From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: "Tobias Böhm" <tobias@aibor.de>, xdp-newbies@vger.kernel.org
Cc: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Subject: Re: cpumap infinite loop
Date: Tue, 05 Mar 2024 18:08:06 +0100 [thread overview]
Message-ID: <871q8od2tl.fsf@toke.dk> (raw)
In-Reply-To: <xgct6vpbwh6c5zjljdc6ypa5hbmcw4bapebdh4eetxwjama3so@iq3xdtavnq4g>
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.
> 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? :)
-Toke
next prev parent reply other threads:[~2024-03-05 17:08 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 [this message]
2024-03-06 9:09 ` Tobias Böhm
2024-03-06 10:20 ` Toke Høiland-Jørgensen
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