Re: [PATCH net 1/1] net: devmem: prevent net-iov / page mixing

[Pavel Begunkov <asml.silence@gmail.com>]

On 7/22/26 20:49, Mina Almasry wrote:
> On Wed, Jul 22, 2026 at 3:59 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
>>
>> We should either have net_iov or page backed frags in a single skb,
>> otherwise it blows up down the stack. Don't allow mixing in
>> zerocopy_fill_skb_from_devmem().
>>
>> Fixes: bd61848900bff ("net: devmem: Implement TX path")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> 
> It's true that we don't support mixing niov types and doing so would
> blow up, but this is an unnecessary defensive check imo. The calling
> code should not (and does not, I hope) have an edge case where it
> tries to mix and match niov types.

Nope, that can easily happen.

> Maybe a DEBUG_NET_WARN_ON check to help catch bugs in the calling code
> may be fine?
> 
> Also we don't really support mixing different niov sub-types in an skb
> (like IO_URING + DMABUF) afaict, so might as well go the extra mile
> and check that it's all devmem niovs specifically.

All those type intermixing rules might be flaky, maybe we should
start with DEBUG_NET_WARN_ON, but at least for tx from a quick
look it's handled by ubuf_info checks.

> And might as well put the check in skb_add_rx_frag_netmem. It's the
> same situation on RX, we don't support mixing there (and I hope no
> code path leads to mixing today).

A DEBUG_NET_WARN_ON there might be a very good idea, I agree.

-- 
Pavel Begunkov