Stray nulls when reading from AF_UNIX sockets

Stray nulls when reading from AF_UNIX sockets

[Anders Blomdell]

Hi,

With recent kernels (2.6.22.x), we are experiencing that random null characters
are read by  our mySQL server (i.e. the number of bytes read by the server is
larger than the number of bytes written). I'm currently investigating the
problem (that occurs every 1-35 hours on a loaded system). A current suspect is
the following code in  net/unix/af_unix.c:

  static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
                                 struct msghdr *msg, size_t size,
                                 int flags)
  {
    ...
    return copied ? : err;
  }

Shouldn't this read:

    return copied ? copied : err;

Or am I missing something?

Please CC me personally.

Regards

Anders Blomdell


-- 
Anders Blomdell                  Email: anders.blomdell@control.lth.se
Department of Automatic Control
Lund University                  Phone:    +46 46 222 4625
P.O. Box 118                     Fax:      +46 46 138118
SE-221 00 Lund, Sweden

Re: Stray nulls when reading from AF_UNIX sockets

[Randy Dunlap]

On Thu, 06 Sep 2007 17:48:07 +0200 Anders Blomdell wrote:

> Hi,
> 
> With recent kernels (2.6.22.x), we are experiencing that random null characters
> are read by  our mySQL server (i.e. the number of bytes read by the server is
> larger than the number of bytes written). I'm currently investigating the
> problem (that occurs every 1-35 hours on a loaded system). A current suspect is
> the following code in  net/unix/af_unix.c:
> 
>   static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,
>                                  struct msghdr *msg, size_t size,
>                                  int flags)
>   {
>     ...
>     return copied ? : err;
>   }
> 
> Shouldn't this read:
> 
>     return copied ? copied : err;
> 
> Or am I missing something?
> 
> Please CC me personally.

It's (?:) a gcc extension.  See

http://gcc.gnu.org/onlinedocs/gcc-4.2.1/gcc/Conditionals.html#Conditionals

---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***

Re: Stray nulls when reading from AF_UNIX sockets

[David Miller]

From: Anders Blomdell <anders.blomdell@control.lth.se>
Date: Thu, 06 Sep 2007 17:48:07 +0200

>     return copied ? : err;
>   }
> 
> Shouldn't this read:
> 
>     return copied ? copied : err;
> 
> Or am I missing something?

These two statements are equivalent, the first version is
a shorthand allowed by gcc.

Re: Stray nulls when reading from AF_UNIX sockets

[Jan Engelhardt]

On Sep 6 2007 12:23, David Miller wrote:
>>     return copied ? : err;
>>   }
>> 
>> Shouldn't this read:
>> 
>>     return copied ? copied : err;
>> 
>> Or am I missing something?
>
>These two statements are equivalent, the first version is
>a shorthand allowed by gcc.

Not only that. With x?x:z, x is evaluated twice,
while with x?:z, x is only evaluated once. That's for stuff when you 
want to, say [dumb example follows],

size_t my_read(..) {
    return read(..) ? : -1
}

and the only other way would be to use a temporary,

size_t my_read(..) {
	size_t x = read(..);
	return x ? x : -1;
}

gcc should be smart enough to also do optimization in the second case..


	Jan
--