[lldb-dev] Missed breakpoint

Greg Clayton via lldb-dev lldb-dev at lists.llvm.org
Fri Mar 18 13:17:00 PDT 2016

The only similar case we ran into was a situation where our kernel was playing tricks and essentially re-loading a module at the same location. So what would happen is:

1 - kernel would load a.out's .text section at 0x10000
2 - LLDB would notice via a module loaded notification and would set a breakpoint at 0x10000 + offset
3 - kernel would load a.out's .text section at 0x10000 but erase all changes, like the breakpoint we set above in step 2
4 - We would get a load notification for a.out's .text section again, but we would see that the location didn't change, so the module doesn't send out a module loaded notification and we would lose the breakpoint for good

What our dynamic loader had to do was to know that this can happen and if you get a load notification for a section, send a module unloaded for a.out first (which would clear the breakpoint), and then send out a module loaded notification and the breakpoint would get set correctly once again when the section was reloaded.

Greg Clayton

> On Mar 18, 2016, at 11:59 AM, Dean De Leo via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> Hi,
> we have been observing that, sporadically, an internal callback set for a breakpoint is not invoked. We are setting an hidden breakpoint in the renderscript plugin through:
> breakpoint = target.CreateBreakpoint(symbol_address, true, false);
> breakpoint->SetCallback(rs_callback, rs_baton, true);
> where symbol_address refers to a runtime function x in the debugged process.
> The callback is properly invoked 99% of the time that the process calls the function x, though occasionally it is missed, while both prior and further calls are recorded.
> This issue seems to occur only in android/mips32, though we are not able to exclude whether other platforms are affected.
> Has anyone experienced a similar issue or can provide any suggestion to what might be worth to watch for that might cause this behaviour?
> Thanks,
> Dean
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