3 ms·

Can you outline how bcc translates that req->rq_disk->disk_name expression to bytecode? AIUI, there are no pointer-dereferencing bytecodes. Is it using the BP

• drzaeus77 11y ago
  Exactly, it is using bpf_probe_read. Internally, BCC uses clang's Rewriter functionality to mangle valid C (but invalid BPF) into valid C with bpf helper functions. The req->rq_disk->disk_name expression would expand into: ({ typeof(char [32]) _val; memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val), (u64)({ typeof(struct gendisk *) _val; memset(&_val, 0, sizeof(_val)); bpf_probe_read(&_val, sizeof(_val), (u64)req + offsetof(struct request, rq_disk)); _val; }) + offsetof(struct gendisk, disk_name)); _val; })); If you are playing with the tools, the BPF() class takes an optional argument debug=, where bit 2 (0x4) will print the rewritten C output for your edification.