On Mon, Aug 1, 2022 at 3:33 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
>
> On Mon, Aug 01, 2022 at 02:16:23PM -0700, Joanne Koong wrote:
> > On Mon, Aug 1, 2022 at 12:38 PM Martin KaFai Lau <kafai@xxxxxx> wrote:
> > >
> > > On Mon, Aug 01, 2022 at 10:52:14AM -0700, Joanne Koong wrote:
> > > > > Since we are on bpf_dynptr_write, what is the reason
> > > > > on limiting it to the skb_headlen() ? Not implying one
> > > > > way is better than another. would like to undertand the reason
> > > > > behind it since it is not clear in the commit message.
> > > > For bpf_dynptr_write, if we don't limit it to skb_headlen() then there
> > > > may be writes that pull the skb, so any existing data slices to the
> > > > skb must be invalidated. However, in the verifier we can't detect when
> > > > the data slice should be invalidated vs. when it shouldn't (eg
> > > > detecting when a write goes into the paged area vs when the write is
> > > > only in the head). If the prog wants to write into the paged area, I
> > > > think the only way it can work is if it pulls the data first with
> > > > bpf_skb_pull_data before calling bpf_dynptr_write. I will add this to
> > > > the commit message in v2
> > > Note that current verifier unconditionally invalidates PTR_TO_PACKET
> > > after bpf_skb_store_bytes(). Potentially the same could be done for
> > > other new helper like bpf_dynptr_write(). I think this bpf_dynptr_write()
> > > behavior cannot be changed later, so want to raise this possibility here
> > > just in case it wasn't considered before.
> >
> > Thanks for raising this possibility. To me, it seems more intuitive
> > from the user standpoint to have bpf_dynptr_write() on a paged area
> > fail (even if bpf_dynptr_read() on that same offset succeeds) than to
> > have bpf_dynptr_write() always invalidate all dynptr slices related to
> > that skb. I think most writes will be to the data in the head area,
> > which seems unfortunate that bpf_dynptr_writes to the head area would
> > invalidate the dynptr slices regardless.
> >
> > What are your thoughts? Do you think you prefer having
> > bpf_dynptr_write() always work regardless of where the data is? If so,
> > I'm happy to make that change for v2 :)
> Yeah, it sounds like an optimization to avoid unnecessarily
> invalidating the sliced data.
>
> To be honest, I am not sure how often the dynptr_data()+dynptr_write() combo will
> be used considering there is usually a pkt read before a pkt write in
> the pkt modification use case. If I got that far to have a sliced data pointer
> to satisfy what I need for reading, I would try to avoid making extra call
> to dyptr_write() to modify it.
>
> I would prefer user can have similar expectation (no need to worry pkt layout)
> between dynptr_read() and dynptr_write(), and also has similar experience to
> the bpf_skb_load_bytes() and bpf_skb_store_bytes(). Otherwise, it is just
> unnecessary rules for user to remember while there is no clear benefit on
> the chance of this optimization.
>
Are you saying that bpf_dynptr_read() shouldn't read from non-linear
part of skb (and thus match more restrictive bpf_dynptr_write), or are
you saying you'd rather have bpf_dynptr_write() write into non-linear
part but invalidate bpf_dynptr_data() pointers?
I guess I agree about consistency and that it seems like in practice
you'd use bpf_dynptr_data() to work with headers and stuff like that
at known locations, and then if you need to modify the rest of payload
you'd do either bpf_skb_load_bytes()/bpf_skb_store_bytes() or
bpf_dynptr_read()/bpf_dynptr_write() which would invalidate
bpf_dynptr_data() pointers (but that would be ok by that time).
> I won't insist though. User can always stay with the bpf_skb_load_bytes()
> and bpf_skb_store_bytes() to avoid worrying about the skb layout.
>
> > >
> > > Thinking from the existing bpf_skb_{load,store}_bytes() and skb->data perspective.
> > > If the user changes the skb by directly using skb->data to avoid calling
> > > load_bytes()/store_bytes(), the user will do the necessary bpf_skb_pull_data()
> > > before reading/writing the skb->data. If load_bytes()+store_bytes() is used instead,
> > > it would be hard to reason why the earlier bpf_skb_load_bytes() can load a particular
> > > byte but [may] need to make an extra bpf_skb_pull_data() call before it can use
> > > bpf_skb_store_bytes() to store a modified byte at the same offset.
