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Message-ID: <87eegmrcie.fsf@henneberg-systemdesign.com>
Date: Wed, 10 Mar 2021 21:15:37 +0100
From: Henneberg - Systemdesign <lists@...neberg-systemdesign.com>
To: Willy Tarreau <w@....eu>
Cc: netdev@...r.kernel.org
Subject: Re: TIOCOUTQ implementation for sockets vs. tty
Willy Tarreau <w@....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
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Jochen Henneberg
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