I now have a few patches [1] adding support for collecting initial
register values from the current thread. While I believe the initial
register collection logic works (it works for GHC), I have encountered
an unfortunate issue while folding this code into libebl. Namely, it
seems that libebl backends are dynamically loaded.
To see why this is problematic, consider this example,
// libdwfl now provides this,
int dwfl_linux_local_attach (Dwfl *dwfl) {
// local_thread_callbacks uses ebl_set_initial_registers_local
// which is now provided by libebl
dwfl_attach_state(dwfl, NULL, pid, &local_thread_callbacks, NULL);
}
// Here is the test-case,
void test() {
Dwfl *dwfl = dwfl_begin (&proc_callbacks);
// Figure out what modules are loaded
dwfl_linux_proc_report(dwfl, getpid());
dwfl_report_end (dwfl, NULL, NULL);
// but the ebl backend, libebl_x86_64.so, only gets loaded here
dwfl_linux_local_attach(dwfl);
// therefore when we begin to unwind we have no debug information
// for libebl, which is where PC sits when we collect the initial
// register values. Unwinding consequently fails
dwfl_getthread_frames (dwfl, getpid(), frame_cb, dwfl) != 0;
}
The issue here is that debug information is not reported to libdwfl as
it is not loaded at the time we report the loaded modules.
At this point I'm a bit unsure of how to proceed. I can think of a few
options,
* teach dwfl_linux_local_attach to report libebl when it gets
loaded. This would be rather ugly as we would either need to
re-read the /proc map data or somehow extract the necessary
information from the dynamic linker
* Expose an entirely different interface from dwfl_getthread_frames
for local unwinding such that PC doesn't sit within libebl when
initial register values are collected.
This would likely involve turning it into an inline function defined
in a header which could grab the initial register values,
// For instance, in the case of x86_64
#if defined(__x86_64__) && defined(__linux__)
static inline void
dwfl_get_frames_here(Dwfl *dwfl,
void (*cb)(Frame *, void*),
void *cbdata)
{
Thread thread = ...; // need to figure out what to put here
Dwarf_Word regs[17];
__asm__ (( "// Grab registers " ));
dwfl_thread_state_registers(thread, 0, 17, regs);
}
Unfortunately this is quite a departure from the current interface
and seems to break the nice separation between libebl and the
libdwfl.
I also don't really know how to correctly handle the
rather subtle distinction between 32-/64-bit programs running on
32-/64-bit kernels.
* Something I haven't yet thought of...
Any ideas?
Cheers,
- Ben
[1]
https://github.com/bgamari/elfutils/compare/master...local-unwind