alignment faults in 3.6

Eric Dumazet eric.dumazet at
Fri Oct 12 06:04:23 EDT 2012

On Fri, 2012-10-12 at 10:03 +0100, Russell King - ARM Linux wrote:

> No.  It is my understanding that various IP option processing can also
> cause the alignment fault handler to be invoked, even when the packet is
> properly aligned, and then there's jffs2/mtd which also relies upon
> alignment faults being fixed up.

Oh well.

We normally make sure we dont have alignment faults on arches that dont

So if you find an offender, please report a bug, because I can guarantee
you we will _fix_ it.

One example of a fix was the following subtle one.

commit 117632e64d2a5f464e491fe221d7169a3814a77b
tcp: take care of misalignments
We discovered that TCP stack could retransmit misaligned skbs if a
malicious peer acknowledged sub MSS frame. This currently can happen
only if output interface is non SG enabled : If SG is enabled, tcp
builds headless skbs (all payload is included in fragments), so the tcp
trimming process only removes parts of skb fragments, header stay
Some arches cant handle misalignments, so force a head reallocation and
shrink headroom to MAX_TCP_HEADER.

Dont care about misaligments on x86 and PPC (or other arches setting

This patch introduces __pskb_copy() which can specify the headroom of
new head, and pskb_copy() becomes a wrapper on top of __pskb_copy()

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