On Tue, 8 Dec 2020, Alexei Starovoitov wrote: > On Tue, Dec 08, 2020 at 10:13:35PM +0000, Alan Maguire wrote: > > > > Does this approach prevent more complex run-time specification of BTF > > object fd though? For example, I've been working on a simple tracer > > focused on kernel debugging; it uses a BPF map entry for each kernel > > function that is traced. User-space populates the map entry with BTF type > > ids for the function arguments/return value, and when the BPF program > > runs it uses the instruction pointer to look up the map entry for that > > function, and uses bpf_snprintf_btf() to write the string representations > > of the function arguments/return values. I'll send out an RFC soon, > > but longer-term I was hoping to extend it to support module-specific > > types. Would a dynamic case like that - where the BTF module fd is looked > > up in a map entry during program execution (rather than derived via > > __btf_builtin_type_id()) work too? Thanks! > > fd has to be resolved in the process context. bpf prog can read fd > number from the map, but that number is meaningless. > Say we allow using btf_obj_id+btf_id, how user space will know these > two numbers? Some new libbpf api that searches for it? > An extension to libbpf_find_vmlinux_btf_id() ? I was hoping that this api > will stay semi-internal. But say it's extended. > The user space will store a pair of numbers into a map and > what program are going to do with it? > If it's printing struct veth_stats contents it should have attached to > a corresponding function in the veth module via fentry or something. > The prog has hard coded logic in C with specific pointer to print. > The prog has its type right there. Why would the prog take a pointer > from one place, but it's type_id from the map? That's not realistic. > Where it would potentially make sense is what I think you're descring > where single kprobe style prog attached to many places and args of > those places are stored in a map and the prog selects them with > map_lookup with key=PT_REGS_IP ? Right, that's exactly it. A pair of generic tracing BPF programs are used, and they attach to kprobe/kretprobes, and when they run they use the arguments plus the map details about BTF ids of those arguments to run bpf_snprintf_btf(), and send perf events to userspace containing the results. > And passes pointers into bpf_snprintf_btf() from PT_REGS_PARM1() ? Exactly. > I see why that is useful, but it's so racy. By the time the map > is populated those btf_obj_id+btf_id could be invalid. > I think instead of doing this in user space the program needs an access > to vmlinux+mods BTFs. Sort-of like proposed bpf helper to return ksym > based on IP there could be a helper to figure out btf_id+btf_obj_POINTER > based on IP. Then there will no need for external map to populate. > Would that solve your use case? That would be fantastic! We could do that from the context passed into a kprobe program as the IP in struct pt_regs points at the function. kretprobes seems a bit trickier as in that case the IP in struct pt_regs is actually set to kretprobe_trampoline rather than the function we're returning from due to how kretprobes work; maybe there's another way to get it in that case though.. Alan
