linux-kernel - Re: [PATCH] kernel: debug: Handle breakpoints in kernel .init.text section

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Date:   Wed, 24 Feb 2021 10:59:20 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     Doug Anderson <dianders@...omium.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Jason Wessel <jason.wessel@...driver.com>,
        Peter Zijlstra <peterz@...radead.org>, stefan.saecherl@....de,
        qy15sije@....cs.fau.de, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kernel: debug: Handle breakpoints in kernel .init.text section

On Tue, 23 Feb 2021 at 18:24, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Tue, Feb 23, 2021 at 02:33:50PM +0530, Sumit Garg wrote:
> > Thanks Doug for your comments.
> >
> > On Tue, 23 Feb 2021 at 05:28, Doug Anderson <dianders@...omium.org> wrote:
> > > > To be clear there is still a very small window between call to
> > > > free_initmem() and system_state = SYSTEM_RUNNING which can lead to
> > > > removal of freed .init.text section breakpoints but I think we can live
> > > > with that.
> > >
> > > I know kdb / kgdb tries to keep out of the way of the rest of the
> > > system and so there's a bias to just try to infer the state of the
> > > rest of the system, but this feels like a halfway solution when really
> > > a cleaner solution really wouldn't intrude much on the main kernel.
> > > It seems like it's at least worth asking if we can just add a call
> > > like kgdb_drop_init_breakpoints() into main.c.  Then we don't have to
> > > try to guess the state...
>
> Just for the record, +1. This would be a better approach.
>
>
> > Sounds reasonable, will post RFC for this. I think we should call such
> > function as kgdb_free_init_mem() in similar way as:
> > - kprobe_free_init_mem()
> > - ftrace_free_init_mem()
>
> As is matching the names...
>
>
> > @@ -378,8 +382,13 @@ int dbg_deactivate_sw_breakpoints(void)
> >         int i;
> >
> >         for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
> > -               if (kgdb_break[i].state != BP_ACTIVE)
> > +               if (kgdb_break[i].state < BP_ACTIVE_INIT)
> > +                       continue;
> > +               if (system_state >= SYSTEM_RUNNING &&
> > +                   kgdb_break[i].state == BP_ACTIVE_INIT) {
> > +                       kgdb_break[i].state = BP_UNDEFINED;
> >                         continue;
> > +               }
> >                 error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
> >                 if (error) {
> >                         pr_info("BP remove failed: %lx\n",
> >
> > >
> > > > +                       kgdb_break[i].state = BP_ACTIVE;
> > > > +               else
> > > > +                       kgdb_break[i].state = BP_ACTIVE_INIT;
> > >
> > > I don't really see what the "BP_ACTIVE_INIT" state gets you.  Why not
> > > just leave it as "BP_ACTIVE" and put all the logic fully in
> > > dbg_deactivate_sw_breakpoints()?
> >
> > Please see my response above.
> >
> > [which was]
> > > "BP_ACTIVE_INIT" state is added specifically to handle this scenario
> > > as to keep track of breakpoints that actually belong to the .init.text
> > > section. And we should be able to again set breakpoints after free
> > > since below change in this patch would mark them as "BP_UNDEFINED":
>
> This answer does not say whether the BP_ACTIVE_INIT state needs to be
> per-breakpoint state or whether we can infer it from the global state.
>
> Changing the state of breakpoints in .init is a one-shot activity
> whether it is triggered explicitly (e.g. kgdb_free_init_mem) or implicitly
> (run the first time dbg_deactivate_sw_breakpoints is called with the system
> state >= running).
>
> As Doug has suggested it is quite possible to unify all the logic to
> handle .init within a single function by running that function when the
> state changes globally.
>

Ah, I see. Thanks for further clarification. Will get rid of
BP_ACTIVE_INIT state.

-Sumit

>
> Daniel.