Re: No direct copy from ctx to map possible, why?

[Yonghong Song <yonghong.song@linux.dev>]

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
>
>
>
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