Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> [Thu, 2020-04-02 13:22 -0700]: > On Fri, Mar 27, 2020 at 12:11:15PM +0100, Toke Høiland-Jørgensen wrote: > > > > Current code is in [0], for those following along. There are two bits of > > kernel support missing before I can get it to where I want it for an > > initial "release": Atomic replace of the dispatcher (this series), and > > the ability to attach an freplace program to more than one "parent". > > I'll try to get an RFC out for the latter during the merge window, but > > I'll probably need some help in figuring out how to make it safe from > > the verifier PoV. > > I have some thoughts on the second part "ability to attach an freplace > to more than one 'parent'". > I think the solution should be more generic than just freplace. > fentry/fexit need to have the same feature. > Few folks already said that they want to attach fentry to multiple > kernel functions. It's similar to what people do with kprobe progs now. > (attach to multiple and differentiate attach point based on parent IP) > Similarly "bpftool profile" needs it to avoid creating new pair of fentry/fexit > progs for every target bpf prog it's collecting stats about. > I didn't add this ability to fentry/fexit/freplace only to simplify > initial implementation ;) I think the time had come. > Currently fentry/fexit/freplace progs have single prog->aux->linked_prog pointer. > It just needs to become a linked list. > The api extension could be like this: > bpf_raw_tp_open(prog_fd, attach_prog_fd, attach_btf_id); > (currently it's just bpf_raw_tp_open(prog_fd)) > The same pair of (attach_prog_fd, attach_btf_id) is already passed into prog_load > to hold the linked_prog and its corresponding btf_id. > I'm proposing to extend raw_tp_open with this pair as well to > attach existing fentry/fexit/freplace prog to another target. > Internally the kernel verify that btf of current linked_prog > exactly matches to btf of another requested linked_prog and > if they match it will attach the same prog to two target programs (in case of freplace) > or two kernel functions (in case of fentry/fexit). > > Toke, Andrey, > if above kinda makes sense from high level description > I can prototype it quickly and then we can discuss details > in the patches ? > Or we can drill further into details and discuss corner cases. That makes sense to me. I've also been thinking of a way to "transition" ext prog from one target program to another, but I had an impression that limiting number of target progs to one for an ext prog is "by design" and hard to change, and was looking at introducing a way to duplicate existing ext prog by its fd but with different attach_prog_fd and attach_btf_id (smth like BPF_PROG_DUP command) instead. But since you're saying that there are actually many use-cases to be able to attach freplace/fexit/fentry to multiple target programs, that works as well. Happy to look at the prototype when it's available. Thanks. -- Andrey Ignatov
