On Mon, Apr 15, 2024 at 5:41 AM Fabian Pfitzner <f.pfitzner@xxxxxxxxxxxxxxxxxx> 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, .....). > > 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. > Have you tried bpf_xdp_output() helper ? > 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 > > > >
