fdtable(cur_fds)->max_fds;
>> +	rcu_read_unlock();
>> +	return max_fds;
>> +}
>> +
>>  /**
>>   * __close_range() - Close all file descriptors in a given range.
>>   *
>> @@ -683,27 +694,23 @@ EXPORT_SYMBOL(__close_fd); /* for ksys_close() */
>>   */
>>  int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
>>  {
>> -	unsigned int cur_max;
>> +	unsigned int cur_max = UINT_MAX;
>>  	struct task_struct *me = current;
>>  	struct files_struct *cur_fds = me->files, *fds = NULL;
>>  
>> -	if (flags & ~CLOSE_RANGE_UNSHARE)
>> +	if (flags & ~(CLOSE_RANGE_UNSHARE | CLOSE_RANGE_CLOEXEC))
>>  		return -EINVAL;
>>  
>>  	if (fd > max_fd)
>>  		return -EINVAL;
>>  
>> -	rcu_read_lock();
>> -	cur_max = files_fdtable(cur_fds)->max_fds;
>> -	rcu_read_unlock();
>> -
>> -	/* cap to last valid index into fdtable */
>> -	cur_max--;
>> -
>>  	if (flags & CLOSE_RANGE_UNSHARE) {
>>  		int ret;
>>  		unsigned int max_unshare_fds = NR_OPEN_MAX;
>>  
>> +		/* cap to last valid index into fdtable */
>> +		cur_max = __get_max_fds(cur_fds) - 1;
>> +
>>  		/*
>>  		 * If the requested range is greater than the current maximum,
>>  		 * we're closing everything so only copy all file descriptors
>> @@ -724,16 +731,31 @@ int __close_range(unsigned fd, unsigned max_fd, unsigned int flags)
>>  			swap(cur_fds, fds);
>>  	}
>>  
>> -	max_fd = min(max_fd, cur_max);
>> -	while (fd <= max_fd) {
>> -		struct file *file;
>> +	if (flags & CLOSE_RANGE_CLOEXEC) {
>> +		struct fdtable *fdt;
>>  
>> -		file = pick_file(cur_fds, fd++);
>> -		if (!file)
>> -			continue;
>> +		spin_lock(&cur_fds->file_lock);
>> +		fdt = files_fdtable(cur_fds);
>> +		cur_max = fdt->max_fds - 1;
>> +		max_fd = min(max_fd, cur_max);
>> +		while (fd <= max_fd)
>> +			__set_close_on_exec(fd++, fdt);
>> +		spin_unlock(&cur_fds->file_lock);
>> +	} else {
>> +		/* Initialize cur_max if needed.  */
>> +		if (cur_max == UINT_MAX)
>> +			cur_max = __get_max_fds(cur_fds) - 1;
>
> The separation between how cur_fd is retrieved in the two branches makes
> the code more difficult to follow imho. Unless there's a clear reason
> why you've done it that way I would think that something like the patch
> I appended below might be a little clearer and easier to maintain(?).

Thanks for the review!

I've opted for this version as in the flags=CLOSE_RANGE_CLOEXEC case we
can read max_fds directly from the fds table and avoid doing it from the
RCU critical section as well.  I'll change it in favor of more readable
code.

Giuseppe