XDP BPF Stack Limit Issues

From: Christian Deacon <gamemann@gflclan.com>
To: xdp-newbies@vger.kernel.org
Subject: XDP BPF Stack Limit Issues
Date: Wed, 16 Dec 2020 09:29:05 -0600	[thread overview]
Message-ID: <ad6ea0ec-c5ce-2887-6f4c-7ed762a0f130@gflclan.com> (raw)

Hey everyone,

I've been trying to implement IPv6 support into an XDP Firewall which 
can be found below.

https://github.com/gamemann/XDP-Firewall

Unfortunately, I've been fighting with the BPF verifier and I'm 
exceeding the BPF stack size of 512 bytes. I linked the above in the 
case others want to see the headers that define things like 
`MAX_FILTERS` inside the XDP program. The error I am receiving is:

```
error: <unknown>:0:0: in function xdp_prog_main i32 (%struct.xdp_md*): 
Looks like the BPF stack limit of 512 bytes is exceeded. Please move 
large on stack variables into BPF per-cpu array map.
```

Which spams anywhere from 3 - 10 times depending on what I try to 
resolve the issue.

I ended up re-writing the entire program trying to use as little 
variables as possible and I got very close to getting the program to 
compile until I added support for the ICMPv6 protocol (once I remove 
this, it compiles and runs without any issues). I'm at a loss on what I 
can do now, though.

The current XDP program code is the following.

https://gist.github.com/gamemann/a0acd9603405c3d7b3c792b5429ced38

 From what the error states, I could try storing variables into a 
per-CPU BPF map. Therefore, I tried storing the ICMP (and at one point 
TCP) information into a BPF map and used the data later on which can be 
found below.

https://gist.github.com/gamemann/663674924e16286b02a835637912c2a5

This still exceeded the BPF stack size. With that said, I'd assume 
performance would be heavily impacted if we stored everything inside a 
BPF map. To my understanding, per-CPU maps cannot be reliably read 
within the XDP program. Therefore, if this would have worked, I'd 
probably want to use a regular non per-CPU map anyways which would 
impact performance.

I also tried BPF calls without luck and was thinking about trying BPF 
tail calls. Though, I don't think this would help. BPF tail calls use 
the same BPF stack to my understanding.

I could try adding even more variables inside the program to a BPF map 
such as the PPS and BPS variables. However, I wanted to see if there 
were any other suggestions from the mailing list on this. I plan to 
write another firewall that'll have a lot more functionality than this 
firewall in XDP and I'm worried I'd run into similar issues there.

Any help would be highly appreciated and thank you for your time!