Re: [PATCH bpf-next v6 9/9] selftests/bpf: Cover metadata access from a modified skb clone

On Tue, Aug 12, 2025 at 11:20 AM -07, Martin KaFai Lau wrote:

On 8/12/25 6:12 AM, Jakub Sitnicki wrote:

quoted

quoted

No strong opinion to either copy the metadata on a clone or set the dynptr
rdonly for a clone. I am ok with either way.

A brain dump:
On one hand, it is hard to comment without visibility on how will it look like
when data_meta can be preserved in the future, e.g. what may be the overhead but
there is flags in bpf_dynptr_from_skb_meta and bpf_dynptr_write, so there is
some flexibility. On the other hand, having a copy will be less surprise on the
clone skb like what we have discovered in this and the earlier email thread but
I suspect there is actually no write use case on the skb data_meta now.

All makes sense.
To keep things simple and consistent, it would be best to have a single
unclone (bpf_try_make_writable) point caused by a write to metadata
through an skb clone.
Today, the unclone in the prologue can already be triggered by a write
to data_meta from a dead branch. Despite being useless, since
pskb_expand_head resets meta_len.
We also need the prologue unclone for bpf_dynptr_slice_rdwr created from
an skb_meta dynptr, because creating a slice does not invalidate packet
pointers by contract.
So I'm thinking it makes sense to unclone in the prologue if we see a
potential bpf_dynptr_write to skb_meta dynptr as well. This could be
done by tweaking check_helper_call to set the seen_direct_write flag:
static int check_helper_call(...)
{
         // ...
        	switch (func_id) {
         // ...
	case BPF_FUNC_dynptr_write:
	{
                 // ...
		dynptr_type = dynptr_get_type(env, reg);
                 // ...
		if (dynptr_type == BPF_DYNPTR_TYPE_SKB ||
		    dynptr_type == BPF_DYNPTR_TYPE_SKB_META)
			changes_data = true;

This looks ok.

quoted

		if (dynptr_type == BPF_DYNPTR_TYPE_SKB_META)
			env->seen_direct_write = true;

The "seen_direct_write = true;" addition will be gone from the verifier
eventually when pskb_expand_head can keep the data_meta (?). Right, there are
existing cases that the prologue call might be unnecessary. However, I don't
think it should be the reason that it can set "seen_direct_write" on top of the
"changes_data". I think it is confusing.

quoted

		break;
	}
         // ...
}
That would my the plan for the next iteration, if it sounds sensible.
As for keeping metadata intact past a pskb_expand_head call, on second
thought, I'd leave that for the next patch set, to keep the patch count
within single digits.

If the plan is to make pskb_expand_head support the data_meta later, just set
the rdonly bit in the bpf_dynptr_from_skb_meta now. Then the future
pskb_expand_head change will be a clean change in netdev and filter.c, and no
need to revert the "seen_direct_write" changes from the verifier.

I was planning to keep the "seen_direct_write" change once
pskb_expand_head is patched to preserve the metadata. That way
bpf_dynptr_write(&meta, ...) could remain just a memmove.

But to be fair, at this point I don't have the code worked out, so who
knows how things are going to eventually play out.

All right. Let me make the skb_meta dynptr read-only for skb clones for
now. That sounds like a clean cut compromise since it's a corner case
which we expect no one cares about at the moment.

Thanks for guidance.