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From: Martin Lau <kafai@fb.com>
To: Jakub Sitnicki <jakub@cloudflare.com>
Cc: John Fastabend <john.fastabend@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"kernel-team@cloudflare.com" <kernel-team@cloudflare.com>
Subject: Re: [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a SOCKMAP
Date: Mon, 25 Nov 2019 22:07:12 +0000	[thread overview]
Message-ID: <20191125220709.jqywizwbr3xwsazi@kafai-mbp> (raw)
In-Reply-To: <87o8x0nsra.fsf@cloudflare.com>

On Mon, Nov 25, 2019 at 11:40:41AM +0100, Jakub Sitnicki wrote:
> On Mon, Nov 25, 2019 at 05:17 AM CET, John Fastabend wrote:
> > Alexei Starovoitov wrote:
> >> On Sat, Nov 23, 2019 at 12:07:48PM +0100, Jakub Sitnicki wrote:
> >> > SOCKMAP now supports storing references to listening sockets. Nothing keeps
> >> > us from using it as an array of sockets to select from in SK_REUSEPORT
> >> > programs.
> >> >
> >> > Whitelist the map type with the BPF helper for selecting socket. However,
> >> > impose a restriction that the selected socket needs to be a listening TCP
> >> > socket or a bound UDP socket (connected or not).
> >> >
> >> > The only other map type that works with the BPF reuseport helper,
> >> > REUSEPORT_SOCKARRAY, has a corresponding check in its update operation
> >> > handler.
> >> >
> >> > Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
> >> > ---
> >
> > [...]
> >
> >> > diff --git a/net/core/filter.c b/net/core/filter.c
> >> > index 49ded4a7588a..e3fb77353248 100644
> >> > --- a/net/core/filter.c
> >> > +++ b/net/core/filter.c
> >> > @@ -8723,6 +8723,8 @@ BPF_CALL_4(sk_select_reuseport, struct sk_reuseport_kern *, reuse_kern,
> >> >  	selected_sk = map->ops->map_lookup_elem(map, key);
> >> >  	if (!selected_sk)
> >> >  		return -ENOENT;
> >> > +	if (!sock_flag(selected_sk, SOCK_RCU_FREE))
> >> > +		return -EINVAL;
If I read it correctly,
this is to avoid the following "if (!reuse)" to return -ENOENT,
and instead returns -EINVAL for non TCP_LISTEN tcp_sock.
It should at least only be done under the "if (!reuse)" then.

Checking SOCK_RCU_FREE to imply TCP_LISTEN is not ideal.
It is not immediately obvious.  Why not directly check
TCP_LISTEN?

Note that the SOCK_RCU_FREE check at the 'slow-path'
reuseport_array_update_check() is because reuseport_array does depend on
call_rcu(&sk->sk_rcu,...) to work, e.g. the reuseport_array
does not hold the sk_refcnt.

> >>
> >> hmm. I wonder whether this breaks existing users...
> >
> > There is already this check in reuseport_array_update_check()
> >
> > 	/*
> > 	 * sk must be hashed (i.e. listening in the TCP case or binded
> > 	 * in the UDP case) and
> > 	 * it must also be a SO_REUSEPORT sk (i.e. reuse cannot be NULL).
> > 	 *
> > 	 * Also, sk will be used in bpf helper that is protected by
> > 	 * rcu_read_lock().
> > 	 */
> > 	if (!sock_flag(nsk, SOCK_RCU_FREE) || !sk_hashed(nsk) || !nsk_reuse)
> > 		return -EINVAL;
> >
> > So I believe it should not cause any problems with existing users. Perhaps
> > we could consolidate the checks a bit or move it into the update paths if we
> > wanted. I assume Jakub was just ensuring we don't get here with SOCK_RCU_FREE
> > set from any of the new paths now. I'll let him answer though.
> 
> That was exactly my thinking here.
> 
> REUSEPORT_SOCKARRAY can't be populated with sockets that don't have
> SOCK_RCU_FREE set. This makes the flag check in sk_select_reuseport BPF
> helper redundant for this map type.
> 
> SOCKMAP, OTOH, allows storing established TCP sockets, which don't have
> SOCK_RCU_FREE flag and shouldn't be used as reuseport targets. The newly
> added check protects us against it.
> 
> I have a couple tests in the last patch for it -
> test_sockmap_reuseport_select_{listening,connected}. Admittedly, UDP is
> not covered.
> 
> Not sure how we could go about moving the checks to the update path for
> SOCKMAP. At update time we don't know if the map will be used with a
> reuseport or a sk_{skb,msg} program.
or make these checks specific to the sockmap's lookup path.



digress a little from this patch,
will the upcoming patches/examples show the use case to have both
TCP_LISTEN and TCP_ESTABLISHED sk in the same sock_map?

  reply	other threads:[~2019-11-25 22:07 UTC|newest]

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-23 11:07 [PATCH bpf-next 0/8] Extend SOCKMAP to store listening sockets Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 1/8] bpf, sockmap: Return socket cookie on lookup from syscall Jakub Sitnicki
2019-11-24  5:32   ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 2/8] bpf, sockmap: Let all kernel-land lookup values in SOCKMAP Jakub Sitnicki
2019-11-24  5:35   ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 3/8] bpf, sockmap: Allow inserting listening TCP sockets into SOCKMAP Jakub Sitnicki
2019-11-24  5:38   ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 4/8] bpf, sockmap: Don't let child socket inherit psock or its ops on copy Jakub Sitnicki
2019-11-24  5:56   ` John Fastabend
2019-11-25 22:38   ` Martin Lau
2019-11-26 15:54     ` Jakub Sitnicki
2019-11-26 17:16       ` Martin Lau
2019-11-26 18:36         ` Jakub Sitnicki
     [not found]           ` <87sglsfdda.fsf@cloudflare.com>
2019-12-11 17:20             ` Martin Lau
2019-12-12 11:27               ` Jakub Sitnicki
2019-12-12 19:23                 ` Martin Lau
2019-12-17 15:06                   ` Jakub Sitnicki
2019-11-26 18:43         ` John Fastabend
2019-11-27 22:18           ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 5/8] bpf: Allow selecting reuseport socket from a SOCKMAP Jakub Sitnicki
2019-11-24  5:57   ` John Fastabend
2019-11-25  1:24   ` Alexei Starovoitov
2019-11-25  4:17     ` John Fastabend
2019-11-25 10:40       ` Jakub Sitnicki
2019-11-25 22:07         ` Martin Lau [this message]
2019-11-26 14:30           ` Jakub Sitnicki
2019-11-26 19:03             ` Martin Lau
2019-11-27 21:34               ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 6/8] libbpf: Recognize SK_REUSEPORT programs from section name Jakub Sitnicki
2019-11-24  5:57   ` John Fastabend
2019-11-23 11:07 ` [PATCH bpf-next 7/8] selftests/bpf: Extend SK_REUSEPORT tests to cover SOCKMAP Jakub Sitnicki
2019-11-24  6:00   ` John Fastabend
2019-11-25 22:30   ` Martin Lau
2019-11-26 14:32     ` Jakub Sitnicki
2019-12-12 10:30     ` Jakub Sitnicki
2019-11-23 11:07 ` [PATCH bpf-next 8/8] selftests/bpf: Tests for SOCKMAP holding listening sockets Jakub Sitnicki
2019-11-24  6:04   ` John Fastabend
2019-11-24  6:10 ` [PATCH bpf-next 0/8] Extend SOCKMAP to store " John Fastabend
2019-11-25  9:22   ` Jakub Sitnicki

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