Socket hangs in read() when putting interface down

Socket hangs in read() when putting interface down

[chris schlund]

Using read() on a socket(AF_INET, SOCK_STREAM) connected via any network
interface (e.g eth0) and putting this interface down (ifconfig) while
the socket is in read() produces a hanger (the read() will not return
anymore).
Even if I install sigaction signal handler I got no signal when the
interface is removed.

This may be ok on a major interface like eth0 but it is a problem when
using a ppp interface via GSM data or GPRS connection.
Killing the pppd while the socket waits in read() produces the same
behaviour. Also a broken connection looks the same.
It is reproducable on several 2.6 kernel versions including the lastest
stable 2.6.21.

On my understanding "ifconfig down" should lead into a read() error.

Unfortunately I am not very familiar with in-depth kernel programming
and I am still wondering about the magic inside the kernel:

ifconfig -> net/dev.c:dev_close() notifies a call chain which probably
could/should handle this.
But I miss the dependency to net/ipv4/tcp.c: tcp_recvmsg() function.
This is where I think my receive is blocking.

Can someone give me a hint where to look?

Thanks and kind regards
Chris


p.s.
This happens only, when socket is in blocking mode.
I am using a simple example like:
s = socket();
connect(s);
while(){
  read(s)
}

Re: Socket hangs in read() when putting interface down

[Rick Jones]

chris schlund wrote:
> Using read() on a socket(AF_INET, SOCK_STREAM) connected via any network
> interface (e.g eth0) and putting this interface down (ifconfig) while
> the socket is in read() produces a hanger (the read() will not return
> anymore).
> Even if I install sigaction signal handler I got no signal when the
> interface is removed.

I'd not think there was a direct tie-in between an application signal 
handler and an interface being downed.

> This may be ok on a major interface like eth0 but it is a problem when
> using a ppp interface via GSM data or GPRS connection.
> Killing the pppd while the socket waits in read() produces the same
> behaviour. Also a broken connection looks the same.
> It is reproducable on several 2.6 kernel versions including the lastest
> stable 2.6.21.
> 
> On my understanding "ifconfig down" should lead into a read() error.

Why?  Can't traffic for the endpoint arrive via some other interface on 
the system?  I would think that getting out of a blocking read() 
requires the application to either have a timer running (and so get out 
of read() with EINTR) or to have keepalives enabled on the TCP 
connection (I'm assuming TCP for an AF_INET, SOCK_STREAM although I 
suppose it could be SCTP)

rick jones

Re: Socket hangs in read() when putting interface down

[chris schlund]

>> This may be ok on a major interface like eth0 but it is a problem when
>> using a ppp interface via GSM data or GPRS connection.
>> Killing the pppd while the socket waits in read() produces the same
>> behaviour. Also a broken connection looks the same.
>> It is reproducable on several 2.6 kernel versions including the lastest
>> stable 2.6.21.
>>
>> On my understanding "ifconfig down" should lead into a read() error.
> 
> Why?  Can't traffic for the endpoint arrive via some other interface on
> the system?  I would think that getting out of a blocking read()
> requires the application to either have a timer running (and so get out
> of read() with EINTR) or to have keepalives enabled on the TCP
> connection (I'm assuming TCP for an AF_INET, SOCK_STREAM although I
> suppose it could be SCTP)
> 
> rick jones

Of course, you are right.
I am so focused to my small embedded world, that I simply have forgotten
how IP-traffic works in big systems (to clarify my first post: I'm using
TCP).

Your hint with the self produced signal to get out of read() was very
helpful for me. Thats what I was missing.

I also learned another think, while reading the list FAQ:
PEBCAC (=Problem exists between chair and computer)
Obviously, thats what best described me!

Thank you very much!

Chris Schlund