> > This should ideally look like a real global variable extern: > > extern const struct rq runqueues __ksym; > > > But that's the case for non-per-cpu variables. You didn't seem to > address per-CPU variables in this patch set. How did you intend to > handle that? We should look at a possible BPF helper to access such > variables as well and how the verifier will prevent direct memory > accesses for such variables. > > We should have some BPF helper that accepts per-CPU PTR_TO_BTF_ID, and > returns PTR_TO_BTF_ID, but adjusted to desired CPU. And verifier > ideally would allow direct memory access on that resulting > PTR_TO_BTF_ID, but not on per-CPU one. Not sure yet how this should > look like, but the verifier probably needs to know that variable > itself is per-cpu, no? > Yes, that's what I was unclear about, so I don't have that part in this patchset. But your explanation helped me organize my thoughts. :) Actually, the verifier can tell whether a var is percpu from the DATASEC, since we have encoded "percpu" DATASEC in btf. I think the following should work: We may introduce a new PTR_TO_BTF_VAR_ID. In ld_imm, libbpf replaces ksyms with btf_id. The btf id points to a KIND_VAR. If the pointed VAR is found in the "percpu" DATASEC, dst_reg is set to PTR_TO_BTF_VAR_ID; otherwise, it will be a PTR_TO_BTF_ID. For PTR_TO_BTF_VAR_ID, reg->btf_id is the id of the VAR. For PTR_TO_BTF_ID, reg->btf_id is the id of the actual kernel type. The verifier would reject direct memory access on PTR_TO_BTF_VAR_ID, but the new BPF helper can convert a PTR_TO_BTF_VAR_ID to PTR_TO_BTF_ID. Hao
