Re: [PATCH net v1 1/3] net: core: propagate unreadable flag in skb_zerocopy

[Pavel Begunkov]

On 8/2/26 13:33, Pavel Begunkov wrote:
> On 8/1/26 13:52, Mina Almasry wrote:
> > When skb_zerocopy() copies devmem payload fragments, it fails to update
> > the target skb's unreadable flag. This causes the target to appear as
> > readable memory.
> >
> > Propagate the unreadable flag if any devmem fragments were copied from
> > the source.
>
>  From a quick look, it doesn't seem like the function would work reliably
> with netmems even with this patch, but that's a different kind of issue
> we can return to later. Curious, did you hit this in prod?

 	skb_shinfo(to)->nr_frags = j;
 
+	if (i > 0 && from->unreadable)
+		to->unreadable = 1;

Taking another look, if j != i, it'd also have the mixing problem,
and that can happen if it hits that head-to-frag copy.

if (!skb_frags_readable(from) && j > 0 && len)
	// goto fault;

Maybe add sth like this before the frag assignment loop?

--
Pavel Begunkov