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Groups > comp.os.linux.development.apps > #678

Re: Linux O_NONBLOCK bug/ quirk

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On 03/28/2014 01:12 PM, Rainer Weikusat wrote:
> Rainer Weikusat <rweikusat@mobileactivedefense.com> writes:
>
> [...]
>
>> a receive operation on a socket in
>> non-blocking mode can actually be blocked forever on Linux, example
>> code:
>>
>> ---------
>> #include <fcntl.h>
>> #include <string.h>
>> #include <sys/socket.h>
>> #include <sys/un.h>
>>
>> int main(void)
>> {
>>      struct sockaddr_un sun;
>>      int fd;
>>
>>      fd = socket(AF_UNIX, SOCK_DGRAM, 0);
>>      sun.sun_family = AF_UNIX;
>>      strncpy(sun.sun_path, "/tmp/bla", sizeof(sun.sun_path));
>>      bind(fd, (struct sockaddr *)&sun, sizeof(sun));
>>
>>      if (fork() == 0) read(fd, &fd, sizeof(fd));
>>
>>      sleep(1);
>>
>>      fcntl(fd, F_SETFL, O_NONBLOCK);
>>      read(fd, &fd, sizeof(fd));
>>
>>      return 0;
>> }
>> --------
>
> This is more serious than I thought. Considering newer APIs, the can
> also be
>
> ------------
> #include <fcntl.h>
> #include <string.h>
> #include <sys/socket.h>
> #include <sys/un.h>
>
> int main(void)
> {
>      struct sockaddr_un sun;
>      int fd;
>
>      fd = socket(AF_UNIX, SOCK_DGRAM, 0);
>      sun.sun_family = AF_UNIX;
>      strncpy(sun.sun_path, "/tmp/bla", sizeof(sun.sun_path));
>      bind(fd, (struct sockaddr *)&sun, sizeof(sun));
>
>      if (fork() == 0) recv(fd, &fd, sizeof(fd), 0);
>
>      sleep(1);
>
>      recv(fd, &fd, sizeof(fd), MSG_DONTWAIT);
>
>      return 0;
> }
> -------------
>
> What happens here is that the 2nd recv stops on a mutex held by the
> first and thus, effectively blocks until a message is received on this
> socket, IOW, the implementation of MSG_DONTWAIT is completely broken for
> datagram sockets (certainly AF_UNIX, likely, AF_INET, too). Further, it
> is not going to be fixed because the so-called 'networking maintainer'
> 'completely disagrees' with the idea that 'non-blocking operation'
> actually means 'the call will not block' (that's the state of 2 days
> ago).
>
If the MSG_DONTWAIT triggers an EAGAIN or  EWOULDBLOCK, it's easier to
maintain a connection in a stable state - as long as the forking
process knows the status of the forked process.  Communicating the 
status of a child process is not normally performed by the socket 
connection (though somebody probably has and will say that it can be 
done) - The wait () and waitpid () man pages are just one place to begin 
reading about communicating between endpoints.  If I'm reading this 
correctly, then the mutex would cause a hang.  But hangs aren't that 
unusual if a program doesn't take care to monitor the forked processes' 
status.  It also doesn't really say anything about the connection's 
state, or whether it's still possible to monitor it.

Mutexes aren't normally exposed as part of the socket API (right?) 
That's one of the reasons that socket close down modes exist at all - 
which you can view with netstat.



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Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-03-27 15:26 +0000
  Re: Linux O_NONBLOCK bug/ quirk crankypuss <crankypuss@nomail.invalid> - 2014-03-28 02:08 -0600
    Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-03-28 12:45 +0000
      Re: Linux O_NONBLOCK bug/ quirk crankypuss <crankypuss@nomail.invalid> - 2014-03-29 05:19 -0600
  Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-03-28 20:12 +0000
    Re: Linux O_NONBLOCK bug/ quirk unixb4coffee <unixb4coffee@gmail.com> - 2014-04-02 11:13 -0700
      Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-04-03 16:36 +0100
        Re: Linux O_NONBLOCK bug/ quirk unixb4coffee <unixb4coffee@gmail.com> - 2014-04-03 11:43 -0700
          Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-04-03 21:31 +0100
  Re: Linux O_NONBLOCK bug/ quirk Lusotec <nomail@nomail.not> - 2014-03-28 23:13 +0000
    Re: Linux O_NONBLOCK bug/ quirk Richard Kettlewell <rjk@greenend.org.uk> - 2014-03-29 11:15 +0000
    Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-03-30 19:42 +0100
    Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-04-16 12:42 +0100
      Re: Linux O_NONBLOCK bug/ quirk Rainer Weikusat <rweikusat@mobileactivedefense.com> - 2014-04-16 13:36 +0100

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