linux-kernel - RE: v4.14-rc3/arm64 DABT exception in atomic_inc() / __skb

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Date:   Thu, 26 Oct 2017 15:24:29 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willem de Bruijn' <willemdebruijn.kernel@...il.com>,
        Wei Wei <dotweiba@...il.com>
CC:     Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: RE: v4.14-rc3/arm64 DABT exception in atomic_inc() / __skb_clone()

From: Willem de Bruijn
> Sent: 25 October 2017 19:50
...
> From skb->dev and netdev_priv, the tun device has flags 0x1002 ==
> IFF_TAP | IFF_NO_PI. This kernel precedes the recent support for
> IFF_NAPI and IFF_NAPI_FRAGS. The allocation most likely happened
> in tun_build_skb from current->task_frag. It would be a previous
> allocation that left alloc_frag->offset unaligned. But perhaps this code
> needs to perform alignment before setting skb->head.
>
> At least on platforms where atomic on dataref must be aligned.

Isn't that true of almost everything?
I'm not even sure x86 always (ever?) manages locked cycles on
misaligned addresses.

	David