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

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Message-ID: <CAAVpQUBRymieDppfa=QsS2Q9u9mkYoq+Rb-iCtKC7t=2bRrpbw@mail.gmail.com>
Date: Tue, 6 Jan 2026 23:41:57 -0800
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	edumazet@...gle.com, pabeni@...hat.com, horms@...nel.org, axboe@...nel.dk, 
	Willem de Bruijn <willemb@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH net] net: do not write to msg_get_inq in callee

On Tue, Jan 6, 2026 at 7:06 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> From: Willem de Bruijn <willemb@...gle.com>
>
> NULL pointer dereference fix.
>
> 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 for that reason.
> But it is more robust to avoid the write, and slightly simpler code.
> And it fixes a bug, see below.
>
> 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 fix to commit 4d1442979e4a ("af_unix: don't post cmsg for
> SO_INQ unless explicitly asked for"), which fixed the inverse.
>
> Also avoid NULL pointer dereference in unix_stream_read_generic if
> state->msg is NULL and msg->msg_get_inq is written. A NULL state->msg
> can happen when splicing as of commit 2b514574f7e8 ("net: af_unix:
> implement splice for stream af_unix sockets").
>
> Also collapse two branches using a bitwise or.
>
> Cc: stable@...r.kernel.org
> Fixes: 4d1442979e4a ("af_unix: don't post cmsg for SO_INQ unless explicitly asked for")
> Link: https://lore.kernel.org/netdev/willemdebruijn.kernel.24d8030f7a3de@gmail.com/
> Signed-off-by: Willem de Bruijn <willemb@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Thanks!