netdev - Re: [PATCH net-next] net: do not write to msg_get

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Message-ID: <CANn89iL+AuhJw7-Ma4hQsgQ5X0vxOwToSr2mgVSbkSauy-TGkg@mail.gmail.com>
Date: Mon, 5 Jan 2026 18:21:48 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com, horms@...nel.org, axboe@...nel.dk, kuniyu@...gle.com, 
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] net: do not write to msg_get_inq in caller

On Mon, Jan 5, 2026 at 5:33 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> msg_get_inq is an input field from caller to callee. Don't set it in
> the callee, as the caller may not clear it on struct reuse.
>
> This is a kernel-internal variant of msghdr only, and the only user
> does reinitialize the field. So this is not critical.
>
> But it is more robust to avoid the write, and slightly simpler code.
>
> Callers set msg_get_inq to request the input queue length to be
> returned in msg_inq. This is equivalent to but independent from the
> SO_INQ request to return that same info as a cmsg (tp->recvmsg_inq).
> To reduce branching in the hot path the second also sets the msg_inq.
> That is WAI.
>
> This is a small follow-on to commit 4d1442979e4a ("af_unix: don't
> post cmsg for SO_INQ unless explicitly asked for"), which fixed the
> inverse.
>
> Also collapse two branches using a bitwise or.
>
> Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.24d8030f7a3de@gmail.com/
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> ---

Patch looks sane to me, but the title is a bit confusing, I guess you meant

"net: do not write to msg_get_inq in callee" ?

Also, unix_stream_read_generic() is currently potentially adding a NULL deref
if u->recvmsg_inq is non zero, but msg is NULL ?

If this is the case  we need a Fixes: tag.