RE: [PATCH] usb: gadget: f_fs: report error if excess data received

[Du, Changbin]

> Hi,
> 
> changbin.du@xxxxxxxxx writes:
> > From: "Du, Changbin" <changbin.du@xxxxxxxxx>
> >
> > Since the buffer size for req is rounded up to maxpacketsize,
> > then we may end up with more data then user space has space
> > for.
> 
> only for OUT direction with the controller you're using ;-)
For sure.

> 
> > If it happen, we can keep the excess data for next i/o, or
> > report an error. But we cannot silently drop data, because
> > USB layer should ensure the data integrality it has transferred,
> > otherwise applications may get corrupt data if it doesn't
> > detect this case.
> 
> and when has this actually happened ? Host should not send more data in
> this case, if it does, it's an error on the host side. Also, returning
> -EOVERFLOW is not exactly correct here, because you'd violate POSIX
> specification of read(), right ?
> 
This can happen if the host side app force kill-restart, not taking care of this
special condition(and we are not documented), or even it is a bug. Usually APPs
may has  a protocol to control the packet size, but protocol mismatch can happen
if either side encounter an error.

Anyway, this is real. If kernel return success and drop data, the error may 
explosion later, or its totally hided (but why some data lost in kernel?
Kernel cannot tell userspace we cannot be trusted sometimes, right?). 
so IMO, if this is an error, we need report an error or fix it, not hide it.

The POSIX didn't say read cannot return "-EOVERFLOW", it says:
" Other errors may occur, depending on the object connected to fd."

If "-EOVERFLOW" is not suitable, EFAULT, or any suggestions?

> > Here, we simply report an error to userspace to let userspace
> > proccess. Actually, userspace applications should negotiate
> 
> no, this violates POSIX. Care to explain what problem are you actually
> facing ?
> 
Why this violates POSIX? Could you give more details?

The problem is device side app sometimes received incorrect data caused
by the dropping. Most times the error can be detected by APP itself, but
sometimes cannot. It depends on the design of its communication protocol.

> --
> Balbi

Best Regards,
Du, Changbin