linux-kernel - Re: [PATCH 2/4] Drivers: hv: Support the newly introduced KVP messages in the driver

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Message-ID: <20120312052221.GE3337@mwanda>
Date:	Mon, 12 Mar 2012 08:22:21 +0300
From:	Dan Carpenter <dan.carpenter@...cle.com>
To:	KY Srinivasan <kys@...rosoft.com>
Cc:	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>,
	Alan Stern <stern@...land.harvard.edu>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>
Subject: Re: [PATCH 2/4] Drivers: hv: Support the newly introduced KVP
 messages in the driver

On Sun, Mar 11, 2012 at 08:53:57PM +0000, KY Srinivasan wrote:
> 
> 
> > -----Original Message-----
> > From: Dan Carpenter [mailto:dan.carpenter@...cle.com]
> > Sent: Sunday, March 11, 2012 2:49 PM
> > To: KY Srinivasan
> > Cc: gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> > devel@...uxdriverproject.org; virtualization@...ts.osdl.org; ohering@...e.com;
> > Alan Stern
> > Subject: Re: [PATCH 2/4] Drivers: hv: Support the newly introduced KVP
> > messages in the driver
> > 
> > On Sun, Mar 11, 2012 at 04:56:06PM +0000, KY Srinivasan wrote:
> > > > Probably that's not enough to make a difference and we'd need to
> > > > introduce a new function.
> > > >
> > > > Btw I don't know if utf16s_to_utf8s() counts the NUL char or not.
> > > > It feels like maybe we could end up with ->value_size equal to
> > > > HV_KVP_EXCHANGE_MAX_VALUE_SIZE + 1.
> > >
> > > The MAX value is set to accommodate the maximum string that will ever
> > > be handled including the string terminator. The function utf16s_to_utf8s()
> > > returns the converted string length but the returned length does not
> > > include the string terminator (like strlen), hence the "+1".
> > >
> > 
> > sprintf() and friends copy the NUL terminator but utf16s_to_utf8s()
> > doesn't so the code isn't right and it does seem like maybe we could
> > end up with a ->value_size equal to HV_KVP_EXCHANGE_MAX_VALUE_SIZE +
> > 1.
> 
> You are right in that utf16s_to_utf8s() does not copy the string
> terminator. This is not an issue in this case since the buffer for
> the utf8 string is zeroed out to begin with (this memory was
> allocated using kzalloc()). The return value of the
> utf16s_to_utf8s() is the length of the utf8s string as what would
> be returned by strlen.


There is no strlen() involved...  It returns the number of bytes
copied to the output string.  It doesn't copy a NUL.  We pass
HV_KVP_EXCHANGE_MAX_VALUE_SIZE bytes as the limit.  So it fills up
the buffer with non-null characters and we have an off-by-one.

> I add one to take into account the string
> terminator character for further processing. As I said before the
> MAX value takes into account the terminating character for all the
> strings handled.

So you're saying that since we control the input string, we'll never
hit the max?  Still, why not pass HV_KVP_EXCHANGE_MAX_VALUE_SIZE - 1
to leave room for the NUL just for correctness?  We'd still add one
to the return value but we wouldn't go over the size of the buffer.

Again, I don't really know how utf16s_to_utf8s() works so I might
have misunderstood.

regards,
dan carpenter


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