Re: [PATCH net] tun: handle copy failure in tun_put_user()

[Jason Wang]

On 01/20/2014 04:43 PM, Michael S. Tsirkin wrote:
> On Sun, Jan 19, 2014 at 07:48:56PM -0800, David Miller wrote:
>> From: Jason Wang <jasowang@xxxxxxxxxx>
>> Date: Mon, 20 Jan 2014 11:16:48 +0800
>>
>>> This patch return the error code of copy helpers in tun_put_user() instead of
>>> ignoring them.
>>>
>>> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
>>> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> I'm not sure we need to worry about this too much.
> But if yes, a bunch of places besides tun should be
> changed.

Yes, I send the patch because the error processing here is different
from what macvtap does. Macvtap just return error in this case and so do
packet socket.
>  Consider for example udp_recvmsg: it
> never seems to return any error except -EAGAIN.
>
> Is this a bug? Man page for recvmsg says:
>      EFAULT The receive buffer pointer(s)  point  outside  the process's  address
>               space.
>
> this isn't very clear: does this mean "all pointers are invalid"
> or "some pointers are invalid"?
> Also, what if pointers themselves are valid but length
> makes us go outside the address space?
>
> I'm guessing the simplest way is to clarify in the man page that
> passing invalid pointers / lengths is not guaranteed
> to result in EFAULT and that Linux makes no guarantees
> about the returned length in this case.
>
> Cc linux-man in case they can suggest some insights on this.
>
>> If you perform some of the copy successfully, you have to report that
>> length rather than just an error.
>>
>> Otherwise userland has no way to determine how much of the data was
>> successfully sourced.
>>
>> I'm not applying this, sorry.
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