On Tue, Mar 15, 2022 at 2:50 PM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Tue, Mar 15, 2022 at 1:22 AM Grant Seltzer Richman > <grantseltzer@xxxxxxxxx> wrote: > > > > Hi there, > > > > If I call `bpf_map_create()` successfully I'll have a file descriptor > > and not a `struct bpf_map`. This stifles me from using a lot of the > > `bpf_map__` API functions, for example `bpf_map__pin()`. What's the > > reason for this? Is there a way to get a `struct bpf_map` that I'm > > missing? > > > > bpf_map_create() is a low-level libbpf API. It does create a map and > returns FD. But it is not "integrated" with struct bpf_map APIs, which > are so-called high-level libbpf APIs. bpf_object/bpf_map/bpf_program > and related APIs are high-level APIs and they expect ELF BPF object > file to declaratively define everything. You can use low-level APIs by > getting FDs from high-level bpf_map/bpf_program using bpf_map__fd() > and bpf_program__fd(), but constructing bpf_map from FD isn't easy or > straightforward and should be avoided, if possible. Thank you for the explanation! > > But one way to do this is to still declaratively define map in BPF > object file, but then do bpf_map__reuse_fd() to substitute already > created map by FD. This way libbpf won't create another map, it will > just use your FD. But definitions of maps have to be compatible in > terms of key/value sizes, max_entries, etc. In short: it's painful. > > > As a bit of aside, I do think we are missing high-level APIs to work > with bpf_map elements, something like bpf_map__lookup_elem() and > bpf_map__update_elem() has been on my mind for a while, but I haven't > gotten time to get to it. It would further minimize the need to drop > down to low-level APIs. Would these just call into bpf_map_update_elem() and bpf_map_lookup_elem()? They'd take a *bpf_map and pass the member fd to the low level functions? > > > > Thanks so much, > > Grant Seltzer > > > > P.s. been a while since I've worked on adding docs, but I will finally > > be getting back to it! > > Great, looking forward!
