On Wed, 3 Aug 2022 18:05:44 -0700 Joanne Koong wrote: > I think the problem is that the skb may be cloned, so a write into any > portion of the paged data requires a pull. If it weren't for this, > then we could do the write and checksumming without pulling (eg kmap > the page, get the csum_partial of the bytes you'll write over, do the > write, get the csum_partial of the bytes you just wrote, then unkmap, > then update skb->csum to be skb->csum - csum of the bytes you wrote > over + csum of the bytes you wrote). I think we would even be able to > provide a direct data slice to non-contiguous pages without needing > the additional copy to a stack buffer (eg kmap the non-contiguous > pages to a contiguous virtual address that we pass back to the bpf > program, and then when the bpf program is finished do the cleanup for > the mappings). The whole read/write/data concept is not a great match for packet parsing. Primary use for packet parsing is that you want to read a header and not have to deal with frags or pulling. In that case you should get a direct pointer or a copy on the stack, transparently. Maybe before I go on talking nonsense I should read up on what dynptr is and what it's trying to achieve. Stan says its like unique_ptr in C++ which tells me.. nothing :) $ git grep dynptr -- Documentation/ $ Any pointers? > Three ideas I'm thinking through as a possible solution: > 1) Enforce that the skb is always uncloned for skb-type bpf progs (we > currently do this just for the skb head, see bpf_unclone_prologue()), > but I'm not sure if the trade-off (pulling all the packet data, even > if it won't be used) is acceptable. > > 2) Don't support cloned skbs for bpf_dynptr_write/data and don't do > any pulling. If the prog wants to use bpf_dynptr_write/data, then they > have to pull it first I think all output skbs from TCP are cloned, so that's not gonna work. > 2) (uglier than #1 and #2) For bpf_dynptr_write()s, pull if the write > is to a paged area and the skb is cloned, otherwise write to the paged > area without pulling; if we do this, then we always have to invalidate > all data slices associated with the skb (even for writes to the head) > since at prog load time, the verifier doesn't know if the pull happens > or not. For bpf_dynptr_data()s, follow the same policy. > > I'm leaning towards 2. What are your thoughts?
