Re: [PATCH] usb: gadget: ffs: don't allow to open with O_NONBLOCK flag

[Michal Nazarewicz <mina86@mina86.com>]

On Wed, Apr 01 2015, Robert Baldyga <r.baldyga@samsung.com> wrote:
> FunctionFS can't support O_NONBLOCK because read/write operatons are
> directly translated into USB requests which are asynchoronous, so we
> can't know how long we will have to wait for request completion. For
> this reason in case of open with O_NONBLOCK flag we return
> -EWOULDBLOCK.

‘can’t’ is a bit strong of a word here though.  It can, but in a few
cases it doesn’t.

It kinda saddens me that this undoes all the lines of code that were put
into the file to support O_NONBLOCK (e.g. FFS_NO_SETUP path of
ffs_ep0_read).

I’m also worried this may break existing applications which, for better
or worse, open the file with O_NONBLOCK.

Most importantly though, this does not stop users from using fcntl to
set O_NONBLOCK, so if you really want to stop O_NONBLOCK from being set,
that path should be checked as well (if possible).

> Signed-off-by: Robert Baldyga <r.baldyga@samsung.com>
> ---
>  drivers/usb/gadget/function/f_fs.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
> index 175c995..1014911 100644
> --- a/drivers/usb/gadget/function/f_fs.c
> +++ b/drivers/usb/gadget/function/f_fs.c
> @@ -538,6 +538,14 @@ static int ffs_ep0_open(struct inode *inode, struct file *file)
>  	if (unlikely(ffs->state == FFS_CLOSING))
>  		return -EBUSY;
>  
> +	/*
> +	 * We are not supporting O_NONBLOCK because read/write operatons are
> +	 * directly translated into USB requests which are asynchoronous, so
> +	 * we can't know how long we will have to wait for request completion.
> +	 */
> +	if (unlikely(file->f_flags & O_NONBLOCK))
> +		return -EWOULDBLOCK;
> +
>  	file->private_data = ffs;
>  	ffs_data_opened(ffs);
>  
> @@ -874,6 +882,14 @@ ffs_epfile_open(struct inode *inode, struct file *file)
>  	if (WARN_ON(epfile->ffs->state != FFS_ACTIVE))
>  		return -ENODEV;
>  
> +	/*
> +	 * We are not supporting O_NONBLOCK because read/write operatons are
> +	 * directly translated into USB requests which are asynchoronous, so
> +	 * we can't know how long we will have to wait for request completion.
> +	 */
> +	if (unlikely(file->f_flags & O_NONBLOCK))
> +		return -EWOULDBLOCK;
> +
>  	file->private_data = epfile;
>  	ffs_data_opened(epfile->ffs);
>  
> -- 
> 1.9.1
>

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michał “mina86” Nazarewicz    (o o)
ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>--ooO--(_)--Ooo--