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

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Message-ID: <CANn89iKeksiX+cimHzm2GsQtfCE_i2ncs4GfO-WmUjRHXcWOXQ@mail.gmail.com>
Date: Mon, 5 Jan 2026 18:58:07 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Jens Axboe <axboe@...nel.dk>, netdev@...r.kernel.org, davem@...emloft.net, 
	kuba@...nel.org, pabeni@...hat.com, horms@...nel.org, 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 6:57 PM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jens Axboe wrote:
> > On 1/5/26 10:42 AM, Willem de Bruijn wrote:
> > > Eric Dumazet wrote:
> > >> 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" ?
> > >
> > > Indeed, thanks. Will fix.
> > >
> > >>
> > >> 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.
> > >
> > > Oh good point. state->msg can be NULL as of commit 2b514574f7e8 ("net:
> > > af_unix: implement splice for stream af_unix sockets"). That commit
> > > mentions "we mostly have to deal with a non-existing struct msghdr
> > > argument".
> >
> > Worth noting that this is currently not possible, as io_uring should
> > be the only one setting ->recvmsg_inq and it would not do that via
> > splice. Should still be fixed of course.
>
> recvmsg_inq is written from setsockopt SO_INQ. Do you mean
> msg_get_inq?
>
> I think this is reachable with a setsockopt + splice:
>
>         do_cmsg = READ_ONCE(u->recvmsg_inq);
>         if (do_cmsg)
>                 msg->msg_get_inq = 1;

It is a bit strange that syzbot did not find it yet.