The 8 threads whose [rbx] did not resolve to a vtable were never in
XObject::Wait. The backtrace grep matched WaitMultiple as a substring, and
there %rbx is the XObject** array (mov %rsi,%rbx) with the count in %ebp, so
[rbx] is objects[0] -- an object pointer, needing a second deref -- not a
vtable. The unwind restored rbx correctly for all 18.
freeze_waitobj.sh now takes the function and frame index from the backtrace and
applies the matching read, and captures twice in one run (healthy and after the
~270s black-screen) so the comparison is within-run. waitobj_report.py tabulates
both and diffs them, discarding any value info symbol cannot resolve.
Re-extracting the same gdb capture per thread rather than by grep qualifies the
previous entry. Of eighteen threads whose frame 3 is XObject::Wait, eight have
[rbx] equal to the XEvent vtable plus sixteen, two equal to the XSemaphore
vtable plus sixteen, and eight hold a pointer into the mmap region that is not a
vtable at all. So the waits are on two distinct kernel types, XEvent and
XSemaphore, and the earlier claim that the object is an XEvent was right for the
majority but not the whole picture.
The eight non-vtable readings are the method checking itself rather than a
failure. A polymorphic object's first word is always a vtable pointer, so those
values are simply not this: rbx could not be restored for those frames and the
unwind returned whatever the register held. A reading counts only if [rbx] lands
in the binary's vtable range and resolves to a "vtable for" symbol; anything
else is discarded rather than interpreted. Ten of eighteen resolve and the rest
are honestly unknown.
That also settles the previous entry's worry that the 0x7ffc and 0x7ffd
addresses looked like stack. They are the shared mmap region, which holds thread
stacks and large allocations alike, so the vtable check rather than the address
range is what separates an object from a stack slot.
The follow-up run that would have added /proc/<pid>/maps classification and a
wider object dump never booted -- EMULATOR GONE at 0s, skip_intro exit 4 --
most likely a stale emulator or lockfile from the preceding gdb session, whose
process tree is parented differently and escaped the usual cleanup. So the map
classification, the multi-word object dump and the frozen-state capture are all
still unrun.
Executed the wait-object read on a live run under gdb. For every thread whose
frame 3 is XObject::Wait, rbx gives a pointer whose first quadword is
0x5555562db8f0, which is the PIE base plus 0xd878f0, and the symbol table has
vtable for xe::kernel::XEvent at 0xd878e0. The stored pointer is the vtable
symbol plus sixteen, since offset-to-top and RTTI come first, so it matches
exactly. The same vtable appears on every sampled waiting thread while the this
pointers differ, meaning many threads waiting on different XEvent instances. The
whole chain needs no DWARF and no rebuild, as the static groundwork predicted.
Two caveats, both recorded rather than smoothed over.
This is a healthy-play snapshot, not the freeze. The capture landed at 195 s of
flight with screen_id reporting flight and a non-black mean, because boot under
gdb costs about 300 s and the entire experiment has to fit inside one call --
a timeout kills the process group and takes the emulator with it, which lost an
earlier attempt outright. So this describes what threads wait on during normal
play, which is the control the frozen capture never had, but it is not the
frozen case.
And the this addresses look like host stack rather than heap. Either xenia
places these objects somewhere unusual, or rbx at frame 3 is not Wait's this
after the unwind and the vtable match is coincidence. An exact plus-sixteen
match on a known symbol is hard to get by accident, but the address range is not
what was expected. The settling check is to dump a few words at rbx and see
whether they look like an XEvent -- vtable, KernelState pointer, handle and type
fields -- or like saved registers.
