[Lldb-commits] [lldb] r202956 - Add support for JIT debugging on Linux using the GDB JIT interface. Patch written with Keno Fischer.

[Andrew MacPherson]

Hi Jim,

I agree that this part of the patch isn't pretty but I wasn't able to find
a way to be notified when a shared library is loaded (exactly as you
suggest). We need to check on launch or attach and then whenever a new
library is loaded, if there's a better way to do that it would be great to
clean this up.

I can't reproduce the crash on Linux but it looks like we need to
unregister the notification callback that's set, that's the only reason I
can think of that you would end up in the ProcessStateChangedCallback with
an invalid baton pointer. I've attached a patch that I'm hoping will
resolve the issue here, it's definitely a bug that it wasn't being
unregistered.

I hadn't intended for the JITLoaderGDB to be enabled on OSX as I wasn't
able to test it there, sorry about that. It's likely not useful there right
now as it would need modifications to the ObjectFileMachO to do some
relocations, and additionally MCJIT in LLVM would need to be modified to
call the GDB hook on OSX (though other JIT hosts may work). That said it
shouldn't be causing crashes if enabled.

Thanks for looking into this.

Cheers,
Andrew



On Thu, Mar 6, 2014 at 4:28 AM, Jim Ingham <jingham at apple.com> wrote:

> This part of the patch worries me.  If I am debugging a process that
> doesn't have this JIT loader symbol, this bit means every time that the
> process stops for any reason you will search the whole world for some
> symbol that won't be found.  That's something we really avoid doing if we
> can, programs get pretty big and this is not the sort of thing you want to
> do.
>
> I don't know how this symbol comes about, is there no event (shared
> library load or something) that you can hook into to find this symbol?
>
> This patch is also causing a crash on Mac OS X just running a program.
>  The crash looks like:
>
> *(lldb) **bt*
> * thread #8: tid = 0xf68e5, name =
> <lldb.process.internal-state(pid=40372)>, function:
> lldb_private::Process::GetTarget() , stop reason = EXC_BAD_ACCESS (code=1,
> address=0x100)
>     frame #0: 0x0000000106f8072c LLDB`lldb_private::Process::GetTarget()
> at Process.h:2516
>     frame #1: 0x0000000108e7aa5a
> LLDB`JITLoaderGDB::GetSymbolAddress(lldb_private::ConstString const&,
> lldb::SymbolType) const at JITLoaderGDB.cpp:368
>     frame #2: 0x0000000108e7a8bf LLDB`JITLoaderGDB::SetJITBreakpoint() at
> JITLoaderGDB.cpp:99
>     frame #3: 0x0000000108e7a6d8
> LLDB`JITLoaderGDB::ProcessStateChangedCallback(void*,
> lldb_private::Process*, lldb::StateType) at JITLoaderGDB.cpp:354
>     frame #4: 0x0000000108b7b29b
> LLDB`lldb_private::Process::SynchronouslyNotifyStateChanged(lldb::StateType)
> at Process.cpp:1223
>     frame #5: 0x0000000108b89762
> LLDB`lldb_private::Process::ShouldBroadcastEvent(lldb_private::Event*) at
> Process.cpp:3846
>     frame #6: 0x0000000108b8454d
> LLDB`lldb_private::Process::HandlePrivateEvent(std::__1::shared_ptr<lldb_private::Event>&)
> at Process.cpp:4141
>     frame #7: 0x0000000108b8a755
> LLDB`lldb_private::Process::RunPrivateStateThread() at Process.cpp:4290
>     frame #8: 0x0000000108b89bfd
> LLDB`lldb_private::Process::PrivateStateThread(void*) at Process.cpp:4221
>     frame #9: 0x00000001087d811a LLDB`ThreadCreateTrampoline(void*) at
> Host.cpp:629
>     frame #10: 0x00007fff815df899 libsystem_pthread.dylib`_pthread_body
>     frame #11: 0x00007fff815df72a libsystem_pthread.dylib`_pthread_start
>     frame #12: 0x00007fff815e3fc9 libsystem_pthread.dylib`thread_start
> *(lldb) **f 2*
> frame #2: 0x0000000108e7a8bf LLDB`JITLoaderGDB::SetJITBreakpoint() at
> JITLoaderGDB.cpp:99
>    96          log->Printf("JITLoaderGDB::%s looking for JIT register
> hook",
>    97                      __FUNCTION__);
>    98
> -> 99      addr_t jit_addr =
> GetSymbolAddress(ConstString("__jit_debug_register_code"),
>    100                                        eSymbolTypeAny);
>    101     if (jit_addr == LLDB_INVALID_ADDRESS)
>    102         return;
> *(lldb) **expr *this*
> (JITLoaderGDB) $13 = {
>   lldb_private::JITLoader = {
>     m_process = 0x0000000000000000 Public:
> lldb_private::ThreadSafeValue<lldb::StateType> @  Private:
> lldb_private::ThreadSafeValue<lldb::StateType> @
>   }
>   m_jit_objects = size=160215376 {
>     [0] = {
>       first = <parent is NULL>
>       second = <parent is NULL>
>     }
>     ...
>   }
>   m_jit_break_id = 0
>   m_notification_callbacks = {
>     baton = 0x0000000000000001
>     initialize = 0x00007fc54b00f3b0
>     process_state_changed = 0x00000001098cb150 (vtable for
> std::__1::__shared_ptr_pointer<lldb_private::Section*,
> std::__1::default_delete<lldb_private::Section>,
> std::__1::allocator<lldb_private::Section> > + 16)
>   }
> }
>
> Looks like the JIT instance that is getting passed in is not good for some
> reason.
>
> Jim
>
>
> On Mar 5, 2014, at 2:12 AM, Andrew MacPherson <andrew.macp at gmail.com>
> wrote:
>
> +void
> +JITLoaderGDB::ProcessStateChangedCallback(void *baton,
> +                                          lldb_private::Process *process,
> +                                          lldb::StateType state)
> +{
> +    JITLoaderGDB* instance = static_cast<JITLoaderGDB*>(baton);
> +
> +    switch (state)
> +    {
> +    case eStateConnected:
> +    case eStateAttaching:
> +    case eStateLaunching:
> +    case eStateInvalid:
> +    case eStateUnloaded:
> +    case eStateExited:
> +    case eStateDetached:
> +        // instance->Clear(false);
> +        break;
> +
> +    case eStateRunning:
> +    case eStateStopped:
> +        // Keep trying to set our JIT breakpoint each time we stop until
> we
> +        // succeed
> +        if (!instance->DidSetJITBreakpoint() && process->IsAlive())
> +            instance->SetJITBreakpoint();
> +        break;
> +
> +    case eStateStepping:
> +    case eStateCrashed:
> +    case eStateSuspended:
> +        break;
> +    }
> +}
> +
>
>
>
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