On 11/9/22 3:10 AM, Toke Høiland-Jørgensen wrote:
Snipping a bit of context to reply to this bit:
Can the xdp prog still change the metadata through xdp->data_meta? tbh, I am not
sure it is solid enough by asking the xdp prog not to use the same random number
in its own metadata + not to change the metadata through xdp->data_meta after
calling bpf_xdp_metadata_export_to_skb().
What do you think the usecase here might be? Or are you suggesting we
reject further access to data_meta after
bpf_xdp_metadata_export_to_skb somehow?
If we want to let the programs override some of this
bpf_xdp_metadata_export_to_skb() metadata, it feels like we can add
more kfuncs instead of exposing the layout?
bpf_xdp_metadata_export_to_skb(ctx);
bpf_xdp_metadata_export_skb_hash(ctx, 1234);
There are several use cases for needing to access the metadata after
calling bpf_xdp_metdata_export_to_skb():
- Accessing the metadata after redirect (in a cpumap or devmap program,
or on a veth device)
- Transferring the packet+metadata to AF_XDP
fwiw, the xdp prog could also be more selective and only stores one of the hints
instead of the whole 'struct xdp_to_skb_metadata'.
- Returning XDP_PASS, but accessing some of the metadata first (whether
to read or change it)
The last one could be solved by calling additional kfuncs, but that
would be less efficient than just directly editing the struct which
will be cache-hot after the helper returns.
Yeah, it is more efficient to directly write if possible. I think this set
allows the direct reading and writing already through data_meta (as a _u8 *).
And yeah, this will allow the XDP program to inject arbitrary metadata
into the netstack; but it can already inject arbitrary *packet* data
into the stack, so not sure if this is much of an additional risk? If it
does lead to trivial crashes, we should probably harden the stack
against that?
As for the random number, Jesper and I discussed replacing this with the
same BTF-ID scheme that he was using in his patch series. I.e., instead
of just putting in a random number, we insert the BTF ID of the metadata
struct at the end of it. This will allow us to support multiple
different formats in the future (not just changing the layout, but
having multiple simultaneous formats in the same kernel image), in case
we run out of space.
This seems a bit hypothetical. How much headroom does it usually have for the
xdp prog? Potentially the hints can use all the remaining space left after the
header encap and the current bpf_xdp_adjust_meta() usage?
We should probably also have a flag set on the xdp_frame so the stack
knows that the metadata area contains relevant-to-skb data, to guard
against an XDP program accidentally hitting the "magic number" (BTF_ID)
in unrelated stuff it puts into the metadata area.
Yeah, I think having a flag is useful. The flag will be set at xdp_buff and
then transfer to the xdp_frame?
After re-reading patch 6, have another question. The 'void
bpf_xdp_metadata_export_to_skb();' function signature. Should it at
least return ok/err? or even return a 'struct xdp_to_skb_metadata *'
pointer and the xdp prog can directly read (or even write) it?
Hmm, I'm not sure returning a failure makes sense? Failure to read one
or more fields just means that those fields will not be populated? We
should probably have a flags field inside the metadata struct itself to
indicate which fields are set or not, but I'm not sure returning an
error value adds anything? Returning a pointer to the metadata field
might be convenient for users (it would just be an alias to the
data_meta pointer, but the verifier could know its size, so the program
doesn't have to bounds check it).
If some hints are not available, those hints should be initialized to
0/CHECKSUM_NONE/...etc. The xdp prog needs a direct way to tell hard failure
when it cannot write the meta area because of not enough space. Comparing
xdp->data_meta with xdp->data as a side effect is not intuitive.
It is more than saving the bound check. With type info of 'struct
xdp_to_skb_metadata *', the verifier can do more checks like reading in the
middle of an integer member. The verifier could also limit write access only to
a few struct's members if it is needed.
The returning 'struct xdp_to_skb_metadata *' should not be an alias to the
xdp->data_meta. They should actually point to different locations in the
headroom. bpf_xdp_metadata_export_to_skb() sets a flag in xdp_buff.
xdp->data_meta won't be changed and keeps pointing to the last
bpf_xdp_adjust_meta() location. The kernel will know if there is
xdp_to_skb_metadata before the xdp->data_meta when that bit is set in the
xdp_{buff,frame}. Would it work?
A related question, why 'struct xdp_to_skb_metadata' needs
__randomize_layout?
The __randomize_layout thing is there to force BPF programs to use CO-RE
to access the field. This is to avoid the struct layout accidentally
ossifying because people in practice rely on a particular layout, even
though we tell them to use CO-RE. There are lots of examples of this
happening in other domains (IP header options, TCP options, etc), and
__randomize_layout seemed like a neat trick to enforce CO-RE usage :)
I am not sure if it is necessary or helpful to only enforce __randomize_layout
in 'struct xdp_to_skb_metadata'. There are other CO-RE use cases (tracing and
non tracing) that already have direct access (reading and/or writing) to other
kernel structures.
It is more important for the verifier to see the xdp prog accessing it as a
'struct xdp_to_skb_metadata *' instead of xdp->data_meta which is a __u8 * so
that the verifier can enforce the rules of access.
Does xdp_to_skb_metadata have a use case for XDP_PASS (like patch 7) or the
xdp_to_skb_metadata can be limited to XDP_REDIRECT only?
XDP_PASS cases where we convert xdp_buff into skb in the drivers right
now usually have C code to manually pull out the metadata (out of hw
desc) and put it into skb.
So, currently, if we're calling bpf_xdp_metadata_export_to_skb() for
XDP_PASS, we're doing a double amount of work:
skb_metadata_import_from_xdp first, then custom driver code second.
In theory, maybe we should completely skip drivers custom parsing when
there is a prog with BPF_F_XDP_HAS_METADATA?
Then both xdp->skb paths (XDP_PASS+XDP_REDIRECT) will be bpf-driven
and won't require any mental work (plus, the drivers won't have to
care either in the future).
> WDYT?
Yeah, not sure if it can solely depend on BPF_F_XDP_HAS_METADATA but it makes
sense to only use the hints (if ever written) from xdp prog especially if it
will eventually support xdp prog changing some of the hints in the future. For
now, I think either way is fine since they are the same and the xdp prog is sort
of doing extra unnecessary work anyway by calling
bpf_xdp_metadata_export_to_skb() with XDP_PASS and knowing nothing can be
changed now.
I agree it would be best if the drivers also use the XDP metadata (if
present) on XDP_PASS. Longer term my hope is we can make the XDP
metadata support the only thing drivers need to implement (i.e., have
the stack call into that code even when no XDP program is loaded), but
for now just for consistency (and allowing the XDP program to update the
metadata), we should probably at least consume it on XDP_PASS.
-Toke
