[llvm-dev] Continuing from dbgtrap on different targets

Joseph Tremoulet via llvm-dev
2020-03-04T20:16:22Z

Hi,

I'm noticing an unexpected difference between targets when I hit a dbgtrap
in the debugger.  Consider this simple llvm function:

define void @do_break() {
  entry:
    call void @llvm.debugtrap()
    ret void
}

If I compile that with llc and use lldb to launch a program that calls it, on
x86_64 linux (Ubuntu 18.04), here's what I see at the stop:

Process 130404 stopped
* thread #1, name = 'doit', stop reason = signal SIGTRAP
    frame #0: 0x0000000000400541 doit`do_break at stub.ll:2:1
(lldb) disas
doit`do_break:
    0x400540 <+0>: int3
->  0x400541 <+1>: retq
(lldb) register read rip
     rip = 0x0000000000400541  doit`do_break + 1 at stub.ll:2:1

Note that rip is reported as pointing to the next instruction after the int3.
If I 'continue' from there, the program continues doing whatever was
after the debugtrap.


If I follow the same steps on aarch64 (also Ubuntu 18.04), I see this:

Process 21586 stopped
* thread #1, name = 'doit', stop reason = signal SIGTRAP
    frame #0: 0x00000000004005dc doit`do_break at stub.ll:1:1
(lldb) disas
doit`do_break:
->  0x4005dc <+0>: brk    #0x1
    0x4005e0 <+4>: ret
(lldb) register read pc
      pc = 0x00000000004005dc  doit`do_break at stub.ll:1:1

Note that here, pc is reported as pointing at the 'brk' instruction
itself.  If I 'continue' from there, I immediately find myself stopped
back at the same point, ad infinitum.

>From what I can tell, GDB also "gets stuck" when it hits this
instruction on aarch64 (and also doesn't on x86_64).

I'm wondering what to make of this / where's the "bug".

  *   Should llvm use a different lowering for dbgtrap on aarch64-linux?  I
don't think so, it seems to be standard
  *   Should the system signal handler be reporting an incremented pc in the
context struct when it hits brk?
     *   And even if so, what should the workaround be for systems without such
a fix?
  *   Should lldb (and gdb for that matter) somehow recognize this case, and
increment pc when stopping or resuming at a brk?
  *   Is this just unsupported, is continuing past a debugtrap UB or otherwise
disallowed?

I'd appreciate any insights here.

Thanks,
-Joseph

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