On Tue, Sep 21, 2021 at 06:27:16AM IST, Alexei Starovoitov wrote: > On Mon, Sep 20, 2021 at 7:15 AM Kumar Kartikeya Dwivedi > <memxor@xxxxxxxxx> wrote: > > > > This change updates the BPF syscall loader to relocate BTF_KIND_FUNC > > relocations, with support for weak kfunc relocations. The next commit > > adds bpftool supports to set up the fd_array_sz parameter for light > > skeleton. > > > > A second map for keeping fds is used instead of adding fds to existing > > loader.map because of following reasons: > > but it complicates signing bpf progs a lot. > Can you explain this in short? (Just want to understand why it would be problem). > > If reserving an area for map and BTF fds, we would waste the remaining > > of (MAX_USED_MAPS + MAX_KFUNC_DESCS) * sizeof(int), which in most cases > > will be unused by the program. Also, we must place some limit on the > > amount of map and BTF fds a program can possibly open. > > That is just (256 + 64)*4 bytes of data. Really not much. > I wouldn't worry about reserving this space. > Ok, I'll probably go with this now, I didn't realise a separate fd would be prohibitive for the signing case, so I thought it would nice to lift the limiation on number of map_fds by packing fd_array fds in another map. > > If setting gen->fd_array to first map_fd offset, and then just finding > > the offset relative to this (for later BTF fds), such that they can be > > packed without wasting space, we run the risk of unnecessarily running > > out of valid offset for emit_relo stage (for kfuncs), because gen map > > creation and relocation stages are separated by other steps that can add > > lots of data (including bpf_object__populate_internal_map). It is also > > prone to break silently if features are added between map and BTF fd > > emits that possibly add more data (just ~128KB to break BTF fd, since > > insn->off allows for INT16_MAX (32767) * 4 bytes). > > I don't follow this logic. > > > Both of these issues are compounded by the fact that data map is shared > > by all programs, so it is easy to end up with invalid offset for BTF fd. > > I don't follow this either. There is only one map and one program. > What sharing are you talking about? What I saw was that the sequence of calls is like this: bpf_gen__map_create add_data - from first emit we add map_fd, we also store gen->fd_array then libbpf would call bpf_object__populate_internal_map which calls bpf_gen__map_update_elem, which also does add_data (can be of arbitrary sizes). emit_relos happens relatively at the end. For each program in the object, this sequence can be repeated, such that the add_data that we do in emit_relos, relative offset from gen->fd_array offset can end up becoming big enough (as all programs in object add data to same map), while gen->fd_array comes from first map creation. However reserving an area works ok (like with the stack). Thanks. -- Kartikeya
