TIOCOUTQ implementation for sockets vs. tty

TIOCOUTQ implementation for sockets vs. tty

[Henneberg - Systemdesign]

Hi,

I have a question regarding the implementation of ioctl TIOCOUTQ for
various sockets compared to the tty implementation.

For several sockets, e. g. AF_BLUETOOTH it is done like this

af_bluetooth.c:
case TIOCOUTQ:
	if (sk->sk_state == BT_LISTEN)
		return -EINVAL;

	amount = sk->sk_sndbuf - sk_wmem_alloc_get(sk);
	if (amount < 0)
		amount = 0;
	err = put_user(amount, (int __user *)arg);
	break;

so the ioctl returns the available space in the send queue if I
understand the code correctly (this is also what I observed from tests).

The tty does this:

n_tty.c:
case TIOCOUTQ:
	return put_user(tty_chars_in_buffer(tty), (int __user *) arg);

so it returns the used space in the send queue. This is also what I
would expect from the manpage description.

Is this mismatch intentional?

Regards
-Jochen

Re: TIOCOUTQ implementation for sockets vs. tty

[Willy Tarreau]

Hi,

On Wed, Mar 10, 2021 at 07:16:34PM +0100, Henneberg - Systemdesign wrote:
> Hi,
> 
> I have a question regarding the implementation of ioctl TIOCOUTQ for
> various sockets compared to the tty implementation.
> 
> For several sockets, e. g. AF_BLUETOOTH it is done like this
> 
> af_bluetooth.c:
> case TIOCOUTQ:
> 	if (sk->sk_state == BT_LISTEN)
> 		return -EINVAL;
> 
> 	amount = sk->sk_sndbuf - sk_wmem_alloc_get(sk);
> 	if (amount < 0)
> 		amount = 0;
> 	err = put_user(amount, (int __user *)arg);
> 	break;
> 
> so the ioctl returns the available space in the send queue if I
> understand the code correctly (this is also what I observed from tests).
> 
> The tty does this:
> 
> n_tty.c:
> case TIOCOUTQ:
> 	return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
> 
> so it returns the used space in the send queue. This is also what I
> would expect from the manpage description.
> 
> Is this mismatch intentional?

At least both man pages (tty_ioctl and tcp(7)) mention that TIOCOUTQ
should return the number of byte in queue.

What I suspect for sockets is that sk_sndbuf grows with allocations
and that sk_wmem_alloc_get() in fact returns the number of unused
allocations thus the difference would be the amount queued. But I
could be wrong and I would tend to read the code the same way as you
did.

Willy

Re: TIOCOUTQ implementation for sockets vs. tty

[Henneberg - Systemdesign]

Willy Tarreau <w@1wt.eu> writes:

> Hi,
>
> On Wed, Mar 10, 2021 at 07:16:34PM +0100, Henneberg - Systemdesign wrote:
>> Hi,
>> 
>> I have a question regarding the implementation of ioctl TIOCOUTQ for
>> various sockets compared to the tty implementation.
>> 
>> For several sockets, e. g. AF_BLUETOOTH it is done like this
>> 
>> af_bluetooth.c:
>> case TIOCOUTQ:
>> 	if (sk->sk_state == BT_LISTEN)
>> 		return -EINVAL;
>> 
>> 	amount = sk->sk_sndbuf - sk_wmem_alloc_get(sk);
>> 	if (amount < 0)
>> 		amount = 0;
>> 	err = put_user(amount, (int __user *)arg);
>> 	break;
>> 
>> so the ioctl returns the available space in the send queue if I
>> understand the code correctly (this is also what I observed from tests).
>> 
>> The tty does this:
>> 
>> n_tty.c:
>> case TIOCOUTQ:
>> 	return put_user(tty_chars_in_buffer(tty), (int __user *) arg);
>> 
>> so it returns the used space in the send queue. This is also what I
>> would expect from the manpage description.
>> 
>> Is this mismatch intentional?
>
> At least both man pages (tty_ioctl and tcp(7)) mention that TIOCOUTQ
> should return the number of byte in queue.
>
> What I suspect for sockets is that sk_sndbuf grows with allocations
> and that sk_wmem_alloc_get() in fact returns the number of unused
> allocations thus the difference would be the amount queued. But I
> could be wrong and I would tend to read the code the same way as you
> did.

I am quite sure that my assumption is right. When I call the ioctl right
before and after writing data to an AF_BLUETOOTH socket the returned
value decreases where it should increase.

-Jochen

>
> Willy


-- 
Henneberg - Systemdesign
Jochen Henneberg
Loehnfeld 26
21423 Winsen (Luhe)
--
Fon: +49 172 160 14 69
www: www.henneberg-systemdesign.com