Re: [PATCH bpf-next v1 1/3] bpf: Add skb dynptrs

[Martin KaFai Lau <kafai@fb.com>]

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.

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.