Re: cpumap infinite loop

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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