[WIP/RFC v4 0/5] Support abstract Unix sockets

Alejandro Colomar alx.manpages at gmail.com
Wed Aug 24 18:15:40 UTC 2022

Hello Maxim!

On 8/24/22 18:00, Maxim Dounin wrote:
>> For having nginx as a reverse proxy in front of a server, such as unit,
>> it's faster to communicate through Unix sockets (UDS), rather than TCP
>> (localhost).
> I would rather say "it's a common misconception that it's faster
> to communicate through Unix sockets".  While it Unix sockets can
> be beneficial for some microbenchmarks,

Yeah, I don't know how much of an improvement it will really bring.  I'm 
going to develop some benchmark soon, as a side effect of another task, 
but it will still be a bit artificial, and not a real load.  But it will 
help see how much of an improvement UDS have in the context of Unit.

> in most production setups
> it makes no difference, yet used to introduce various issues.

The only one I can think of is security, since you don't have file 
permissions, but one could also argue that the security can't be lower 
than that of TCP sockets.

>> Nginx already supports UDS.  But UDS has a problem: the
>> kernel creates a file in the fs, and it's not always trivial to clean up
>> those files.  Then, if the application is restarted, there's no
>> SO_REUSEADDR to allow reusing the socket file, so the application will
>> just fail.
>> This happens in nginx Unit, which creates listener sockets from a
>> privileged thread, and then uses them from unprivileged threads.  When
>> the unprivileged thread stops using the socket, it can't remove the
>> file, and doing so would require huge complexity to implement.  It's
>> easier to just tell the kernel we want an abstract UDS (AUDS), so that
>> there's no file at all.  Then if the user restarts Unit, it'll be able
>> to recreate the AUDS.
>> A user reported this problem with normal UDS and we concluded that the
>> easiest solution would be to add support for AUDS.  His set-up is a
>> kubernetes pod, where a container uses nginx and another container uses
>> Unit.  Communicating through an AUDS would be trivial and fast.
> So, you are trying to implement abstract namespace sockets as a
> bandaid for Unit bug, which is not able to properly remove Unix
> sockets in some cases and fails to restart, correct? 

Yeah, it helps avoid fixing that bug.  But that's why the Linux kernel 
implemented that feature in the first place: so that programs don't need 
to cleanup the filesystem in these cases, recognizing that it may not be 
always trivial.

> This does
> not look like a valid reason to me, especially given that a)
> abstract namespace sockets are Linux-only, and

Since other systems can use localhost, I don't think it's so important 
to support abstract sockets in them (it's not losing a big feature, but 
rather a minor improvement).  What Unit (and some other web servers do) 
is just report an error in the configuration.

> b) there are
> multiple approaches to filing the abstract socket address.

I don't understand this last point.  Do you mean the NUL and @ 
alternatives?  NUL is the one supported by the kernel, and so the most 
obvious one.  Since NUL is not usable by most tools, most programs use 
the alternative syntax @.  Some programs only allow @; but since it 
doesn't hurt to allow NUL, and it makes for a very easy implementation, 
I'd do it that way.



Alejandro Colomar
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