RE: [PATCH 2/2] gdbserver: track current process as well as current thread

["Aktemur, Tankut Baris" <tankut.baris.aktemur@intel.com>]

> > > The recent commit 421490af33bf ("gdbserver/linux: Access memory even
> > > if threads are running") caused a regression in
> > > gdb.threads/access-mem-running-thread-exit.exp with gdbserver, which I
> > > somehow missed.  Like so:
> > >
> > >  (gdb) print global_var
> > >  Cannot access memory at address 0x555555558010
> > >  (gdb) FAIL: gdb.threads/access-mem-running-thread-exit.exp: non-stop: access mem (print
> > global_var after writing, inf=2, iter=1)
> > >
> > > The problem starts with GDB telling GDBserver to select a thread, via
> > > the Hg packet, which GDBserver complies with, then that thread exits,
> > > and GDB, without knowing the thread is gone, tries to write to memory,
> > > through the context of the previously selected Hg thread.
> > >
> > > GDBserver's GDB-facing memory access routines, gdb_read_memory and
> > > gdb_write_memory, call set_desired_thread to make GDBserver re-select
> > > the thread that GDB has selected with the Hg packet.  Since the thread
> > > is gone, set_desired_thread returns false, and the memory access
> > > fails.
> > >
> > > Now, to access memory, it doesn't really matter which thread is
> > > selected.  All we should need is the target process.  Even if the
> > > thread that GDB previously selected is gone, and GDB does not yet know
> > > about that exit, it shouldn't matter, GDBserver should still know
> > > which process that thread belonged to.
> > >
> > > Fix this by making GDBserver track the current process separately,
> > > like GDB also does.  Add a new set_desired_process routine that is
> > > similar to set_desired_thread, but just sets the current process,
> > > leaving the current thread as NULL.  Use it in the GDB-facing memory
> > > read and write routines, to avoid failing if the selected thread is
> > > gone, but the process is still around.
> > >
> > > Change-Id: I4ff97cb6f42558efbed224b30d5c71f6112d44cd
> > > ---
> > >  gdbserver/gdbthread.h  |  1 +
> > >  gdbserver/inferiors.cc | 26 +++++++++++++++++++------
> > >  gdbserver/server.cc    |  4 ++--
> > >  gdbserver/target.cc    | 44 ++++++++++++++++++++++++++++++++++++++++--
> > >  gdbserver/target.h     | 15 +++++++++++++-
> > >  5 files changed, 79 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/gdbserver/gdbthread.h b/gdbserver/gdbthread.h
> > > index 10ae1cb7c87..8b897e73d33 100644
> > > --- a/gdbserver/gdbthread.h
> > > +++ b/gdbserver/gdbthread.h
> > > @@ -252,6 +252,7 @@ class scoped_restore_current_thread
> > >
> > >  private:
> > >    bool m_dont_restore = false;
> > > +  process_info *m_process;
> > >    thread_info *m_thread;
> > >  };
> > >
> > > diff --git a/gdbserver/inferiors.cc b/gdbserver/inferiors.cc
> > > index 678d93c77a1..3d0a8b0e716 100644
> > > --- a/gdbserver/inferiors.cc
> > > +++ b/gdbserver/inferiors.cc
> > > @@ -26,6 +26,11 @@
> > >  std::list<process_info *> all_processes;
> > >  std::list<thread_info *> all_threads;
> > >
> > > +/* The current process.  */
> > > +static process_info *current_process_;
> > > +
> > > +/* The current thread.  This is either a thread of CURRENT_PROCESS, or
> > > +   NULL.  */
> > >  struct thread_info *current_thread;
> > >
> > >  /* The current working directory used to start the inferior.
> > > @@ -130,6 +135,7 @@ clear_inferiors (void)
> > >    clear_dlls ();
> > >
> > >    switch_to_thread (nullptr);
> > > +  current_process_ = nullptr;
> > >  }
> > >
> > >  struct process_info *
> > > @@ -153,6 +159,8 @@ remove_process (struct process_info *process)
> > >    free_all_breakpoints (process);
> > >    gdb_assert (find_thread_process (process) == NULL);
> > >    all_processes.remove (process);
> > > +  if (current_process () == process)
> > > +    switch_to_process (nullptr);
> > >    delete process;
> > >  }
> > >
> > > @@ -205,8 +213,7 @@ get_thread_process (const struct thread_info *thread)
> > >  struct process_info *
> > >  current_process (void)
> > >  {
> > > -  gdb_assert (current_thread != NULL);
> > > -  return get_thread_process (current_thread);
> > > +  return current_process_;
> >
> > A bit late I know, but I wonder if the assert, or something similar,
> > should have been retained here?
> >
> > The comment on this function currently (though Baris has a patch
> > proposed to change this), says this function should only be called in a
> > context where the result will not be nullptr.  Given that, not of the
> > (many) existing uses check the return value of this function against
> > nullptr.
> >
> > Happy to roll a patch to add the assert back, just wondered if there was
> > a reason for its removal.
> >
> > Thanks,
> > Andrew
> 
> Hi Andrew,
> 
> I think the current process can in fact be null in some brief periods.
> For instance, in 'handle_detach' there is
> 
>       if (extended_protocol || target_running ())
>         {
>           /* There is still at least one inferior remaining or
>              we are in extended mode, so don't terminate gdbserver,
>              and instead treat this like a normal program exit.  */
>           cs.last_status.set_exited (0);
>           cs.last_ptid = ptid_t (pid);
> 
>           switch_to_thread (nullptr);
>         }
> 
> So, if the current process exits, gdbserver reports the event to GDB and
> sets the current thread to nullptr, which also sets the current process
> to nullptr.
> 
> I believe an invariant is this:
> 
>   (current_thread == nullptr) || (current_process () == get_thread_process (current_thread))

Kindly pinging for/in the context of
https://sourceware.org/pipermail/gdb-patches/2023-April/199116.html

-Baris



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