[lldb-dev] gdb-remote command broken in TOT

Todd Fiala tfiala at google.com
Tue May 13 07:56:21 PDT 2014

Hey Matthew,

> However is there any real need to restrict the fallback case to just

Keep in mind that qC in gdbserver does implement the protocol correctly and
returns the thread id, not the proc id. On Linux the first thread happens
to get a thread id that is identical to the proc id.  So when launching an
exe, the active thread id returned just happens to be the proc id as well
out of happy coincidence. This is not true on other OSes and not
guaranteed true when attaching to a Linux process if the active thread is
not the first thread.  Given that lldb was not caching the proc id until my
change last week, it means that lldb using qC against a gdbserver (or gdb
remote protocol implementing qC per spec) is definitely getting the buggy
behavior I was fixing (i.e. different proc ids throughout the life of the
process when running against a multi-threaded inferior, and entirely
invalid results for proc id when run on OSes where the thread ids have no
correlation to the proc id, as on MacOSX).

Thus, reverting to using qC when qProcessInfo isn't implemented is really
just reverting to buggy and inconsistent behavior, although it may allow
lldb to run where it can't now (and could before) and the proc id reported
just happens to be right some of the time.

I think the real fix here may be if $qProcessInfo isn't present, then we
probably need to settle for not assuming we can get the process id, and
just make sure lldb can handle running without it in that case.

Greg, any thoughts on that?


On Tuesday, May 13, 2014, Matthew Gardiner <mg11 at csr.com> wrote:e thread if

> Todd Fiala wrote:
>> This is a change I made.
>> qC is supposed to return the thread id, not the process id.
>> qProcessInfo is being used now to collect the process id.  We added a
>> fallback so that if the process info request fails, on Apple/iOS we fall
>> back to the older qC (which, as indicated before, used to return the
>> process id erroneously, see the gdb remote documentation).
> After doing a bit more analysis, I see that you added a fallback such that
> if the process info request fails, we fall back to traditional gdbserver
> qC. This is completely reasonable, and permits our custom stubs to support
> qProcessInfo.
> However is there any real need to restrict the fallback case to just
> Apple/iOS? that is:
>     const llvm::Triple &triple = GetProcessArchitecture().GetTriple();
>     if ((triple.getVendor() == llvm::Triple::Apple) &&
>         (triple.getOS() == llvm::Triple::IOS))
>     {
> Surely, all stubs which fail to implement qProcessInfo should be permitted
> to fallback to qC?
> I've just commented out the above "if" in my working copy, and now the
> initial "gdb-remote host:port" works again - in so far as the current
> inferior process number is reported rather than 0. (Admittedly other things
> don't quite work as desired later on in the debug-session, e.g. "next". But
> presumably an Apple/iOS stub would either fail similarly, or work as
> expected due to some localised conditional treatment, (i.e. "if apple
> then...")).
> So, to continue to interoperate with traditional gdbserver implementations
> can we let all older stubs continue to fallback to qC?
> thanks
> Matt
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