netdev - Re: [PATCH] net: devmem: Remove dst (ENODEV) check in net_devmem_get

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Message-ID: <CANn89iL5nN6ZX+gJpEBXRiSdadaHvSPPDQ3QnUArBi1fnB0ddg@mail.gmail.com>
Date: Tue, 28 Oct 2025 00:03:19 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Shivaji Kant <shivajikant@...gle.com>
Cc: netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Mina Almasry <almasrymina@...gle.com>, Stanislav Fomichev <sdf@...ichev.me>, 
	Pavel Begunkov <asml.silence@...il.com>, Pranjal Shrivastava <praan@...gle.com>, 
	Vedant Mathur <vedantmathur@...gle.com>
Subject: Re: [PATCH] net: devmem: Remove dst (ENODEV) check in net_devmem_get_binding

On Mon, Oct 27, 2025 at 11:07 PM Shivaji Kant <shivajikant@...gle.com> wrote:
>
> The Devmem TX binding lookup function, performs a strict
> check against the socket's destination cache (`dst`) to
> ensure the bound `dmabuf_id` corresponds to the correct
> network device (`dst->dev->ifindex == binding->dev->ifindex`).
>
> However, this check incorrectly fails and returns `-ENODEV`
> if the socket's route cache entry (`dst`) is merely missing
> or expired (`dst == NULL`). This scenario is observed during
> network events, such as when flow steering rules are deleted,
> leading to a temporary route cache invalidation.
>
> The parent caller, `tcp_sendmsg_locked()`, is already
> responsible for acquiring or validating the route (`dst_entry`).
> If `dst` is `NULL`, `tcp_sendmsg_locked()` will correctly
> derive the route before transmission.
>
> This patch removes the `dst` validation from
> `net_devmem_get_binding()`. The function now only validates
> the existence of the binding and its TX vector, relying on the
> calling context for device/route correctness. This allows
> temporary route cache misses to be handled gracefully by the
> TCP/IP stack without ENODEV error on the Devmem TX path.
>
> Reported-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Vedant Mathur <vedantmathur@...gle.com>
> Suggested-by: Eric Dumazet <edumazet@...gle.com>
> Fixes: bd61848900bf ("net: devmem: Implement TX path")
> Signed-off-by: Shivaji Kant <shivajikant@...gle.com>
> ---
>  net/core/devmem.c | 27 ++++++++++++++++++++++++---
>  1 file changed, 24 insertions(+), 3 deletions(-)
>

Patch looks good, but the title should be improved a bit.

"Remove dst (ENODEV) check in net_devmem_get_binding"

It is not about removing the check, more about refreshing the dst if necessary ?

Please wait ~24 hours before sending an updated version, to  give time
for other reviewers to chime in.

Thanks !