Re: [PATCH] net: unix: clarify BSD behavior comment in unix_release_sock()

[Sunday Adelodun <adelodunolaoluwa@yahoo.com>]

On 10/19/25 00:52, Kuniyuki Iwashima wrote:
> From: Sunday Adelodun <adelodunolaoluwa@yahoo.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 not a pending fix, and remove the outdated FIXME marker.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@yahoo.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 So, the comment looks outdated.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<---

Thanks Kuniyuki for the detailed explanation and example script.
I'll go ahead and send a v2 that removes the entire comment as you 
suggested.
After that, I’ll work on a follow-up patch to add a selftest for this 
behavior.

Thanks again!
Sunday