Re: [PATCH] [PR gdb/13463] linux-nat: Stop lwp before xfer if running

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 27 August 2013 15:33, Pedro Alves <palves@redhat.com> wrote:
> On 08/08/2013 08:59 PM, simon.marchi@polymtl.ca wrote:
>> (err, resending with setting From: correctly, sorry about that)
>>
>> Hi,
>>
>> This is an attempt at finding a solution for PR 13463:
>> http://sourceware.org/bugzilla/show_bug.cgi?id=13463
>>
>> IMO, this is a serious bug (at least on x86 Linux). Inserting breakpoints
>> while the inferior is running does not work. It can be triggered very easily:
>>
>>     1. Start a simple program in non-stop/async mode.
>>     2. Put a breakpoint anywhere in the program.
>>
>> You should get an error like
>>
>>     Cannot access memory at address 0x400504
>>
>> What happens is that GDB tries to ptrace read/write the inferior's memory
>> while the process is not stopped, which is not supported.
>>
>> The obvious/naive solution is to stop the process and resume it whenever
>> we want to do a memory transfer while it is executing.
>
> Yeah, gdbserver does this.  Starts with prepare_to_access_memory.  A
> prepare -> do -> "unprepare" sequence is possibly more efficient,
> as we can batch several "do"s over a single pause sequence.  I've considered
> before pushing that prepare/unprepare sequence all the way to gdb core,
> in order to batch e.g., the whole of prologue analysis + breakpoint
> insertion, or of shared library list reading, etc., but I thought all
> the way through all that.

I saw the gdbserver version, it seems to be handled quite cleanly.
Should I add an equivalent to prepare_to_access_memory to gdb? This
way other platforms could do apply the same fix if the bug applies to
them. Either way, this gives me a sense of the code duplication
between gdb and gdbserver. Scary!

>> I gave it a shot,
>> and it seems to work for me, but there is probably some cases it does not
>> cover, maybe other things it breaks or some better way to do it. I ran a
>> make check and gdb.sum was identical before and after (minus the time).
>
> Points I believe should be handled:
>
> - it should not resume threads that were meant to be stopped,
>   or were already stopped.  target_resume blindly does that.

I believe my fix handles already stopped threads correctly. If the
thread is meant to be stopped, should I simply stop it and not resume
it after, will it be in a valid state? What is the right way to check
if a thread is meant to be stopped?

> - it's quite possible the thread you just tried to stop
>   disappears/exit just as you tried to stop it, and then the
>   xfer fails.
>
> On the latter issue, and considering that there's no real
> guarantee some other thread could be simultaneously poking
> the same address gdbserver is (there's no ptrace atomic write),
> gdbserver takes the easy route and always pauses all threads.
> (the opposite end of the scale would be if gdb/gdbserver
> force-cloned a thread in the inferior to use it as proxy to
> peek/poke through.)

I wouldn't consider it a critical bug if the read/write failed because
the thread stopped right before we peek/poke it, as it does not
"break" a feature. I suppose it could be addressed in a different
patch.

Thanks,

Simon

>
>>
>> What do you think about it?
>>
>>       * linux-nat.c (linux_nat_xfer_partial): Interrupt during the
>>       memory transfer, if it is running.
>>
>> ---
>>  gdb/linux-nat.c |   18 ++++++++++++++++--
>>  1 files changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
>> index 45a6e5f..b8c0d1c 100644
>> --- a/gdb/linux-nat.c
>> +++ b/gdb/linux-nat.c
>> @@ -294,8 +294,9 @@ static void linux_nat_async (void (*callback)
>>                             void *context),
>>                            void *context);
>>  static int kill_lwp (int lwpid, int signo);
>> -
>> +static int linux_nat_stop_lwp (struct lwp_info *lwp, void *data);
>>  static int stop_callback (struct lwp_info *lp, void *data);
>> +static void cleanup_target_stop(void *arg);
>>
>>  static void block_child_signals (sigset_t *prev_mask);
>>  static void restore_child_signals_mask (sigset_t *prev_mask);
>> @@ -4205,11 +4206,24 @@ linux_nat_xfer_partial (struct target_ops *ops, enum target_object object,
>>    old_chain = save_inferior_ptid ();
>>
>>    if (is_lwp (inferior_ptid))
>> -    inferior_ptid = pid_to_ptid (GET_LWP (inferior_ptid));
>> +    {
>> +      struct lwp_info* lwp;
>> +      inferior_ptid = pid_to_ptid (GET_LWP (inferior_ptid));
>> +
>> +      lwp = find_lwp_pid (inferior_ptid);
>> +      if (target_async_permitted && lwp != NULL && !lwp->stopped)
>> +     {
>> +       make_cleanup (cleanup_target_stop, &lwp->ptid);
>> +       linux_nat_stop_lwp (lwp, NULL);
>> +       stop_wait_callback (lwp, NULL);
>> +     }
>> +    }
>> +
>>
>>    xfer = linux_ops->to_xfer_partial (ops, object, annex, readbuf, writebuf,
>>                                    offset, len);
>>    do_cleanups (old_chain);
>>    return xfer;
>>  }
>>
>
> --
> Pedro Alves