Re: [PATCH mptcp-next v3] mptcp: fix skb_ext leak in fallback mode

[Matthieu Baerts <matttbe@kernel.org>]

Hi Geliang,

On 02/09/2026 10:39, Geliang Tang wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> In fallback mode, MPTCP sockets behave as plain TCP and should not allocate
> SKB_EXT_MPTCP for transmitted skbs. The current code unconditionally
> allocates the extension, causing memory leaks when skbs are freed without
> releasing it.

I'm surprised by this. Do you have kmemleak reports?

Or is it linked to this issue?

  https://github.com/multipath-tcp/mptcp_net-next/issues/583

I'm asking because I would think that such issues would have been caught
by kmemleak, plus the fix is quite invasive.

> Fix by short-circuiting __mptcp_add_ext() in fallback mode and skipping all
> DSS bookkeeping in mptcp_sendmsg_frag(). Also allow TCP coalescing when
> mpext is NULL in fallback mode.
> 
> This latent bug will be exposed once TLS ULP support is added to fallback
> MPTCP sockets, as each sendmsg via the TLS path would leak one skb_ext
> object.

Do you mean this bug is only visible with the new feature? If yes, then
no need to threat it as a fix.

Then, I'm also wondering if this patch shouldn't be specific to this
case: maybe less code should be modified to deal with fallback sockets
once this happens? (I'm probably missing context to really help here)

Cheers,
Matt
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