Re: [PATCH v3 2/3] kgdb: Add NOKPROBE labels on the trap handler functions

[Doug Anderson]

Hi,

On Sun, Sep 27, 2020 at 2:16 PM Daniel Thompson
<daniel.thompson@xxxxxxxxxx> wrote:
>
> Currently kgdb honours the kprobe blocklist but doesn't place its own
> trap handling code on the list. Add labels to discourage attempting to
> use kgdb to debug itself.
>
> Not every functions that executes from the trap handler needs to be
> marked up: relatively early in the trap handler execution (just after
> we bring the other CPUs to a halt) all breakpoints are replaced with
> the original opcodes. This patch marks up code in the debug_core that
> executes between trap entry and the breakpoints being deactivated
> and, also, code that executes between breakpoint activation and trap
> exit.
>
> To be clear these changes are not sufficient to make recursive trapping
> impossible since cover all the library calls made during kgdb's
> entry/exit logic. However going much further whilst we are sharing the
> kprobe blocklist risks reducing the capabilities of kprobe and this
> would be a bad trade off (especially so given kgdb's users are currently
> conditioned to avoid recursive traps).
>
> Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> ---
>  kernel/debug/debug_core.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)

I didn't go on any more hunts for missing functions since this seems
fine to me.  It's mostly just trying to make it a little harder for
someone to shoot themselves in the foot, after all.  ;-)

Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

-Doug