Re: [PATCH 1/4] Vectorize aio_read/aio_write methods

[Andrew Morton <akpm@osdl.org>]

Badari Pulavarty <pbadari@us.ibm.com> wrote:
>
> On Thu, 2006-05-11 at 11:47 -0700, Andrew Morton wrote:
> > Badari Pulavarty <pbadari@us.ibm.com> wrote:
> > >
> > >  static ssize_t ep_aio_read_retry(struct kiocb *iocb)
> > >  {
> > >  	struct kiocb_priv	*priv = iocb->private;
> > > -	ssize_t			status = priv->actual;
> > > +	ssize_t			len, total;
> > >  
> > >  	/* we "retry" to get the right mm context for this: */
> > > -	status = copy_to_user(priv->ubuf, priv->buf, priv->actual);
> > > -	if (unlikely(0 != status))
> > > -		status = -EFAULT;
> > > -	else
> > > -		status = priv->actual;
> > > +
> > > +	/* copy stuff into user buffers */
> > > +	total = priv->actual;
> > > +	len = 0;
> > > +	for (i=0; i < priv->count; i++) {
> > > +		ssize_t this = min(priv->iv[i].iov_len, total);
> > > +
> > > +		if (copy_to_user(priv->iv[i].iov_buf, priv->buf, this))
> > > +			break;
> > > +
> > > +		total -= this;
> > > +		len += this;
> > > +		if (total <= 0)
> > > +			break;
> > > +	}
> > > +
> > > +	if (unlikely(len == 0))
> > > +		len = -EFAULT;
> > 
> > This is still wrong, isn't it?  Or am I looking at the same patch?
> > 
> > There's no way in which `total' can go negative, so it'd be nicer to just
> > test it for equality with zero.  Because if it goes unexpectedly negative,
> > we _want_ the kernel to malfunction, rather than mysteriously covering
> > things up.
> > 
> > The final test there should be
> > 
> > 	if (unlikely(total != 0))
> > 
> > yes?
> 
> No. The original check is correct - we want to return EFAULT if
> copy_to_user() failed and we haven't copied anything so far.
> If we copied anything so far, we should return, that many bytes.
> (like short-io).

oic.  And we're sure that we cannot call into this code if someone's trying
a zero-sized read?

Either way, the below (which is faster!) will fix, yes?

--- 25/drivers/usb/gadget/inode.c~vectorize-aio_read-aio_write-methods-fix	Thu May 11 11:53:41 2006
+++ 25-akpm/drivers/usb/gadget/inode.c	Thu May 11 13:19:45 2006
@@ -567,18 +567,18 @@ static ssize_t ep_aio_read_retry(struct 
 	for (i = 0; i < priv->count; i++) {
 		ssize_t this = min(priv->iv[i].iov_len, total);
 
-		if (copy_to_user(priv->iv[i].iov_buf, priv->buf, this))
+		if (copy_to_user(priv->iv[i].iov_buf, priv->buf, this)) {
+			if (len == 0)
+				len = -EFAULT;
 			break;
+		}
 
 		total -= this;
 		len += this;
-		if (total <= 0)
+		if (total == 0)
 			break;
 	}
 
-	if (unlikely(len == 0))
-		len = -EFAULT;
-
 	kfree(priv->buf);
 	kfree(priv);
 	aio_put_req(iocb);
_

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