linux-kernel - Re: [PATCH v11 06/21] vfs: Add copy_to_iter(), copy_from_iter() and iov_iter

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Date:	Thu, 16 Oct 2014 09:59:03 -0400
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Matthew Wilcox <matthew.r.wilcox@...el.com>,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH v11 06/21] vfs: Add copy_to_iter(), copy_from_iter() and
 iov_iter_zero()

On Thu, Oct 16, 2014 at 03:33:55PM +0200, Mathieu Desnoyers wrote:
> > +static size_t copy_to_iter_iovec(void *from, size_t bytes, struct iov_iter *i)
> > +{
[...]
> > +	left = __copy_to_user(buf, from, copy);
> 
> How comes this function uses __copy_to_user without any access_ok()
> check ? This has security implications.

The access_ok() check is done higher up the call-chain if it's appropriate.
These functions can be (intentionally) called to access kernel addresses,
so it wouldn't be appropriate to do that here.

> > +static size_t copy_page_to_iter_bvec(struct page *page, size_t offset,
> > +					size_t bytes, struct iov_iter *i)
> > +{
> > +	void *kaddr = kmap_atomic(page);
> > +	size_t wanted = copy_to_iter_bvec(kaddr + offset, bytes, i);
> 
> missing newline.
> 
> > +	kunmap_atomic(kaddr);
> > +	return wanted;
> > +}

Are you seriously suggesting that:

static size_t copy_page_to_iter_bvec(struct page *page, size_t offset,
                                        size_t bytes, struct iov_iter *i)
{
        void *kaddr = kmap_atomic(page);
        size_t wanted = copy_to_iter_bvec(kaddr + offset, bytes, i);

        kunmap_atomic(kaddr);
        return wanted;
}

is more readable than without the newline?  I can see the point of the
rule for functions with a lot of variables, or a lot of lines, but I
don't see the point of it for such a small function.

In any case, this patch is now upstream, so I shan't be proposing any
stylistic changes for it.
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