From: "Tobias Böhm" <tobias@aibor.de>
To: "Toke Høiland-Jørgensen" <toke@redhat.com>
Cc: xdp-newbies@vger.kernel.org,
Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Subject: Re: cpumap infinite loop
Date: Wed, 6 Mar 2024 10:09:06 +0100 [thread overview]
Message-ID: <72h5h7dwvgim5hephweenq63wsqbdaseeu2rimdog2yzlyouqx@e5z5di3v6k7l> (raw)
In-Reply-To: <871q8od2tl.fsf@toke.dk>
On Tue, 05 Mar 2024 at 06:08 PM, Toke Høiland-Jørgensen wrote:
> Tobias Böhm <tobias@aibor.de> writes:
>
> > Hello,
> >
> > I was playing around a bit with cpumaps and wondered what happens when
> > the attached program just does another CPU redirect to itself.
> >
> > I ended up having an infinite loop. The working example can be found
> > here: https://github.com/aibor/cpumap-loop
> >
> > Now, I wonder if there is a way to detect and break this loop. I took a
> > look at the xdp_md->rx_queue_index values. When executed by a NIC event,
> > the value is the NIC ID, so a fairly low number. After CPU redirection
> > the values I saw were far above the range of NIC queue IDs. I couldn't
> > figure out if it is just a random memory value or if this value still
> > has a (maybe different) meaning after CPU redirection. Maybe somehow
> > related to the CPU queue?
>
> It's random. The rxq data structure is not initialised on the stack, so
> it's basically whatever was in that memory. Interestingly, there's a
> TODO comment in there to fix this:
>
> https://elixir.bootlin.com/linux/latest/source/kernel/bpf/cpumap.c#L195
>
> Not sure what the intention was here. +Lorenzo, who wrote that code.
> Returning the contents of a random uninitialised stack variable is
> probably not a good idea, though, we should zero out the data structure.
> I'll send a patch for that.
Thank you for the explanation and the patch. :)
> > If the field is set to a meaningful value I can make assumptions about
> > it and would be able to detect previous CPU redirection, I guess.
> >
> > I'd appreciate any pointers and tips how I could detect such a loop. Or
> > maybe there is a way to prevent it in the first place other than "just
> > being careful"?
>
> Well, you kinda have to go out of your way to construct a loop like
> this. How are you envisioning this would happen accidentally? :)
I totally agree that it is pretty unlikely to create such a loop by accident.
Especially since the map programs usually are rather simple.
My example was driven by pure curiosity, exploring the possibilities of
redirect map programs. And since I saw infinite recursion is possible I
was looking for options for reliable termination conditions. This made me
wonder if I can detect if the program was invoked by the device or by map
redirection.
next prev parent reply other threads:[~2024-03-06 9:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-01 23:04 cpumap infinite loop Tobias Böhm
2024-03-05 17:08 ` Toke Høiland-Jørgensen
2024-03-06 9:09 ` Tobias Böhm [this message]
2024-03-06 10:20 ` Toke Høiland-Jørgensen
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