[lldb-dev] [RFC] Fast Conditional Breakpoints (FCB)

[Pedro Alves via lldb-dev]

Hi,

Very interesting.

One comment below, about something that jumped at me when
I skimmed the proposal.

On 8/14/19 9:52 PM, Ismail Bennani via lldb-dev wrote:
> 
> Since the x86_64 ISA has variable instruction size, LLDB moves enough
> instructions in the trampoline to be able to overwrite them with a jump to the
> trampoline.

If I understood you correctly, you meant to say that LLDB moves
enough instructions _at the breakpoint address_ to be able to 
overwrite them with a jump to the trampoline?

It's the plural (instructionS) that jumped at me.
If so, how do you plan to handle the case of some thread currently
executing one of the instructions that you're overwriting?

Say, you're using a 5 bytes jmp instruction to jump to the
trampoline, so you need to replace 5 bytes at the breakpoint address.
But the instruction at the breakpoint address is shorter than
5 bytes.  Like:

ADDR | BEFORE           | AFTER
---------------------------------------
0000 | INSN1 (1 byte)   | JMP (5 bytes)
0001 | INSN2 (2 bytes)  |                   <<< thread T's PC points here
0002 |                  |
0003 | INSN3 (2 bytes)  |

Now once you resume execution, thread T is going to execute a bogus
instruction at ADDR 0001.

GDB does something similar to this for fast tracepoints (replaces
the tracepointed instruction with a jump to a trampoline area
that does the tracepoint collection, all without traps), and because
of the above, GDB currently keeps it simple and only allows setting
fast tracepoints at addresses with instructions longer than
the jump-to-trampoline jump instruction used.

Thanks,
Pedro Alves