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

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Message-ID: <willemdebruijn.kernel.2124bbf561b5e@gmail.com>
Date: Mon, 05 Jan 2026 12:42:43 -0500
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Eric Dumazet <edumazet@...gle.com>, 
 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

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".

Okay. Will resubmit to net with a Fixes tag (after the usual 24hrs).

Thanks!