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Date: Thu, 23 Jan 2020 09:21:06 +0100 From: Florian Westphal <fw@...len.de> To: Daniel Borkmann <daniel@...earbox.net> Cc: Florian Westphal <fw@...len.de>, Praveen Chaudhary <praveen5582@...il.com>, pablo@...filter.org, davem@...emloft.net, kadlec@...filter.org, netfilter-devel@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Zhenggen Xu <zxu@...kedin.com>, Andy Stracner <astracner@...kedin.com> Subject: Re: [PATCH v3] [net]: Fix skb->csum update in inet_proto_csum_replace16(). Daniel Borkmann <daniel@...earbox.net> wrote: > On 1/22/20 12:43 PM, Florian Westphal wrote: > > Daniel Borkmann <daniel@...earbox.net> wrote: > > > > @@ -449,9 +464,6 @@ void inet_proto_csum_replace16(__sum16 *sum, struct sk_buff *skb, > > > > if (skb->ip_summed != CHECKSUM_PARTIAL) { > > > > *sum = csum_fold(csum_partial(diff, sizeof(diff), > > > > ~csum_unfold(*sum))); > > > > - if (skb->ip_summed == CHECKSUM_COMPLETE && pseudohdr) > > > > - skb->csum = ~csum_partial(diff, sizeof(diff), > > > > - ~skb->csum); > > > > > > What is the technical rationale in removing this here but not in any of the > > > other inet_proto_csum_replace*() functions? You changelog has zero analysis > > > on why here but not elsewhere this change would be needed? > > > > Right, I think it could be dropped everywhere BUT there is a major caveat: > > > > At least for the nf_nat case ipv4 header manipulation (which uses the other > > helpers froum utils.c) will eventually also update iph->checksum field > > to account for the changed ip addresses. > > > > And that update doesn't touch skb->csum. > > > > So in a way the update of skb->csum in the other helpers indirectly account > > for later ip header checksum update. > > > > At least that was my conclusion when reviewing the earlier incarnation > > of the patch. > > Mainly asking because not inet_proto_csum_replace16() but the other ones are > exposed via BPF and they are all in no way fundamentally different to each > other, but my concern is that depending on how the BPF prog updates the csums > things could start to break. :/ I'm reasonably sure removing the skb->csum update from the other helpers will also break ipv4 nat :) So, AFAIU from what you're saying above the patch seems fine as-is and just needs a more verbose commit message explaining why replace16() doesn't update skb->csum while all the other ones do. Is that correct?
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