netdev - Re: [PATCH net] net: adjust skb->truesize in ___pskb

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Message-ID: <CAF=yD-+0JrZZERqPfVNn=JQMZiT8M9nyrDjq6=GCQ+B+FwtGKg@mail.gmail.com>
Date:   Thu, 27 Apr 2017 14:43:11 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] net: adjust skb->truesize in ___pskb_trim()

On Wed, Apr 26, 2017 at 1:08 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> On Wed, Apr 26, 2017 at 6:07 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> From: Eric Dumazet <edumazet@...gle.com>
>>
>> Andrey found a way to trigger the WARN_ON_ONCE(delta < len) in
>> skb_try_coalesce() using syzkaller and a filter attached to a TCP
>> socket.
>>
>> As we did recently in commit 158f323b9868 ("net: adjust skb->truesize in
>> pskb_expand_head()") we can adjust skb->truesize from ___pskb_trim(),
>> via a call to skb_condense().
>>
>> If all frags were freed, then skb->truesize can be recomputed.
>>
>> This call can be done if skb is not yet owned, or destructor is
>> sock_edemux().
>
> Hi Eric,
>
> I still see the warning even with your patch.

Can this happen if sk_trim_filter_cap trims the skb to free some,
but not all, of the frags? If skb->data_len remains larger than
skb->end - skb->tail, skb_condense will not adjust the truesize.