BPF helper functions not working with XDP offloading

[sshetty78]

I am currently trying to implement XDP programs using some of the basic bpf helper functions. But basic bpf helper functions such as bpf_ktime_get_ns,
bpf_trace_printk are erroring out with error - "unsupported function".

I have attached the error screenshots, along with the NIC info. Linux version that I am using supports these bpf helper functions according to the ebpf guide found on the website.

Linux version - Linux charybdis 5.4.0-96-generic #109-Ubuntu SMP Wed Jan 12 16:49:16 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

What could be the reason for this issue? Are there any additional libraries that I am missing? Functions like bpf_map_lookup and bpf_get_prandom32 is working well

https://paste.pics/5f95e51e0bb0cb04f419c9b3af6c1150
https://paste.pics/27f2143bfe97c4f22eb9fdd3c8272778

[tohojo]

sshetty78 ***@***.***> writes:

I am currently trying to implement XDP programs using some of the basic bpf helper functions. But basic bpf helper functions such as bpf_ktime_get_ns, bpf_trace_printk are erroring out with error - "unsupported function". I have attached the error screenshots, along with the NIC info. Linux version that I am using supports these bpf helper functions according to the ebpf guide found on the website. Linux version - Linux charybdis 5.4.0-96-generic #109-Ubuntu SMP Wed Jan 12 16:49:16 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux What could be the reason for this issue? Are there any additional libraries that I am missing? Functions like bpf_map_lookup and bpf_get_prandom32 is working well

Offloaded XDP is limited to the helper functions supported by the NIC. Which is not a lot of them; in particular, anything relying on kernel infrastructure (such as printk, which uses the trace pipe, and ktime_get_ns which uses the internal kernel clock) won't work...

[sshetty78]

Thanks for the clarification. Does this mean it should work on XDP-generic?

[tohojo]

sshetty78 ***@***.***> writes:

Thanks for the clarification. Does this mean it should work on XDP-generic?

Yes, and even in XDP driver mode (on drivers that support it). I generally wouldn't recommend using XDP generic mode; you'll miss out of a lot of the performance benefit of XDP, but won't gain the benefits of having an skb. So the TC hook is most likely a better fit if all you're using is XDP generic. However, if you have a mixed environment where some drivers support driver mode and some don't, XDP generic can be useful to allow you to run the same programs on all interfaces.

[sshetty78]

I am unable to use these functions even in XDP_FLAGS_SKB_MODE mode. I am seeing the same error even when I load XDP program using XDP_FLAGS_SKB_MODE flag. Is there anything else in the framework that could be causing the functions to be unsupported?

[tohojo]

Both of those should definitely work in generic mode. Are you by any chance still setting the prog_ifindex field when loading the program? That will cause the verifier to treat the program as though it's about to be offloaded, regardless of the mode you set (you don't actually specify SKB_MODE when loading the program, only when attaching).