linux-kernel - Re: [PATCH] net: unix: clarify BSD behavior comment in unix_release

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Message-ID: <20251018235325.897059-1-kuniyu@google.com>
Date: Sat, 18 Oct 2025 23:52:02 +0000
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: adelodunolaoluwa@...oo.com
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org, 
	kuba@...nel.org, kuniyu@...gle.com, linux-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, pabeni@...hat.com, skhan@...uxfoundation.org
Subject: Re: [PATCH] net: unix: clarify BSD behavior comment in unix_release_sock()

From: Sunday Adelodun <adelodunolaoluwa@...oo.com>
Date: Fri, 17 Oct 2025 14:30:45 +0100
> The long-standing comment in unix_release_sock() mentioned a "FIXME" about
> BSD sending ECONNRESET to connected sockets upon closure, while Linux waits
> for the last reference. This behavior has existed since early UNIX socket
> implementations and is intentional.
> 
> Update the comment to clarify that this is a deliberate design difference,
> not a pending fix, and remove the outdated FIXME marker.
> 
> Signed-off-by: Sunday Adelodun <adelodunolaoluwa@...oo.com>
> ---
>  net/unix/af_unix.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 768098dec231..c21230a69f42 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -734,14 +734,13 @@ static void unix_release_sock(struct sock *sk, int embrion)
>  	/* ---- Socket is dead now and most probably destroyed ---- */
>  
>  	/*
> -	 * Fixme: BSD difference: In BSD all sockets connected to us get
> -	 *	  ECONNRESET and we die on the spot. In Linux we behave
> -	 *	  like files and pipes do and wait for the last
> -	 *	  dereference.
> +	 * Note: BSD sends ECONNREST to all sockets connected to a closing peer
> +	 * and terminates immediately.

I ran a script below on Mac (I hope the behvaviour does
not differ from FreeBSD), and I only see ECONNRESET on
SOCK_DGRAM test case.

Even after close()ing a SOCK_STREAM socket, its peer can
read 0, not ECONNRESET.

So, the comment looks outdated.

---8<---
$ python3 a.py
test 1
b'hello'
b''
test 2
b''
b''
test 3
[Errno 54] Connection reset by peer
---8<---


> Linux, however, intentionally behaves more
> +	 * like pipes - waiting for the final dereference before destruction.

Note that Linux also sets ECONNRESET if the close()d socket
has unread data or is not yet accept()ed.  You can find this
a few lines above of the diff.

---8<---
$ python3 a.py
test 1
b'hello'
b''
test 2
[Errno 104] Connection reset by peer
test 3
[Errno 11] Resource temporarily unavailable
---8<---


>  	 *
> -	 * Can't we simply set sock->err?
> -	 *
> -	 *	  What the above comment does talk about? --ANK(980817)
> +	 * This behaviour is by design and aligns with Linux's file semantics.
> +	 * Historical note: this difference from BSD has been present since the
> +	 * early UNIX socket implementation and is not considered a bug.
>  	 */

So, I'd remove the entire comment, and if needed, add a
selftest and update man page.

Thanks!


---8<---
import os
from socket import *


def test1():
    print("test 1")
    server = socket(AF_UNIX, SOCK_STREAM)
    server.bind(b'test')
    server.listen()

    client = socket(AF_UNIX, SOCK_STREAM)
    client.connect(server.getsockname())

    child, _ = server.accept()

    child.send(b'hello')
    child.close()

    try:
        client.setblocking(False)
        print(client.recv(20))
        print(client.recv(20))
    except Exception as e:
        print(e)

    client.close()
    server.close()
    os.remove('test')


def test2():
    print("test 2")
    server = socket(AF_UNIX, SOCK_STREAM)
    server.bind(b'test')
    server.listen()

    client = socket(AF_UNIX, SOCK_STREAM)
    client.connect(server.getsockname())

    child, _ = server.accept()

    client.send(b'hello')
    child.close()

    try:
        client.setblocking(False)
        print(client.recv(20))
        print(client.recv(20))
    except Exception as e:
        print(e)

    client.close()
    server.close()
    os.remove('test')


def test3():
    print("test 3")
    server = socket(AF_UNIX, SOCK_DGRAM)
    server.bind(b'test')

    client = socket(AF_UNIX, SOCK_DGRAM)
    client.connect(server.getsockname())

    client.send(b'hello')
    server.close()

    try:
        client.setblocking(False)
        print(client.recv(20))
        print(client.recv(20))
    except Exception as e:
        print(e)

    client.close()
    os.remove('test')


test1()
test2()
test3()
---8<---