On Thu, May 18, 2023 at 5:26 PM Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > > On Thu, May 18, 2023 at 11:26 AM Yonghong Song <yhs@xxxxxxxx> wrote: > > > I wonder now when the address will be stored as number (not string) we > > > could somehow generate relocation records and have the module loader > > > do the relocation automatically > > > > > > not sure how that works for vmlinux when it's loaded/relocated on boot > > > > Right, actual module address will mostly not match the one in dwarf. > > Some during module btf load, we should modify btf address as well > > for later use? Yes, may need to reuse some routines used in initial > > module relocation. > > > Few thoughts: > > Initially I felt that single FUNC with multiple DECL_TAG(addr) > is better, since BTF for all funcs is the same and it's likely > one static inline function that the compiler decided not to inline > (like cpumask_weight), so when libbpf wants to attach prog to it > the kernel should automatically attach in all places. > But then noticed that actually different functions with > the same name and proto will be deduplicated into one. > Their bodies at different locations will be different. > Example: seq_show. > In this case it's better to let libbpf pick the exact one to attach. > Then realized that even the same function like cpumask_weight() > might have different body at different locations due to optimizations. > I don't think dwarf contains enough info to distinguish all the combinations. > > Considering all that it's better to keep one BTF kind_func -> one addr. > If it's extended the way Alan is proposing with kind_flag > the dedup logic will not combine them due to different addresses. I've discussed this w/ Alexei and Yonghong offline, so will summarize what I said here. I don't think that we should go the route of adding kflag to BTF_KIND_FUNC. As Yonghong pointed out, previously only vlen and kind determined byte size of the type, and so adding a third variable (kflag), which would apply only to BTF_KIND_FUNC, seems like an unnecessary new complication. I propose to go with an entirely new kind instead, we have plenty of them left. This new kind will be pretty kernel-specific, so could be targeted for kernel use cases better without adding unnecessary complications to Clang. BTF_KIND_FUNCs generated by Clang for .bpf.o files don't need addr, they are meaningless and Clang doesn't know anything about addresses anyways. So we can keep Clang unchanged and more backwards compatible. But now that this new kind (BTF_KIND_KERNEL_FUNC? KFUNC would be misleading, unfortunately) is kernel-specific and generated by pahole only, besides addr we can add some flags field and use them to mark function as defined as kfunc or not, or (as a hypothetical example) traceable or not, or maybe we even have inline flag some day, etc. Something that makes sense mostly for kernel functions. Having said all that, given we are going to break all existing BTF-aware tools again with a new kind, we should really couple all this work with making BTF self-describing as discussed in [0], so that future changes like this won't break older bpftool and other similar tools, unnecessarily. Which, btw, is another reason to not use kflag to determine the size of btf_type. Proposed solution in [0] assumes that kind + vlen defines the size. We should probably have dedicated discussion for self-describing BTF, but I think both changes have to be done in the same release window. [0] https://lore.kernel.org/bpf/CAEf4BzYjWHRdNNw4B=eOXOs_ONrDwrgX4bn=Nuc1g8JPFC34MA@xxxxxxxxxxxxxx/#t > > Also turned out that the kernel doesn't validate decl_tag string. > The following code loads without error: > __attribute__((btf_decl_tag("\x10\xf0"))); > > I'm not sure whether we want to tighten decl_tag validation and how. > If we keep it as-is we can use func+decl_tag approach > to add 4 bytes of addr in the binary format (if 1st byte is not zero). > But it feels like a hack, since the kernel needs to be changed > anyway to adjust the addresses after module loading and kernel relocation. > So func with kind_flag seems like the best approach. > > Regarding relocation of address in the kernel and modules... > We just need to add base_addr to all addrs-es recorded in BTF. > Both for kernel and for module BTFs. > Shouldn't be too complicated. yep, KASLR seems simple enough to handle by the kernel itself at boot time.
