Re: [PATCH net v3 0/3] net: don't strip zerocopy frag markers from a forwarded skb

[Willem de Bruijn]

Ilya Maximets wrote:
> On 8/22/26 2:09 AM, Jakub Kicinski wrote:
> > On Fri, 21 Aug 2026 23:45:41 +0200 Ilya Maximets wrote:
> >> Unfortunately, this needs a rebase now that a conflicting change
> >> for skb_zerocopy() was merged:
> > 
> > Ugh, I was supposed to merge this first, wasn't I? Sorry.
> 
> Not a huge deal, I guess, the conflict is mechanical and the patches
> are simple.  I can take care of manual backports once we get the
> 'failed to apply' emails.  Just a bit of busy work.
> 
> > I was hoping for Willem to TAL since skb_tx_error() is a tx ZC
> > thing, now I realized that he wasn't CCed :S (please do so on v4)
> 
> FWIW, I CCed a few people on v1 to have a conversation about a proper
> fix, but that wasn't fruitful.  So, if I were Norbert, I wouldn't
> include them for the new versions either as doing so always feels like
> me being annoying. :)

Having a look now.
 
> For now, the plan is to get v4 of these targeted fixes into net and
> stable and then remove skb_tx_error() entirely once net-next is open,
> as it seems to have lost all of its prior meaning.

The original use case in tun_net_xmit introduced in commit
149d36f7187c ("tun: report orphan frags errors to zero copy callback")
still exists. Not sure you can remove the function entirely.

The bug is hit when this function is called with a cloned or shared
skb. The original zerocopy path through tun_net_xmit was probably
expected to not have this problem.

Commit 0110d6f22f39 ("tun: orphan an skb on tx") explains why this
orphan in tun_net_xmit was needed: a (vhost) zerocopy skb injected in
tap1 arriving at tap2 and not being read there indefinitely. Such
loops are not allowed for zerocopy.

Commit 868eefeb17d4 ("tun: orphan frags on xmit") then added the
frags orphan. Since an skb can be cloned on this tap to tap path,
e.g., with a packet socket, I think the bug goes back to the original
commit that introduced the first caller of skb_tx_error, commit
149d36f7187c ("tun: report orphan frags errors to zero copy callback")

If respinning it may be worthile to link to this thread.