Checksum helpers doesn't work as expected

[ali-heidari]

Hi everyone,

I am developing XDP program to manage network packets. By applying DNAT/FullNAT/Protocol changes need to re calculate ip & protocol checksum. Using bpf_l3_csum_replace & bpf_l4_csum_replace gives incorrect checksum, also struggling with mismatched types as functions accept SKB parameters!

Can I know how exactly should use these replace helpers using XDPContext?

I recalculated checksum from scratch by csum differences! Can I pull request my own helper? Where is the best place to commit my helper? Here Bindings?

[aureliar8]

Hi !

I'm pretty sure bpf_l3_csum_replace and bpf_l3_csum_replace do work, as I use them in my project. But what is true is that they have a very un-intuitive and error prone API, and that there's very little doc & example on how to correctly use them.

This is something that could be improved.

As you pointed out, those two helper are only availiable for tc program, and not for xdp ones. For xdp the only thing that you can do is do the checksum computation & diff in your code and then use direct packet access to set the new checksum.

I agree that having a "blessed" aya crate for do this computation would be nice. (Or maybe in network-type)

[ali-heidari]

Hi @aureliar8

True, can't blame these 2 functions. You mentioned correct points.

Somehow i got to that state to calculate checksum by myself. I wasn't sure where is the right place to add helper function, but having a crate specially for checksum calculations is good idea. about network-type, I'll check their repository.

I made the works btw, just wanted to be sure I went the right path, for xdp programs, those helpers won't be the best options?

Thank you,

[aureliar8]

Those helper can't be used for xdp program, and that's not a aya limitation, it's a kernel one

https://docs.ebpf.io/linux/helper-function/bpf_l3_csum_replace/

So yeah on xdp you have no other choice that to do the checksum computation on your own code

[hsqStephenZhang]

@ali-heidari
Hi, you can check which helper functions are supported by bpftool feature probe kernel, bpf_xx_csum_replace did not show in xdp programs.