Re: 'g' packet reply is too long error when target changes number of registers

On 02/03/2014 02:11 PM, [email protected] wrote:
> Hello,
> 
> I'm trying to debug an ARM Aarch64 target with gdb-cross and I get a
> 'g' packet reply is too long error in the following scenario:
> 
> * I debug my ARM target through a probe that has a gdbserver running on it
> (gdbproxy).
> * First I load a custom target description in gdb that reflects the current 
> hardware I am debugging
> * I connect to the probe with "target remote probe_ip"
> * Then I configure the probe to connect to the target (using the same target
> description as the one loaded in GDB)
> * When I ask for the register Info (info reg), I get the 'g' packet reply 
> error.
> 

> Because the probe had no knowledge of the target that will be debugged 
> beforehand, the "target remote" command will force the probe to reply with 
> info about a smaller number of registers (according to the default description)
> than gdb expects.

This sounds odd.  Why not?  Simply configure it before connecting
with GDB?  It sounds quite wrong to be changing the target behind
GDB's back when GDB is _already_ debugging it.  Not just the size
of the g/G packets may change inadvertently, but the layout as well.
If the target description changes with your re-configuration, it
sounds to me like GDB should fetch/recompute the whole target
description.  Today, I think that can only be done with a
disconnect/reconnect.

> This causes rsa->sizeof_g_packet to be reduced.
> 
> After configuration, the probe will send all the register information of the 
> target. This causes the 'g'packet reply error because now, more registers 
> are being sent.

> 
> Why is rsa->sizeof_g_packet shrunk when a smaller packet is received.

  /* If this is smaller than we guessed the 'g' packet would be,
     update our records.  A 'g' reply that doesn't include a register's
     value implies either that the register is not available, or that
     the 'p' packet must be used.  */
  if (buf_len < 2 * rsa->sizeof_g_packet)
    {

If we don't shrink it, then we'd send too much when writing
registers, for example, I think?

-- 
Pedro Alves