Re: Helper functions available for XDP?

Yonghong Song

On Tue, Jul 24, 2018 at 9:19 AM, Andrew Wang <andrw.wng@...> wrote:
Then in this case is it possible to replace TCP checksum on ingress with
XDP? How do I test if the skb is writable?
There is no skb in XDP. The helper bpf_l4_csum_replace cannot be used.
You need to use other csum functions and can directly modify the packet data.

On Tue, Jul 24, 2018 at 12:04 PM, Yonghong Song <ys114321@...> wrote:

On Tue, Jul 24, 2018 at 1:54 AM, Fran├žois <fser@...> wrote:
On Mon, Jul 23, 2018 at 10:08:08PM +0200, Paul Chaignon wrote:
On Mon, Jul 23, 2018 at 02:21:05PM -0400, Andrew Wang wrote:

I am writing a bpf program for packet processing and have loaded my
function at BPF.XDP.

I'm updating the destination IPv6 address and want to update the TCP
checksum, but when I try to call the helper functions "bpf_csum_diff"
"bpf_l4_csum_replace", I get a "unknown func <function name>". Are
functions not available for XDP program types?
XDP programs cannot use bpf_l4_csum_replace, but they can use
bpf_csum_diff since commit 205c380 ("bpf: add csum_diff helper to xdp
well") which landed in v4.16-rc1.
I was thinking maybe those helpers were not available because no skb
was construct. But after checking, the helper you're mentionning is in
the tc_cls. Also, the csum_diff helper lies in the same switch
statement as l4_csum_replace.

Can you elaborate on why one can be used and not the other?
bpf_csum_diff only accesses packet data and can be used in both cls_act
and xdp.
bpf_l4_csum_replace accesses skb data structure, e.g., it needs to
replace the csum
and hence needs to test whether skb is writable. It also different
calculation based on
the existing checksum state (skb->ip_summed), so bpf_l4_csum_replace
cannot be
used in xdp.


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