On Fri, Sep 6, 2019 at 2:47 AM Shmulik Ladkani <shmu...@metanetworks.com> wrote:
>
> On Thu, 5 Sep 2019 17:51:20 -0400
> Willem de Bruijn <willemdebruijn.ker...@gmail.com> wrote:
>
> > On Thu, Sep 5, 2019 at 2:36 PM Shmulik Ladkani <shmu...@metanetworks.com>
> > wrote:
> > >
> > > + if (mss != GSO_BY_FRAGS &&
> > > + (skb_shinfo(head_skb)->gso_type & SKB_GSO_DODGY)) {
> > > + /* gso_size is untrusted.
> > > + *
> > > + * If head_skb has a frag_list with a linear non head_frag
> > > + * item, and head_skb's headlen does not fit requested
> > > + * gso_size, fall back to copying the skbs - by disabling
> > > sg.
> > > + *
> > > + * We assume checking the first frag suffices, i.e if
> > > either of
> > > + * the frags have non head_frag data, then the first frag
> > > is
> > > + * too.
> > > + */
> > > + if (list_skb && skb_headlen(list_skb) &&
> > > !list_skb->head_frag &&
> > > + (mss != skb_headlen(head_skb) - doffset)) {
> >
> > I thought the idea was to check skb_headlen(list_skb), as that is the
> > cause of the problem. Is skb_headlen(head_skb) a good predictor of
> > that? I can certainly imagine that it is, just not sure.
>
> Yes, 'mss != skb_headlen(HEAD_SKB)' seems to be a very good predictor,
> both for the test reproducer, and what's observered on a live system.
>
> We *CANNOT* use 'mss != skb_headlen(LIST_SKB)' as the test condition.
> The packet could have just a SINGLE frag_list member, and that member could
> be a "small remainder" not reaching the full mss size - so we could hit
> the test condition EVEN FOR NON gso_size mangled frag_list skbs -
> which is not desired.
The last segment. Yes, good point.
> Also, is we test 'mss != skb_headlen(list_skb)' and execute 'sg=false'
> ONLY IF 'list_skb' is *NOT* the last item, this is still bogus.
> Imagine a gso_size mangled packet having just head_skb and a single
> "small remainder" frag. This packet will hit the BUG_ON, as the
> 'sg=false' solution is now skipped according to the revised condition.
Right, I wouldn't suggest that.
But I wonder whether it is a given that head_skb has headlen.
Btw, it seems slightly odd to me tot test head_frag before testing
headlen in the v2 patch.
