[lldb-dev] Reading memory while inferior is running

[Greg Clayton]

It is fine to not require that a process be stopped in order to read memory. Are you still doing your python based approach? We are currently enforcing that a process must be stopped in order to read memory in SBProcess::ReadMemory() where we acquire the run lock and make sure the process stays stopped. This can be changed, but we will probably need to add code the lldb_private::Process that asks the current process if it can handle reading memory while running. 

So the code would need to go from this:

    if (process_sp)
    {
        Process::StopLocker stop_locker;
        if (stop_locker.TryLock(&process_sp->GetRunLock()))
        {
            Mutex::Locker api_locker (process_sp->GetTarget().GetAPIMutex());
            bytes_read = process_sp->ReadMemory (addr, dst, dst_len, sb_error.ref());
        }
        else
        {
            if (log)
                log->Printf ("SBProcess(%p)::ReadMemory() => error: process is running",
                             static_cast<void*>(process_sp.get()));
            sb_error.SetErrorString("process is running");
        }
    }


to something like:


    if (process_sp)
    {
        Process::StopLocker stop_locker;
        if (process_sp->CanReadMemoryWhileRunning() || stop_locker.TryLock(&process_sp->GetRunLock()))
        {
            Mutex::Locker api_locker (process_sp->GetTarget().GetAPIMutex());
            bytes_read = process_sp->ReadMemory (addr, dst, dst_len, sb_error.ref());
        }
        else
        {
            if (log)
                log->Printf ("SBProcess(%p)::ReadMemory() => error: process is running",
                             static_cast<void*>(process_sp.get()));
            sb_error.SetErrorString("process is running");
        }
    }

The lldb_private::Process::CanReadMemoryWhileRunning() would need to be a virtual function that is added to lldb_private::Process which defaults to return false. Then any process plug-ins that do allow this can override this function and return true.

Greg


> On Dec 21, 2014, at 6:28 AM, Mario Zechner <badlogicgames at gmail.com> wrote:
> 
> Hi,
> 
> i know this has been asked before [1], but i just wanted to explore whether it's a waste of time to patch this into our LLDB fork. I talked about our architecture in [2]. It boils down to having to suspend/resume the inferior to quickly read a tiny bit of memory. This causes us quite a bit of problems as outlined in the other thread.
> 
> Our memory reads would be just that: fetch a block of memory from a known, non-stack address. We can resolve atomicity issues on our end. Do you think this would be feasible?
> 
> Thanks,
> Mario
>   
> 
> [1] http://lists.cs.uiuc.edu/pipermail/lldb-dev/2014-June/004139.html
> [2] http://lists.cs.uiuc.edu/pipermail/lldb-dev/2014-December/006138.html
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