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[Qemu-trivial] [PATCH] gdbstub: set listen backlog to 1

From: Peter Wu
Subject: [Qemu-trivial] [PATCH] gdbstub: set listen backlog to 1
Date: Wed, 4 May 2016 11:32:17 +0200

Avoid possible connection drops on Linux (when tcp_syncookies is
disabled) or fallbacks to SYN cookies with the following kernel warning:

    TCP: request_sock_TCP: Possible SYN flooding on port 1234. Sending cookies. 
 Check SNMP counters.

Since Linux 4.4 (ef547f2ac16b "tcp: remove max_qlen_log"), a backlog of
zero is really treated as the "queue length for completely established
sockets waiting to be accepted" (listen(2)). This is apparently a valid
interpretation of an "implementation-defined minimum value" for a
backlog value of 0 (listen(3p)). Previous kernels would use 8 as
minimum value, but that is no longer the case.

Signed-off-by: Peter Wu <address@hidden>

This was also reported at Linux, but the author of that patch argued that it is
intended behavior[1].

Kind regards,

 [1]: https://lkml.kernel.org/r/address@hidden
 gdbstub.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdbstub.c b/gdbstub.c
index b126bf5..f181dc3 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -1630,7 +1630,7 @@ static int gdbserver_open(int port)
         return -1;
-    ret = listen(fd, 0);
+    ret = listen(fd, 1);
     if (ret < 0) {

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