On Mon, Aug 01, 2022 at 04:23:16PM -0700, Martin KaFai Lau wrote: > On Mon, Aug 01, 2022 at 03:58:41PM -0700, Andrii Nakryiko wrote: > > On Mon, Aug 1, 2022 at 3:33 PM Martin KaFai Lau <kafai@xxxxxx> wrote: > > > > > > On Mon, Aug 01, 2022 at 02:16:23PM -0700, Joanne Koong wrote: > > > > On Mon, Aug 1, 2022 at 12:38 PM Martin KaFai Lau <kafai@xxxxxx> wrote: > > > > > > > > > > On Mon, Aug 01, 2022 at 10:52:14AM -0700, Joanne Koong wrote: > > > > > > > Since we are on bpf_dynptr_write, what is the reason > > > > > > > on limiting it to the skb_headlen() ? Not implying one > > > > > > > way is better than another. would like to undertand the reason > > > > > > > behind it since it is not clear in the commit message. > > > > > > For bpf_dynptr_write, if we don't limit it to skb_headlen() then there > > > > > > may be writes that pull the skb, so any existing data slices to the > > > > > > skb must be invalidated. However, in the verifier we can't detect when > > > > > > the data slice should be invalidated vs. when it shouldn't (eg > > > > > > detecting when a write goes into the paged area vs when the write is > > > > > > only in the head). If the prog wants to write into the paged area, I > > > > > > think the only way it can work is if it pulls the data first with > > > > > > bpf_skb_pull_data before calling bpf_dynptr_write. I will add this to > > > > > > the commit message in v2 > > > > > Note that current verifier unconditionally invalidates PTR_TO_PACKET > > > > > after bpf_skb_store_bytes(). Potentially the same could be done for > > > > > other new helper like bpf_dynptr_write(). I think this bpf_dynptr_write() > > > > > behavior cannot be changed later, so want to raise this possibility here > > > > > just in case it wasn't considered before. > > > > > > > > Thanks for raising this possibility. To me, it seems more intuitive > > > > from the user standpoint to have bpf_dynptr_write() on a paged area > > > > fail (even if bpf_dynptr_read() on that same offset succeeds) than to > > > > have bpf_dynptr_write() always invalidate all dynptr slices related to > > > > that skb. I think most writes will be to the data in the head area, > > > > which seems unfortunate that bpf_dynptr_writes to the head area would > > > > invalidate the dynptr slices regardless. > > > > > > > > What are your thoughts? Do you think you prefer having > > > > bpf_dynptr_write() always work regardless of where the data is? If so, > > > > I'm happy to make that change for v2 :) > > > Yeah, it sounds like an optimization to avoid unnecessarily > > > invalidating the sliced data. > > > > > > To be honest, I am not sure how often the dynptr_data()+dynptr_write() combo will > > > be used considering there is usually a pkt read before a pkt write in > > > the pkt modification use case. If I got that far to have a sliced data pointer > > > to satisfy what I need for reading, I would try to avoid making extra call > > > to dyptr_write() to modify it. > > > > > > I would prefer user can have similar expectation (no need to worry pkt layout) > > > between dynptr_read() and dynptr_write(), and also has similar experience to > > > the bpf_skb_load_bytes() and bpf_skb_store_bytes(). Otherwise, it is just > > > unnecessary rules for user to remember while there is no clear benefit on > > > the chance of this optimization. > > > > > > > Are you saying that bpf_dynptr_read() shouldn't read from non-linear > > part of skb (and thus match more restrictive bpf_dynptr_write), or are > > you saying you'd rather have bpf_dynptr_write() write into non-linear > > part but invalidate bpf_dynptr_data() pointers? > The latter. Read and write without worrying about the skb layout. > > Also, if the prog needs to call a helper to write, it knows the bytes are > not in the data pointer. Then it needs to bpf_skb_pull_data() before > it can call write. However, after bpf_skb_pull_data(), why the prog > needs to call the write helper instead of directly getting a new > data pointer and write to it? If the prog needs to write many many > bytes, a write helper may then help. After another thought, other than the non-linear handling, bpf_skb_store_bytes() / dynptr_write() is more useful in the 'BPF_F_RECOMPUTE_CSUM | BPF_F_INVALIDATE_HASH' flags. That said, my preference is still to have the same expectation on non-linear data for both dynptr_read() and dynptr_write(). Considering the user can fall back to use bpf_skb_load_bytes() and bpf_skb_store_bytes(), I am fine with the current patch also. > > > > > I guess I agree about consistency and that it seems like in practice > > you'd use bpf_dynptr_data() to work with headers and stuff like that > > at known locations, and then if you need to modify the rest of payload > > you'd do either bpf_skb_load_bytes()/bpf_skb_store_bytes() or > > bpf_dynptr_read()/bpf_dynptr_write() which would invalidate > > bpf_dynptr_data() pointers (but that would be ok by that time). > imo, read, write and then go back to read is less common. > writing bytes without first reading them is also less common.
