netdev - RE: ICMP packets - ll_temac with Microblaze

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Date:	Wed, 21 Dec 2011 16:39:22 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Eric Dumazet" <eric.dumazet@...il.com>,
	"Jun Zhao" <mypopydev@...il.com>
Cc:	<monstr@...str.eu>, "David Miller" <davem@...emloft.net>,
	"John Williams" <john.williams@...alogix.com>,
	<netdev@...r.kernel.org>
Subject: RE: ICMP packets - ll_temac with Microblaze

> If not, a malicious attacker could send 1-byte frames and exhaust your
> kernel memory.

That used to happen in SVR4 - it was possible for single byte TCP (etc)
data to be queued at a STREAM head (which counted actual data bytes)
in a 2k message block.
Actually we also managed to use all kernel memory queueing zero sized
STREAMS messages.

> If you want to reduce it, you might use copybreak : Some drivers copy
> the data into a small skb instead of providing a jumbo frame to upper
> stack.

Many, many moons ago I wrote an ethernet driver that received into
an array of 128 (mostly) 512byte buffers. Full sized frames would have
multiple rx ring entries, but could almost always be copied into a
correctly
sized buffer with a single aligned copy (cache-line aligned if useful).
This was significantly faster than other schemes - especially
on systems where the iommu needed setting to allow the ethernet
hardware to access memory.

I don't know if the linux skb buffers would allow the ethernet
driver to use (say) 1600 byte rx buffers, and link them together
when a long frame arrives. Most ethernet HW I've seen will
fragment long receives.

The painful hardware is that which enforces a 4n byte alignment
on the rx buffer! - on systens that can't do misaligned accesses.
2 bytes of junk would be fine!

	David

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