Re: [PATCH net] net: Avoid checksumming unreadable skb tail on trim

[Björn Töpel]

On Fri, 22 May 2026 at 17:39, Breno Leitao <leitao@xxxxxxxxxx> wrote:
>
> On Fri, May 22, 2026 at 02:06:40PM +0200, Björn Töpel wrote:
> > pskb_trim_rcsum_slow() keeps CHECKSUM_COMPLETE valid by subtracting
> > the checksum of the bytes removed from the skb tail. That assumes the
> > removed bytes can be read.
> >
> > io_uring zcrx skbs may contain unreadable net_iov frags. With fbnic
> > header/data split, small TCP/IPv4 packets can carry Ethernet padding
> > in such a frag. ip_rcv_core() trims the skb to iph->tot_len before TCP
> > sees it, and the CHECKSUM_COMPLETE adjustment then calls
> > skb_checksum() on the padding.
> >
> > This is exposed by IPv4 because small TCP/IPv4 frames can be shorter
> > than the Ethernet minimum payload. TCP/IPv6 frames are large enough in
> > the normal zcrx path, so they do not hit the same padding trim.
> >
> > Keep the existing checksum adjustment for readable skbs. If the
> > remaining packet is fully linear, drop CHECKSUM_COMPLETE and let the
> > stack validate the packet after trimming. If unreadable payload would
> > remain, fail the trim; the checksum cannot be adjusted without reading
> > the trimmed tail.
> >
> > Also clear skb->unreadable when trimming removes all frags.
> >
> > Fixes: 65249feb6b3d ("net: add support for skbs with unreadable frags")
> > Signed-off-by: Björn Töpel <bjorn@xxxxxxxxxx>
>
> Reviewed-by: Breno Leitao <leitao@xxxxxxxxxx>

Thanks, Breno!

> > +static int pskb_trim_rcsum_complete(struct sk_buff *skb, unsigned int len)
> > +{
> > +     int delta = skb->len - len;
>
> I suppose this is not unsigned/size_t because csum_block_sub() requires
> an 'int', is this right?

Yup, both skb_checksum() length and csum_block_sub() offset are int,
and it also keeps the existing parent function's local type unchanged
after the move.

> >  /* Note : use pskb_trim_rcsum() instead of calling this directly
> >   */
>
> I think you can make this a one-line comment, or just a proper kdoc.

Yes, but note that this was not an addition from me. I can fix it up
if there are more changes required!


Thanks!
Björn