Re: [RFC] Observer mode

[Michael Snyder <msnyder at vmware dot com>]

Stan Shebs wrote:
> Here is another patch that pushes the envelope a bit, and for which I 
> would like to get some prior agreement that it's a good idea.
>
> The basic idea here is that GDB is to be used to look at targets 
> installed in the field and in use.  In such a situation it may connect 
> to the target and examine state, but not alter that state or interrupt 
> anything.  In our particular target, tracing is allowed, but only using 
> fast tracepoints, because they are done using jumps and thus act as if 
> they were part of the program; taking a trap, even for a slow 
> tracepoint, disrupts thread execution unacceptably.  We have been 
> calling this "observer mode", and the customer's GDB is actually 
> configured to launch in this mode, to reduce the chance of accidental 
> stopping - the debugger user has to ask specifically to change it to 
> normal debugging mode.
>
> The patch for this is conceptually straightforward, merely introducing a 
> set of user-settable variables, such as may-write-memory, which gate all 
> user attempts to write memory, being tested at the target vector level.  
> Later patches in the series add a "set observer" variable which 
> set/clears all of the individual permissions together, and also ensure 
> that non-stop mode is on.
>
> Although enabling/disabling at the target vector is somewhat brute 
> force, it seems to work reasonably well in practice; GDB doesn't get 
> knotted into weird states, and the error messages from failed 
> modification attempts (direct or indirect) are reasonable.
>
> If people like this idea, I'll update the old patch and resubmit.

How different is this from async mode?

> 2009-05-19  Stan Shebs  <stan@codesourcery.com>
>  
>     * target.h (target_insert_breakpoint): Change macro to function.
>     (target_remove_breakpoint): Ditto.
>     (target_stop): Ditto.
>     * target.c (may_write_registers): New global.
>     (may_write_memory): New global.
>     (may_insert_breakpoints): New global.
>     (may_insert_tracepoints): New global.
>     (may_insert_fast_tracepoints): New global.
>     (may_stop): New global.
>     (target_xfer_partial): Test for write permission.
>     (target_store_registers): Ditto.
>     (target_insert_breakpoint): New function.
>     (target_remove_breakpoint): New function.
>     (target_stop): New function.
>     (_initialize_targets): Add new set/show variables.
>     * tracepoint.c (download_target): Check permission,
>     return a success/fail result.
>     (trace_start_command): Don't start tracing if no
>     tracepoints were downloaded.
>
>     gdb/doc/
>     * gdb.texinfo (Observation Mode): New section.