On Fri, 14 Jan 2022, Andrii Nakryiko wrote: > > The one piece that seems to be missing from my perspective - and this may > > be in more recent versions - is uprobe function attachment by name. Most of > > the work is already done in libusdt so it's reasonably doable I think - at a > > minimum it would require an equivalent to the find_elf_func_offset() > > function in my patch 1. Now the name of the library libusdt suggests its > > focus is on USDT of course, but I think having userspace function attach > > by name too would be great. Is that part of your plans for this work? > > True, uprobes don't supprot attaching by function name, which is quite > annoying. It's certainly not a focus for libusdt (or whatever it will > end up being called when open-sources). But if it's not much code and > complexity we should probably just add that to libbpf directly for > uprobes. > I've been looking at this, and I've got the following cases working: - local symbols in a binary. This involves symbol table lookup and relative offset calcuation. - shared object symbols in a shared object. In this case, the symbol table values suffice, no adjustment needed. The former works using the program headers (instead of /proc/pid/maps for offset computation), so can be run for all processes, lifting the limitation in the RFC which only supported name lookup for a specific process. Around a hundred lines for this makes it worthwhile I think. There is one more case, which is a shared library function in a binary - where I specify "malloc" as the function and /usr/bin/foo as the binary path. In this case, for dynamic symbols we can't just look up the symbol table in the binary, since the associated values are 0. Ideally it would be nice if the user could just specify "malloc" and not need to use libc as the binary path argument, but getting this working is proving to be trickier. I've tried making use of PLT section information but no luck yet (the idea being we try to use the trampoline address of malloc@@PLT instead, but I'm still trying to figure out how to extract that). So I'm wondering if we just fail lookup for that case, assuming the user will specify the shared library path if they want to trace a shared library function. What do you think? Thanks! Alan
