netdev - Re: [PATCH 1/4] inet: Add skb_copy_datagram

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Message-ID: <20141106173012.GY7996@ZenIV.linux.org.uk>
Date:	Thu, 6 Nov 2014 17:30:12 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, bcrl@...ck.org,
	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH 1/4] inet: Add skb_copy_datagram_iter

On Thu, Nov 06, 2014 at 04:28:18PM +0800, Herbert Xu wrote:
> +		if (copy_to_iter(skb->data + offset, copy, to))
> +			goto fault;

Sorry, no - copy_to_iter() returns the number of bytes copied, not 0 or -EFAULT.

> +			vaddr = kmap(page);
> +			err = copy_to_iter(vaddr + frag->page_offset +
> +					   offset - start, copy, to);
> +			kunmap(page);
> +			if (err)
> +				goto fault;

And that one should be
			copied = copy_page_to_iter(page, frag->page_offset +
					   offset - start, copy, to);
			if (copied != copy)
				goto fault;

Don't bother with kmap(), vaddr and all that shite.  The primitive is
	copy_page_to_iter(page, offset_in_page, nbytes, iter)
it does all needed kmap itself and it's smart enough to use kmap_atomic
when it can get away with that.  Similar for copy_page_from_iter().

Both of those (as well as copy_{to,from}_iter()) advance iov_iter and return
the number of bytes actually copied.  So the check for EFAULT is "it has copied
less than you've asked it to copy *and* you haven't run out that iov_iter".
The second part is guaranteed to be true in this case - your code makes sure
that 'copy' is no more than the space left in iterator.

In general, this check would be spelled
			if (copied != copy && iov_iter_count(to))
				goto fault;
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