Re: [Xen-devel] [PATCH] netvm: check for page == NULL when propogating the skb->pfmemalloc flag

On Tue, Aug 14, 2012 at 11:05:22AM +0100, Mel Gorman wrote:

On Mon, Aug 13, 2012 at 11:41:44AM -0400, Konrad Rzeszutek Wilk wrote:

quoted

On Wed, Aug 08, 2012 at 03:50:46PM -0700, David Miller wrote:

quoted

From: Mel Gorman <mgorman@suse.de>
Date: Tue, 7 Aug 2012 09:55:55 +0100

quoted

Commit [c48a11c7: netvm: propagate page->pfmemalloc to skb] is responsible
for the following bug triggered by a xen network driver

 ...

quoted

The problem is that the xenfront driver is passing a NULL page to
__skb_fill_page_desc() which was unexpected. This patch checks that
there is a page before dereferencing.

Reported-and-Tested-by: Konrad Rzeszutek Wilk <redacted>
Signed-off-by: Mel Gorman <mgorman@suse.de>

That call to __skb_fill_page_desc() in xen-netfront.c looks completely bogus.
It's the only driver passing NULL here.

It looks to be passing a valid page pointer (at least by looking
at the code) so I am not sure how it got turned in a NULL.

Are we looking at different code bases? I see this and I was assuming it
was the source of the bug.

	__skb_fill_page_desc(skb, 0, NULL, 0, 0);

Yes! Well, that is embarrassing. I was looking at the first invocation of 
__skb_fill_page_desc (which is in xennet_alloc_rx_buffers) <sigh>

-- 
Mel Gorman
SUSE Labs

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