netdev - Re: Fw: [Bug 205339] New: epoll can fail to report a socket readable after enabling SO

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Date:   Mon, 28 Oct 2019 14:22:13 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org, njs@...ox.com
Subject: Re: Fw: [Bug 205339] New: epoll can fail to report a socket readable
 after enabling SO_OOBINLINE

Please Stephen CC the reporter when you forward a bugzilla bug to the list

On 10/28/19 8:11 AM, Stephen Hemminger wrote:
> 
> 
> Begin forwarded message:
> 
> Date: Mon, 28 Oct 2019 02:55:44 +0000
> From: bugzilla-daemon@...zilla.kernel.org
> To: stephen@...workplumber.org
> Subject: [Bug 205339] New: epoll can fail to report a socket readable after enabling SO_OOBINLINE
> 
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=205339
> 
>             Bug ID: 205339
>            Summary: epoll can fail to report a socket readable after
>                     enabling SO_OOBINLINE
>            Product: Networking
>            Version: 2.5
>     Kernel Version: 5.0
>           Hardware: All
>                 OS: Linux
>               Tree: Mainline
>             Status: NEW
>           Severity: low
>           Priority: P1
>          Component: Other
>           Assignee: stephen@...workplumber.org
>           Reporter: njs@...ox.com
>         Regression: No

> Created attachment 285671
>   --> https://bugzilla.kernel.org/attachment.cgi?id=285671&action=edit  
> reproducer
> 
> Consider the following sequence of events:
> 
> 1. OOB data arrives on a socket.
> 2. The socket is registered with epoll with EPOLLIN
> 3. The socket has SO_OOBINLINE toggled from False → True
> 
> In this case, the socket is now readable, and select() reports that it's
> readable, but epoll does *not* report that it's readable.
> 
> This is a pretty minor issue, but it seems like an unambiguous bug so I figured
> I'd report it.
> 
> Weirdly, this doesn't appear to be a general problem with SO_OOBINLINE+epoll.
> For example, this very similar sequence works correctly:
> 
> 1. The socket is registered with epoll with EPOLLIN
> 2. OOB data arrives on the socket.
> 3. The socket has SO_OOBINLINE toggled from False → True
> 
> After step 2, epoll reports the socket as not readable, and then after step 3
> it reports it as readable, as you'd expect.
> 
> In the attached reproducer script, "scenario 4" is the buggy one, and "scenario
> 3" is the very similar non-buggy one. Output on Ubuntu 19.04, kernel
> 5.0.0-32-generic, x86-64:
> 
> -- Scenario 1: no data --
> select() says: sock is NOT readable
> epoll says: sock is NOT readable
> reality: NOT readable
> 
> -- Scenario 2: OOB data arrives --
> select() says: sock is NOT readable
> epoll says: sock is NOT readable
> reality: NOT readable
> 
> -- Scenario 3: register -> OOB data arrives -> toggle SO_OOBINLINE=True --
> select() says: sock is readable
> epoll says: sock is readable
> reality: read succeeded
> 
> -- Scenario 4: OOB data arrives -> register -> toggle SO_OOBINLINE=True --
> select() says: sock is readable
> epoll says: sock is NOT readable
> reality: read succeeded
> 

I really wonder how much energy we should put in maintaining this archaic thing.

We do not have a single packetdrill test at Google using URG stuff.