Re: [PATCH 1/2] [gdbserver] assert on step if !can_hardware_single_step

[Pedro Alves <palves at redhat dot com>]

On 04/07/2015 04:52 PM, Yao Qi wrote:
> From: Yao Qi <yao.qi@linaro.org>
> 
> GDB sends vCont;s by mistake to GDBserver on arm target which doesn't
> have single step at all.  However, it is hard to find the problem from
> the debugging log.  With this patch applied, the problem is easy to
> identify, like:
> 
> (gdb) PASS: gdb.threads/non-stop-fair-events.exp: signal_thread=2: switch to thread 6 to step it
> step&^M
> (gdb) PASS: gdb.threads/non-stop-fair-events.exp: signal_thread=2: set 6 thread stepping
> thread /home/yao/SourceCode/gnu/gdb/git/gdb/gdbserver/linux-low.c:3686: A problem internal to GDBserver has been detected.^M
> linux_resume_one_lwp_throw: Assertion `step == 0' failed.
> 
> gdb/gdbserver:
> 
> 2015-04-02  Yao Qi  <yao.qi@linaro.org>
> 
> 	* linux-low.c (linux_resume_one_lwp_throw): Assert on step.
> ---
>  gdb/gdbserver/linux-low.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/gdb/gdbserver/linux-low.c b/gdb/gdbserver/linux-low.c
> index e4c5420..bc6ab1ae 100644
> --- a/gdb/gdbserver/linux-low.c
> +++ b/gdb/gdbserver/linux-low.c
> @@ -3682,6 +3682,9 @@ linux_resume_one_lwp_throw (struct lwp_info *lwp,
>    if (the_low_target.prepare_to_resume != NULL)
>      the_low_target.prepare_to_resume (lwp);
>  
> +  if (!can_hardware_single_step ())
> +    gdb_assert (step == 0);


Yeah, I have something like that on my x86 software single-step
branch, on the native side, and also in infrun.c.  See:

 https://github.com/palves/gdb/commits/palves/x86_software_single_step
 https://github.com/palves/gdb/commit/52940835548c124a80bd6f381f1a463eda9bab4c

( I just realized/recalled the top commit fixes the exact same as
your patch #2 :-) )

So I think your patch is a good idea.  :-)  But as you're suggesting
it for inclusion, I have to raise the bar a little.  I think gdbserver
crashing/exiting due to bad input from gdb isn't ideal.  This isn't
gdbserver's fault after all.  I think this should be an error instead.
Or perhaps even better, this could stay as an assert in the backend,
if server.c errors out earlier, even, while parsing the
vCont;s / s packets.

(
One nit, as step is a boolean, I think:

   gdb_assert (!step);

would read more naturally.
)

Thanks,
Pedro Alves