Infinity progress update

Yesterday I had my first Infinity milestone. I have a glibc with notes implementing libthread_db’s td_ta_map_lwp2thr and a GDB that loads and executes them. I did a side-by-side test calling the Infinity libpthread::map_lwp2thr function every time libthread_db’s td_ta_map_lwp2thr was called and comparing the results… and it worked! You could see the DWARF if you’re interested.

I’ve had something of a look at all the five functions I need to implement now and I’m pretty sure I have a handle on what infrastructure is needed to support them. The one thing I’m missing at present is function calls (infinity-infinity and infinity-other), which I’ll likely handle with a new instruction (DW_OP_GNU_i8call or something). That’s probably the only extension I need. It’s definitely necessary because one of the arguments to td_ta_thr_iter is a function that td_ta_thr_iter then calls. Infinity basically needs first-class functions for this.

I also have a pretty good idea of how to hook it into GDB’s Linux thread_db code. I’ll write a bunch of functions in GDB’s common code with the same signatures as libthread_db’s functions and pass them to the existing thread_db code by pointer. Doing it that way means the existing thread_db code in GDB and gdbserver can be left unmodified. I’ll implement the bookkeeping stuff there too, probably using the thread_agent type to pass around whatever context I need.

At the moment I’m using GDB’s existing DWARF interpreter to execute the notes, but I may well end up writing a new one for it. Firstly because the existing interpreter is deeply embedded into the GDB side: it uses gdbarch, values, types, etc, none of which are currently in common code. And secondly because the existing interpreter is written to evaluate lots of different DWARF expressions once (or, a small number of times). It’s input is raw DWARF, which it decodes instruction-by-instruction. The use case for the Infinity interpreter is executing a small number of expressions lots of times, so some pre-analysis would be beneficial. The instructions could be expanded, with operands decoded and possibly translated, and common multi-instruction cases could be combined. Stuff like calls to builtin functions (ps_get_thread_area, ps_getpid) could be replaced with intrinsics. And, with some restrictions on the bytecode, I could pre-compute the stack depth and do one overflow check at function entry rather than overflow checks for every push. That kind of thing.

I’m expecting the addition of a second DWARF interpreter to GDB to be contentious, but they’ll optimized for different things and doing different things. For example, Infinity could work better with some type tracking (and will likely need it to make function calls secure) but it’s different from what GDB’s existing interpreter needs and it’s difficult to see how to combine the two without ending up with something that’s not very good at either. Not to mention that getting it to a point it can be moved to common code would likely slow it down a ton.

Anyway, that’s where I’m at.