| Message ID | 1462312458-2077-1-git-send-email-peter@lekensteyn.nl |
|---|---|
| State | Rejected, archived |
| Delegated to: | David Miller |
| Headers | show |
On Tue, 2016-05-03 at 23:54 +0200, Peter Wu wrote: > When applications use listen() with a backlog of 0, the kernel would > set the maximum connection request queue to zero. This causes false > reports of SYN flooding (if tcp_syncookies is enabled) or packet drops > otherwise. > > Prior kernels enforce a minimum size of 8, so do that now as well. > > Fixes: ef547f2ac16b ("tcp: remove max_qlen_log") > Signed-off-by: Peter Wu <peter@lekensteyn.nl> > --- > Hi, > > This patch fixes a regression from Linux 4.4. Use of "qemu-arm -g 1234" > would trigger the following warning in dmesg: > > TCP: request_sock_TCP: Possible SYN flooding on port 1234. Sending cookies. Check SNMP counters. > > For some users the "tcp: remove max_qlen_log" change already broke > applications[1]. While listen(3p) says that a backlog argument of 0 sets > the length to an "implementation-defined minimum value", I doubt that > "0" should be considered a valid value (as demonstrated in the above two > real-world applications that worked fine before). It is a hint anyway. > > This patch was tested on top of Linux v4.5 and removes the warning which > would otherwise be present (due to the inet_csk_reqsk_queue_is_full() > check in tcp_conn_request). > > I also looked at modifying the backlog value in inet_listen, but that > might have other unintended effects: > > - If TFO is enabled and tcp_fastopen==0x400, listen(fd, 0) currently > disables TFO (also possible via setsockopt). Forcing a minimum breaks > this path (unlikely to be a problem though since TFO users likely set > a much higher backlog). > - sk->sk_max_ack_backlog is also reported via tcp statistics and seems > really to be the hint rather than the actual interpreted value. > > Kind regards, > Peter > > [1]: https://lkml.kernel.org/r/CANn89i+OKfw896-N5KsNDEikzUidR8yX1JC089hJnGGfDQ0mzw@mail.gmail.com > --- > include/net/inet_connection_sock.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h > index 49dcad4..ca0fdbc 100644 > --- a/include/net/inet_connection_sock.h > +++ b/include/net/inet_connection_sock.h > @@ -296,7 +296,7 @@ static inline int inet_csk_reqsk_queue_young(const struct sock *sk) > > static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk) > { > - return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog; > + return inet_csk_reqsk_queue_len(sk) >= max(8U, sk->sk_max_ack_backlog); > } > > void inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req); Well, I believe I already gave my opinion on this. listen backlog is not a hint. This is a limit. It is the limit of outstanding children in accept queue. If backlog is 0, no child can be put in the accept queue. It is therefore Working As Intented.
On Tue, May 03, 2016 at 05:25:44PM -0700, Eric Dumazet wrote: > On Tue, 2016-05-03 at 23:54 +0200, Peter Wu wrote: > > When applications use listen() with a backlog of 0, the kernel would > > set the maximum connection request queue to zero. This causes false > > reports of SYN flooding (if tcp_syncookies is enabled) or packet drops > > otherwise. > > > > Prior kernels enforce a minimum size of 8, so do that now as well. > > > > Fixes: ef547f2ac16b ("tcp: remove max_qlen_log") > > Signed-off-by: Peter Wu <peter@lekensteyn.nl> > > --- > > Hi, > > > > This patch fixes a regression from Linux 4.4. Use of "qemu-arm -g 1234" > > would trigger the following warning in dmesg: > > > > TCP: request_sock_TCP: Possible SYN flooding on port 1234. Sending cookies. Check SNMP counters. > > > > For some users the "tcp: remove max_qlen_log" change already broke > > applications[1]. While listen(3p) says that a backlog argument of 0 sets > > the length to an "implementation-defined minimum value", I doubt that > > "0" should be considered a valid value (as demonstrated in the above two > > real-world applications that worked fine before). It is a hint anyway. > > > > This patch was tested on top of Linux v4.5 and removes the warning which > > would otherwise be present (due to the inet_csk_reqsk_queue_is_full() > > check in tcp_conn_request). > > > > I also looked at modifying the backlog value in inet_listen, but that > > might have other unintended effects: > > > > - If TFO is enabled and tcp_fastopen==0x400, listen(fd, 0) currently > > disables TFO (also possible via setsockopt). Forcing a minimum breaks > > this path (unlikely to be a problem though since TFO users likely set > > a much higher backlog). > > - sk->sk_max_ack_backlog is also reported via tcp statistics and seems > > really to be the hint rather than the actual interpreted value. > > > > Kind regards, > > Peter > > > > [1]: https://lkml.kernel.org/r/CANn89i+OKfw896-N5KsNDEikzUidR8yX1JC089hJnGGfDQ0mzw@mail.gmail.com > > --- > > include/net/inet_connection_sock.h | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h > > index 49dcad4..ca0fdbc 100644 > > --- a/include/net/inet_connection_sock.h > > +++ b/include/net/inet_connection_sock.h > > @@ -296,7 +296,7 @@ static inline int inet_csk_reqsk_queue_young(const struct sock *sk) > > > > static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk) > > { > > - return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog; > > + return inet_csk_reqsk_queue_len(sk) >= max(8U, sk->sk_max_ack_backlog); > > } > > > > void inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req); > > Well, I believe I already gave my opinion on this. > > listen backlog is not a hint. This is a limit. > > It is the limit of outstanding children in accept queue. > > If backlog is 0, no child can be put in the accept queue. > > It is therefore Working As Intented. Alright, this is actually described in the Linux manual (listen(2)): Now it specifies the queue length for completely established sockets waiting to be accepted, instead of the number of incomplete connection requests. The POSIX manual (listen(3p)) says: A backlog argument of 0 may allow the socket to accept connections, in which case the length of the listen queue may be set to an implementation-defined minimum value. Not accepting a connection is apparently valid due to the wording ("may allow"). Fair enough, please drop this patch. Applications will have to be fixed then. Kind regards, Peter
On 05/03/2016 05:25 PM, Eric Dumazet wrote: > On Tue, 2016-05-03 at 23:54 +0200, Peter Wu wrote: >> When applications use listen() with a backlog of 0, the kernel would >> set the maximum connection request queue to zero. This causes false >> reports of SYN flooding (if tcp_syncookies is enabled) or packet drops >> otherwise. >> > Well, I believe I already gave my opinion on this. > > listen backlog is not a hint. This is a limit. > > It is the limit of outstanding children in accept queue. > > If backlog is 0, no child can be put in the accept queue. > > It is therefore Working As Intented. Dropping the connection attempt makes sense, but is entering/claiming synflood really indicated in the case of a zero-length accept queue? rick
On Wed, 2016-05-04 at 10:24 -0700, Rick Jones wrote: > Dropping the connection attempt makes sense, but is entering/claiming > synflood really indicated in the case of a zero-length accept queue? This is a one time message. This is how people can learn about their user space bugs, or too small backlog ;) Being totally silent would be not so nice.
On 05/04/2016 10:34 AM, Eric Dumazet wrote: > On Wed, 2016-05-04 at 10:24 -0700, Rick Jones wrote: > >> Dropping the connection attempt makes sense, but is entering/claiming >> synflood really indicated in the case of a zero-length accept queue? > > This is a one time message. > > This is how people can learn about their user space bugs, or too small > backlog ;) > > Being totally silent would be not so nice. > Assuming Peter's assertion about just drops when syncookies are not enabled is accurate, should there be some one-time message in that case too? rick
On Wed, 2016-05-04 at 11:05 -0700, Rick Jones wrote: > Assuming Peter's assertion about just drops when syncookies are not > enabled is accurate, should there be some one-time message in that case too? We have plenty of drop points in the kernel without a message in syslog, but proper SNMP counter updates. Look at my recent commit 9caad864151e525 ("tcp: increment sk_drops for listeners") "ss -tm state listening" will show you the drop count per listener.
diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h index 49dcad4..ca0fdbc 100644 --- a/include/net/inet_connection_sock.h +++ b/include/net/inet_connection_sock.h @@ -296,7 +296,7 @@ static inline int inet_csk_reqsk_queue_young(const struct sock *sk) static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk) { - return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog; + return inet_csk_reqsk_queue_len(sk) >= max(8U, sk->sk_max_ack_backlog); } void inet_csk_reqsk_queue_drop(struct sock *sk, struct request_sock *req);
When applications use listen() with a backlog of 0, the kernel would set the maximum connection request queue to zero. This causes false reports of SYN flooding (if tcp_syncookies is enabled) or packet drops otherwise. Prior kernels enforce a minimum size of 8, so do that now as well. Fixes: ef547f2ac16b ("tcp: remove max_qlen_log") Signed-off-by: Peter Wu <peter@lekensteyn.nl> --- Hi, This patch fixes a regression from Linux 4.4. Use of "qemu-arm -g 1234" would trigger the following warning in dmesg: TCP: request_sock_TCP: Possible SYN flooding on port 1234. Sending cookies. Check SNMP counters. For some users the "tcp: remove max_qlen_log" change already broke applications[1]. While listen(3p) says that a backlog argument of 0 sets the length to an "implementation-defined minimum value", I doubt that "0" should be considered a valid value (as demonstrated in the above two real-world applications that worked fine before). It is a hint anyway. This patch was tested on top of Linux v4.5 and removes the warning which would otherwise be present (due to the inet_csk_reqsk_queue_is_full() check in tcp_conn_request). I also looked at modifying the backlog value in inet_listen, but that might have other unintended effects: - If TFO is enabled and tcp_fastopen==0x400, listen(fd, 0) currently disables TFO (also possible via setsockopt). Forcing a minimum breaks this path (unlikely to be a problem though since TFO users likely set a much higher backlog). - sk->sk_max_ack_backlog is also reported via tcp statistics and seems really to be the hint rather than the actual interpreted value. Kind regards, Peter [1]: https://lkml.kernel.org/r/CANn89i+OKfw896-N5KsNDEikzUidR8yX1JC089hJnGGfDQ0mzw@mail.gmail.com --- include/net/inet_connection_sock.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)