[Bug server/15682] New: z/Z breakpoint reinsertions, dprintf-style gdb->agent

[Bug server/15682] New: z/Z breakpoint reinsertions, dprintf-style gdb->agent

[palves at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15682

            Bug ID: 15682
           Summary: z/Z breakpoint reinsertions, dprintf-style gdb->agent
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: server
          Assignee: unassigned at sourceware dot org
          Reporter: palves at redhat dot com

I noticed this bit in gdbserver's server.c, where we handle z/Z packets:

        if (insert && the_target->insert_point != NULL)
          {
        /* Insert the breakpoint.  If it is already inserted, nothing
           will take place.  */
        res = (*the_target->insert_point) (type, addr, len);

        /* GDB may have sent us a list of *point parameters to be
           evaluated on the target's side.  Read such list here.  If we
           already have a list of parameters, GDB is telling us to drop
           that list and use this one instead.  */
        if (!res && (type == '0' || type == '1'))
          {
            /* Remove previous conditions.  */
            clear_gdb_breakpoint_conditions (addr);
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            process_point_options (addr, &dataptr);
          }
          }


That only clears previous conditions, but leaves other *point options behind.
That means that it leaves agent commands for dprintfs, if GDB flips
between dprintf-style gdb to agent (in always-inserted mode).  That should
clear all print options, not just conditions.

I noticed that while reviewing Hui's "set dprintf-style agent" patch at
<http://sourceware.org/ml/gdb-patches/2013-04/msg00592.html>, and playing
with breakpoint always-inserted mode.  But I only noticed it by inspection.
Trying to trigger the bug with:

set breakpoint always-inserted on
set displaced-stepping on (to minimize insert/removes)
dprintf 50, "hello"
b 51  (in a loop)

and then

(gdb) set dprintf-style agent
(gdb) c
*prints "hello" on the gdbserver console*
*stops at line 51*
(gdb) set dprintf-style gdb
(gdb) c
*prints "hello" on gdb's console ONLY*
*stops at line 51*

Looking deeper, what happens is that gdbserver always deletes the previous
breakpoint (the_target->insert_point ultimately will end up here):

int
set_gdb_breakpoint_at (CORE_ADDR where)
{
  struct breakpoint *bp;

  if (breakpoint_data == NULL)
    return 1;

  /* If we see GDB inserting a second breakpoint at the same address,
     then the first breakpoint must have disappeared due to a shared
     library unload.  On targets where the shared libraries are
     handled by userspace, like SVR4, for example, GDBserver can't
     tell if a library was loaded or unloaded.  Since we refcount
     breakpoints, if we didn't do this, we'd just increase the
     refcount of the previous breakpoint at this address, but the trap
     was not planted in the inferior anymore, thus the breakpoint
     would never be hit.  */
  bp = find_gdb_breakpoint_at (where);
  if (bp != NULL)
    {
      delete_gdb_breakpoint_at (where);

      /* Might as well validate all other breakpoints.  */
      validate_breakpoints ();
    }


Now, if GDBserver does this, then the first reaction would be to just delete
the clear_gdb_breakpoint_conditions call.  But, given non-stop, I think it's
wrong to be deleting the breakpoint like that even temporarily.  Thread may
miss it.  We either need to pause the whole inferior temporarily, or, I think
might be better, just rely on validate_breakpoints to detect the shared library
unload case.

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[Bug server/15682] z/Z breakpoint reinsertions, dprintf-style gdb->agent

[luis_gustavo at mentor dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15682

Luis Machado <luis_gustavo at mentor dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |luis_gustavo at mentor dot com

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[Bug server/15682] z/Z breakpoint reinsertions, dprintf-style gdb->agent

[luis_gustavo at mentor dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=15682

--- Comment #1 from Luis Machado <luis_gustavo at mentor dot com> ---
I'll investigate this one.

Thanks for reporting.

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