Re: [PATCH v3 2/2] io_uring: add support for IORING_OP_GETDENTS

[Pavel Begunkov <asml.silence@gmail.com>]

On 18/02/2021 12:27, Lennert Buytenhek wrote:
> IORING_OP_GETDENTS behaves much like getdents64(2) and takes the same
> arguments, but with a small twist: it takes an additional offset
> argument, and reading from the specified directory starts at the given
> offset.
> 
> For the first IORING_OP_GETDENTS call on a directory, the offset
> parameter can be set to zero, and for subsequent calls, it can be
> set to the ->d_off field of the last struct linux_dirent64 returned
> by the previous IORING_OP_GETDENTS call.
> 
> Internally, if necessary, IORING_OP_GETDENTS will vfs_llseek() to
> the right directory position before calling vfs_getdents().
> 
> IORING_OP_GETDENTS may or may not update the specified directory's
> file offset, and the file offset should not be relied upon having
> any particular value during or after an IORING_OP_GETDENTS call.
> 
> Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
> ---
>  fs/io_uring.c                 | 73 +++++++++++++++++++++++++++++++++++
>  include/uapi/linux/io_uring.h |  1 +
>  2 files changed, 74 insertions(+)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 056bd4c90ade..6853bf48369a 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -635,6 +635,13 @@ struct io_mkdir {
>  	struct filename			*filename;
>  };
>  
[...]
> +static int io_getdents(struct io_kiocb *req, unsigned int issue_flags)
> +{
> +	struct io_getdents *getdents = &req->getdents;
> +	bool pos_unlock = false;
> +	int ret = 0;
> +
> +	/* getdents always requires a blocking context */
> +	if (issue_flags & IO_URING_F_NONBLOCK)
> +		return -EAGAIN;
> +
> +	/* for vfs_llseek and to serialize ->iterate_shared() on this file */
> +	if (file_count(req->file) > 1) {

Looks racy, is it safe? E.g. can be concurrently dupped and used, or just
several similar IORING_OP_GETDENTS requests.

> +		pos_unlock = true;
> +		mutex_lock(&req->file->f_pos_lock);
> +	}
> +
> +	if (req->file->f_pos != getdents->pos) {
> +		loff_t res = vfs_llseek(req->file, getdents->pos, SEEK_SET);

I may be missing the previous discussions, but can this ever become
stateless, like passing an offset? Including readdir.c and beyond. 

> +		if (res < 0)
> +			ret = res;
> +	}
> +
> +	if (ret == 0) {
> +		ret = vfs_getdents(req->file, getdents->dirent,
> +				   getdents->count);
> +	}
> +
> +	if (pos_unlock)
> +		mutex_unlock(&req->file->f_pos_lock);
> +
> +	if (ret < 0) {
> +		if (ret == -ERESTARTSYS)
> +			ret = -EINTR;
> +		req_set_fail_links(req);
> +	}
> +	io_req_complete(req, ret);
> +	return 0;
> +}
[...]

-- 
Pavel Begunkov