On 4/14/24 2:34 PM, Fabian Pfitzner wrote:
Hello,
is there a specific reason why it is not allowed to copy data from ctx
directly into a map via the bpf_map_update_elem helper?
I develop a XDP program where I need to store incoming packets
(including the whole payload) into a map in order to buffer them.
I thought I could simply put them into a map via the mentioned helper
function, but the verifier complains about expecting another type as
"ctx" (R3 type=ctx expected=fp, pkt, pkt_meta, .....).
Looks like you intend to copy packet data. So from the above, 'expected=fp,pkt,pkt_meta...', you can just put the first argument
with xdp->data, right?
Verifer rejects to 'ctx' since 'ctx' contents are subject to verifier rewrite. So actual 'ctx' contents/layouts may not match uapi definition.
I was able to circumvent this error by first putting the packet onto
the stack (via xdp->data) and then write it into the map.
The only limitation with this is that I cannot store packets larger
than 512 bytes due to the maximum stack size.
I was also able to circumvent this by slicing chunks, that are smaller
than 512 bytes, out of the packet so that I can use the stack as a
clipboard before putting them into the map. This is a really ugly
solution, but I have not found a better one yet.
So my question is: Why does this limitation exist? I am not sure if
its only related to XDP programs as this restriction is defined inside
of the bpf_map_update_elem_proto struct (arg3_type restricts this), so
I think it is a general limitation that affects all program types.
Best regards,
Fabian Pfitzner
