Re: [PATCH 1/3] Drivers: hv: Support the newly introduced KVPmessages in the driver

[Dan Carpenter]

On Fri, Mar 16, 2012 at 06:33:35AM +0000, KY Srinivasan wrote:
> 
> 
> > -----Original Message-----
> > From: Dan Carpenter [mailto:dan.carpenter@xxxxxxxxxx]
> > Sent: Friday, March 16, 2012 1:46 AM
> > To: KY Srinivasan
> > Cc: gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> > devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxx; ohering@xxxxxxxx;
> > Alan Stern
> > Subject: Re: [PATCH 1/3] Drivers: hv: Support the newly introduced KVP
> > messages in the driver
> > 
> > On Thu, Mar 15, 2012 at 05:48:43PM -0700, K. Y. Srinivasan wrote:
> > >  	/*
> > >  	 * The windows host expects the key/value pair to be encoded
> > >  	 * in utf16.
> > >  	 */
> > >  	keylen = utf8s_to_utf16s(key_name, strlen(key_name),
> > UTF16_HOST_ENDIAN,
> > > -				(wchar_t *) kvp_data->data.key,
> > > +				(wchar_t *) kvp_data->key,
> > >  				HV_KVP_EXCHANGE_MAX_KEY_SIZE / 2);
> > > -	kvp_data->data.key_size = 2*(keylen + 1); /* utf16 encoding */
> > > +	kvp_data->key_size = 2*(keylen + 1); /* utf16 encoding */
> > > +
> > 
> > I feel like a jerk for asking this, but is the output length correct
> > here?  It seems like we could go over again.  Also utf8s_to_utf16s()
> > can return negative error codes, why do we ignore those?
> 
> We are returning the strings back to the host here. There are checks elsewhere
> in the code to ensure that all strings we return to the host can be accommodated
> in the available space. For the most part these are strings that the host gave us in the 
> first place that have already been validated.  Furthermore, there are checks on the 
> host side to ensure that the returned size parameters are consistent with the protocol 
> definitions for the key value pair. For instance let us say somehow we got into a 
> situation where the converted utf16 string occupied the entire MAX sized array 
> without any room for the terminating character and we set the length parameter 
> to 2 more than the MAX value as this code would do. The host would simply discard the 
> message as an illegal message. This would be more appropriate than sending a 
> truncated key or value.
> 

Uh...  Looking at it again, this code is clearly off by one.  If
we're not going to hit the limit, then we're not going to truncate,
so that's not a concern.  Let's just use the correct limit here.

The problem is that off-by-ones tend to reproduce by copy and paste.
It's best to never introduce any, even harmless ones.

Either that or add a comment.  /* Don't care about wrong limitter
because we trust the input. */.

> With regards to the negative values, negative values indicate a failure of some sort
> in the conversion. Since the host is the recipient here, host will correctly deal with the
> transaction by discarding the tuple.  

I'm not super familiar with this subsystem.  Where can I find code
for rejecting bad transactions?  It seems like an easy thing to
handle the error in both places.  It makes auditing the code a lot
simpler.

regards,
dan carpenter

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