[RFC] Zero-length write() does not generate a datagram on connected socket

[RFC] Zero-length write() does not generate a datagram on connected socket

[Stephen Hemminger]

The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
describes an issue where write() can't be used to generate a zero-length
datagram (but send, and sendto do work).

I think the following is needed:

--- a/net/socket.c	2007-08-20 09:54:28.000000000 -0700
+++ b/net/socket.c	2007-09-24 15:31:25.000000000 -0700
@@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
 	if (pos != 0)
 		return -ESPIPE;
 
-	if (iocb->ki_left == 0)	/* Match SYS5 behaviour */
-		return 0;
+	if (unlikely(iocb->ki_left == 0)) {
+		struct socket *sock = iocb->ki_filp->private_data;
+		if (sock->type == SOCK_STREAM)
+			return 0;
+	}
 
 	x = alloc_sock_iocb(iocb, &siocb);
 	if (!x)

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Herbert Xu]

Stephen Hemminger <shemminger@linux-foundation.org> wrote:
> The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> describes an issue where write() can't be used to generate a zero-length
> datagram (but send, and sendto do work).
> 
> I think the following is needed:
> 
> --- a/net/socket.c      2007-08-20 09:54:28.000000000 -0700
> +++ b/net/socket.c      2007-09-24 15:31:25.000000000 -0700
> @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
>        if (pos != 0)
>                return -ESPIPE;
> 
> -       if (iocb->ki_left == 0) /* Match SYS5 behaviour */
> -               return 0;
> +       if (unlikely(iocb->ki_left == 0)) {
> +               struct socket *sock = iocb->ki_filp->private_data;
> +               if (sock->type == SOCK_STREAM)
> +                       return 0;
> +       }

I'm not sure whether all STREAM protocols treat zero-length
sends as no-ops.  What about SCTP?

Put it another way, do we really need to keep the short-circuit
for SOCK_STREAM?

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Stephen Hemminger]

On Wed, 26 Sep 2007 11:18:39 +0800
Herbert Xu <herbert@gondor.apana.org.au> wrote:

> Stephen Hemminger <shemminger@linux-foundation.org> wrote:
> > The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> > describes an issue where write() can't be used to generate a zero-length
> > datagram (but send, and sendto do work).
> > 
> > I think the following is needed:
> > 
> > --- a/net/socket.c      2007-08-20 09:54:28.000000000 -0700
> > +++ b/net/socket.c      2007-09-24 15:31:25.000000000 -0700
> > @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
> >        if (pos != 0)
> >                return -ESPIPE;
> > 
> > -       if (iocb->ki_left == 0) /* Match SYS5 behaviour */
> > -               return 0;
> > +       if (unlikely(iocb->ki_left == 0)) {
> > +               struct socket *sock = iocb->ki_filp->private_data;
> > +               if (sock->type == SOCK_STREAM)
> > +                       return 0;
> > +       }
> 
> I'm not sure whether all STREAM protocols treat zero-length
> sends as no-ops.  What about SCTP?
> 
> Put it another way, do we really need to keep the short-circuit
> for SOCK_STREAM?
> 
> Cheers,

Stream is defined as sequence of bytes. So short circuit makes sense
If the application wants message boundaries it needs to use SOCK_SEQPACKET.
I was paranoid about possible breakage in TCP or SCTP. But since
send(s, buf, 0, 0) already filters through, I guess it doesn't matter.

-- 
Stephen Hemminger <shemminger@linux-foundation.org>

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Rick Jones]

Herbert Xu wrote:
> Stephen Hemminger <shemminger@linux-foundation.org> wrote:
> 
>>The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
>>describes an issue where write() can't be used to generate a zero-length
>>datagram (but send, and sendto do work).
>>
>>I think the following is needed:
>>
>>--- a/net/socket.c      2007-08-20 09:54:28.000000000 -0700
>>+++ b/net/socket.c      2007-09-24 15:31:25.000000000 -0700
>>@@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
>>       if (pos != 0)
>>               return -ESPIPE;
>>
>>-       if (iocb->ki_left == 0) /* Match SYS5 behaviour */
>>-               return 0;
>>+       if (unlikely(iocb->ki_left == 0)) {
>>+               struct socket *sock = iocb->ki_filp->private_data;
>>+               if (sock->type == SOCK_STREAM)
>>+                       return 0;
>>+       }
> 
> 
> I'm not sure whether all STREAM protocols treat zero-length
> sends as no-ops.  What about SCTP?

I asked Vlad that very question, since SCTP can preserve message 
boundaries. He tells me that a zero-length message is not part of SCTP.

rick jones

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Vlad Yasevich]

Herbert Xu wrote:
> Stephen Hemminger <shemminger@linux-foundation.org> wrote:
>> The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
>> describes an issue where write() can't be used to generate a zero-length
>> datagram (but send, and sendto do work).
>>
>> I think the following is needed:
>>
>> --- a/net/socket.c      2007-08-20 09:54:28.000000000 -0700
>> +++ b/net/socket.c      2007-09-24 15:31:25.000000000 -0700
>> @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
>>        if (pos != 0)
>>                return -ESPIPE;
>>
>> -       if (iocb->ki_left == 0) /* Match SYS5 behaviour */
>> -               return 0;
>> +       if (unlikely(iocb->ki_left == 0)) {
>> +               struct socket *sock = iocb->ki_filp->private_data;
>> +               if (sock->type == SOCK_STREAM)
>> +                       return 0;
>> +       }
> 
> I'm not sure whether all STREAM protocols treat zero-length
> sends as no-ops.  What about SCTP?

0 byte writes are not allowed in SCTP.  A no-op is fine, otherwise
SCTP would return an error.

-vlad

> 
> Put it another way, do we really need to keep the short-circuit
> for SOCK_STREAM?
> 
> Cheers,

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[David Miller]

From: Stephen Hemminger <shemminger@linux-foundation.org>
Date: Mon, 24 Sep 2007 15:34:35 -0700

> The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> describes an issue where write() can't be used to generate a zero-length
> datagram (but send, and sendto do work).
> 
> I think the following is needed:
> 
> --- a/net/socket.c	2007-08-20 09:54:28.000000000 -0700
> +++ b/net/socket.c	2007-09-24 15:31:25.000000000 -0700
> @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
>  	if (pos != 0)
>  		return -ESPIPE;
>  
> -	if (iocb->ki_left == 0)	/* Match SYS5 behaviour */
> -		return 0;
> +	if (unlikely(iocb->ki_left == 0)) {
> +		struct socket *sock = iocb->ki_filp->private_data;
> +		if (sock->type == SOCK_STREAM)
> +			return 0;
> +	}
>  
>  	x = alloc_sock_iocb(iocb, &siocb);
>  	if (!x)

We should simply remove the check completely.

There is no need to add special code for different types of protocols
and sockets.

As is hinted in the bugzilla, the exact same thing can happen with a
suitably constructed sendto() or sendmsg() call.  write() on a socket
is a sendmsg() with a NULL msg_control and a single entry iovec, plain
and simple.

It's how BSD and many other systems behave, and I double checked
Steven's Volume 2 just to make sure.

So I'm going to check in the following to fix this bugzilla.  There is
a similarly ugly test for len==0 in sys_read() on sockets.  If someone
would do some research on the validity of that thing I'd really
appreciate it :-)

diff --git a/net/socket.c b/net/socket.c
index 7d44453..b09eb90 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -777,9 +777,6 @@ static ssize_t sock_aio_write(struct kiocb *iocb, const struct iovec *iov,
 	if (pos != 0)
 		return -ESPIPE;
 
-	if (iocb->ki_left == 0)	/* Match SYS5 behaviour */
-		return 0;
-
 	x = alloc_sock_iocb(iocb, &siocb);
 	if (!x)
 		return -ENOMEM;

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Stephen Hemminger]

On Thu, 27 Sep 2007 13:53:34 -0700 (PDT)
David Miller <davem@davemloft.net> wrote:

> From: Stephen Hemminger <shemminger@linux-foundation.org>
> Date: Mon, 24 Sep 2007 15:34:35 -0700
> 
> > The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> > describes an issue where write() can't be used to generate a zero-length
> > datagram (but send, and sendto do work).
> > 
> > I think the following is needed:
> > 
> > --- a/net/socket.c	2007-08-20 09:54:28.000000000 -0700
> > +++ b/net/socket.c	2007-09-24 15:31:25.000000000 -0700
> > @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
> >  	if (pos != 0)
> >  		return -ESPIPE;
> >  
> > -	if (iocb->ki_left == 0)	/* Match SYS5 behaviour */
> > -		return 0;
> > +	if (unlikely(iocb->ki_left == 0)) {
> > +		struct socket *sock = iocb->ki_filp->private_data;
> > +		if (sock->type == SOCK_STREAM)
> > +			return 0;
> > +	}
> >  
> >  	x = alloc_sock_iocb(iocb, &siocb);
> >  	if (!x)
> 
> We should simply remove the check completely.
> 
> There is no need to add special code for different types of protocols
> and sockets.
> 
> As is hinted in the bugzilla, the exact same thing can happen with a
> suitably constructed sendto() or sendmsg() call.  write() on a socket
> is a sendmsg() with a NULL msg_control and a single entry iovec, plain
> and simple.
> 
> It's how BSD and many other systems behave, and I double checked
> Steven's Volume 2 just to make sure.
> 
> So I'm going to check in the following to fix this bugzilla.  There is
> a similarly ugly test for len==0 in sys_read() on sockets.  If someone
> would do some research on the validity of that thing I'd really
> appreciate it :-)

Read of zero length should be a no-op for SOCK_STREAM but
for SOCK_DATAGRAM or SOCK_SEQPACKET it might be useful as a 
remote wait for event.

-- 
Stephen Hemminger <shemminger@linux-foundation.org>

Re: [RFC] Zero-length write() does not generate a datagram on connected socket

[Michael Kerrisk]

On 9/28/07, Stephen Hemminger <shemminger@linux-foundation.org> wrote:
> On Thu, 27 Sep 2007 13:53:34 -0700 (PDT)
> David Miller <davem@davemloft.net> wrote:
>
> > From: Stephen Hemminger <shemminger@linux-foundation.org>
> > Date: Mon, 24 Sep 2007 15:34:35 -0700
> >
> > > The bug http://bugzilla.kernel.org/show_bug.cgi?id=5731
> > > describes an issue where write() can't be used to generate a zero-length
> > > datagram (but send, and sendto do work).
> > >
> > > I think the following is needed:
> > >
> > > --- a/net/socket.c  2007-08-20 09:54:28.000000000 -0700
> > > +++ b/net/socket.c  2007-09-24 15:31:25.000000000 -0700
> > > @@ -777,8 +777,11 @@ static ssize_t sock_aio_write(struct kio
> > >     if (pos != 0)
> > >             return -ESPIPE;
> > >
> > > -   if (iocb->ki_left == 0) /* Match SYS5 behaviour */
> > > -           return 0;
> > > +   if (unlikely(iocb->ki_left == 0)) {
> > > +           struct socket *sock = iocb->ki_filp->private_data;
> > > +           if (sock->type == SOCK_STREAM)
> > > +                   return 0;
> > > +   }
> > >
> > >     x = alloc_sock_iocb(iocb, &siocb);
> > >     if (!x)
> >
> > We should simply remove the check completely.
> >
> > There is no need to add special code for different types of protocols
> > and sockets.
> >
> > As is hinted in the bugzilla, the exact same thing can happen with a
> > suitably constructed sendto() or sendmsg() call.  write() on a socket
> > is a sendmsg() with a NULL msg_control and a single entry iovec, plain
> > and simple.
> >
> > It's how BSD and many other systems behave, and I double checked
> > Steven's Volume 2 just to make sure.
> >
> > So I'm going to check in the following to fix this bugzilla.  There is
> > a similarly ugly test for len==0 in sys_read() on sockets.  If someone
> > would do some research on the validity of that thing I'd really
> > appreciate it :-)
>
> Read of zero length should be a no-op for SOCK_STREAM but
> for SOCK_DATAGRAM or SOCK_SEQPACKET it might be useful as a
> remote wait for event.

Hmm -- I hadn't checked the behavior for zero-length read() on other
systems.  i will try to do that soonish (probably only Minday or so).

Cheers,

Michael