Re: [PATCH 3/3] io_uring: add support for getdents

[Hao Xu <hao.xu@linux.dev>]

On 7/11/23 20:15, Dominique Martinet wrote:
> Hao Xu wrote on Tue, Jul 11, 2023 at 07:40:27PM +0800:
>> diff --git a/io_uring/fs.c b/io_uring/fs.c
>> index f6a69a549fd4..77f00577e09c 100644
>> --- a/io_uring/fs.c
>> +++ b/io_uring/fs.c
>> @@ -291,3 +298,56 @@ void io_link_cleanup(struct io_kiocb *req)
>>   	putname(sl->oldpath);
>>   	putname(sl->newpath);
>>   }
>> +
>> +int io_getdents_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
>> +{
>> +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
>> +
>> +	if (READ_ONCE(sqe->off) != 0)
>> +		return -EINVAL;
>> +
>> +	gd->dirent = u64_to_user_ptr(READ_ONCE(sqe->addr));
>> +	gd->count = READ_ONCE(sqe->len);
>> +
>> +	return 0;
>> +}
>> +
>> +int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
>> +{
>> +	struct io_getdents *gd = io_kiocb_to_cmd(req, struct io_getdents);
>> +	struct file *file;
>> +	unsigned long getdents_flags = 0;
>> +	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
>> +	bool should_lock = false;
>> +	int ret;
>> +
>> +	if (force_nonblock) {
>> +		if (!(req->file->f_mode & FMODE_NOWAIT))
>> +			return -EAGAIN;
>> +
>> +		getdents_flags = DIR_CONTEXT_F_NOWAIT;
>> +	}
>> +
>> +	file = req->file;
>> +	if (file && (file->f_mode & FMODE_ATOMIC_POS)) {
> 
> If file is NULL here things will just blow up in vfs_getdents anyway,
> let's remove the useless check
> 
>> +		if (file_count(file) > 1)
> 
> I was curious about this so I found it's basically what __fdget_pos does
> before deciding it should take the f_pos_lock, and as such this is
> probably correct... But if someone can chime in here: what guarantees
> someone else won't __fdget_pos (or equivalent through this) the file
> again between this and the vfs_getdents call?
> That second get would make file_count > 1 and it would lock, but lock
> hadn't been taken here so the other call could get the lock without
> waiting and both would process getdents or seek or whatever in
> parallel.
> 

Hi Dominique,

This file_count(file) is atomic_read, so I believe no race condition here.

> 
> That aside I don't see any obvious problem with this.
>