netdev - Re: [PATCH net] net: implement lockless setsockopt(SO_PEEK

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Date: Mon, 19 Feb 2024 14:56:11 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <willemdebruijn.kernel@...il.com>
CC: <daan.j.demeyer@...il.com>, <davem@...emloft.net>, <dsahern@...nel.org>,
	<edumazet@...gle.com>, <eric.dumazet@...il.com>, <kuba@...nel.org>,
	<kuniyu@...zon.com>, <martin.lau@...nel.org>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>
Subject: Re: [PATCH net] net: implement lockless setsockopt(SO_PEEK_OFF)

From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Date: Mon, 19 Feb 2024 12:20:39 -0500
> Eric Dumazet wrote:
> > On Mon, Feb 19, 2024 at 5:07 PM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > Eric Dumazet wrote:
> > > > syzbot reported a lockdep violation [1] involving af_unix
> > > > support of SO_PEEK_OFF.
> > > >
> > > > Since SO_PEEK_OFF is inherently not thread safe (it uses a per-socket
> > > > sk_peek_off field), there is really no point to enforce a pointless
> > > > thread safety in the kernel.
> > >
> > > Would it be sufficient to just move the setsockopt, so that the
> > > socket lock is not taken, but iolock still is?
> > 
> > Probably, if we focus on the lockdep issue rather than the general
> > SO_PEEK_OFF mechanism.
> > 
> > We could remove unix_set_peek_off() in net-next,
> > unless someone explains why keeping a locking on iolock is needed.

Probably to avoid a small race where setsockopt() does not take effect.

                                    sk_peek_offset_bwd
  setsockopt                        |- off = READ_ONCE(sk_peek_off)
  `- WRITE_ONCE(sk_peek_off, val)   |
                                    `- WRITE_ONCE(sk_peek_off, off - val)

> 
> Since calling SO_PEEK_OFF and recvmsg concurrently is inherently not
> thread-safe, fine to remove it all.

Agreed, we can do that locklessly.

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