On Tue, Nov 16, 2021 at 8:42 AM Mauricio Vásquez <mauricio@xxxxxxxxxx> wrote: > > The result of the CO-RE relocations can be useful for some use cases > like BTFGen[0]. This commit adds a new ‘record_core_relos’ option to > save the result of such relocations and a couple of functions to access > them. > > [0]: https://github.com/kinvolk/btfgen/ > > Signed-off-by: Mauricio Vásquez <mauricio@xxxxxxxxxx> > Signed-off-by: Rafael David Tinoco <rafael.tinoco@xxxxxxxxxxx> > Signed-off-by: Lorenzo Fontana <lorenzo.fontana@xxxxxxxxxx> > Signed-off-by: Leonardo Di Donato <leonardo.didonato@xxxxxxxxxx> > --- > tools/lib/bpf/libbpf.c | 63 ++++++++++++++++++++++++++++++++++++++- > tools/lib/bpf/libbpf.h | 49 +++++++++++++++++++++++++++++- > tools/lib/bpf/libbpf.map | 2 ++ > tools/lib/bpf/relo_core.c | 28 +++++++++++++++-- > tools/lib/bpf/relo_core.h | 21 ++----------- > 5 files changed, 140 insertions(+), 23 deletions(-) > Ok, I've meditated on this patch set long enough. I still don't like that libbpf will be doing all this just for the sake of BTFGen's use case. In the end, I think giving bpftool access to internal APIs of libbpf is more appropriate, and it seems like it's pretty easy to achieve. It might actually clean up gen_loader parts a bit as well. So we'll need to coordinate all this with Alexei's current work on CO-RE for kernel as well. But here's what I think could be done to keep libbpf internals simple. We split bpf_core_apply_relo() into two parts: 1) calculating the struct bpf_core_relo_res and 2) patching the instruction. If you look at bpf_core_apply_relo, it needs prog just for prog_name (which we can just pass everywhere for logging purposes) and to extract one specific instruction to be patched. This instruction is passed at the very end to bpf_core_patch_insn() after bpf_core_relo_res is calculated. So I propose to make those two explicitly separate steps done by libbpf. So bpf_core_apply_relo() (which we should rename to bpf_core_calc_relo() or something like that) won't do any modification to the program instructions. bpf_object__relocate_core() will do bpf_core_calc_relo() first, if that's successful, it will pass the result into bpf_core_patch_insn(). Simple and clean, unless I missed some complication (happens all the time, but..) At this point, we can teach bpftool to just take btf_ext (from BPF object file), iterate over all CO-RE relos and call only bpf_core_calc_relo() (no instruction patching). Using bpf_core_relo_res bpftool will know types and fields that need to be preserved and it will be able to construct minimal btf. So interface for bpftool looks like this: bpftool gen distill_btf (or whatever the name) <file.bpf.o> <distilled_raw.btf> BTFGen as a solution, then, can use bpftool to process each pair of btf + bpf object. Thoughts? [...]
