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 :)
>
> 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.
