linux-kernel - Re: [PATCH] kgdb: Fix to kill breakpoints on initmem after boot

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Date:   Fri, 26 Feb 2021 12:32:07 +0530
From:   Sumit Garg <sumit.garg@...aro.org>
To:     Daniel Thompson <daniel.thompson@...aro.org>
Cc:     kgdb-bugreport@...ts.sourceforge.net,
        Andrew Morton <akpm@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Douglas Anderson <dianders@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>, stefan.saecherl@....de,
        qy15sije@....cs.fau.de,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] kgdb: Fix to kill breakpoints on initmem after boot

+ stable ML

On Thu, 25 Feb 2021 at 21:26, Daniel Thompson
<daniel.thompson@...aro.org> wrote:
>
> On Wed, Feb 24, 2021 at 01:46:52PM +0530, Sumit Garg wrote:
> > Currently breakpoints in kernel .init.text section are not handled
> > correctly while allowing to remove them even after corresponding pages
> > have been freed.
> >
> > Fix it via killing .init.text section breakpoints just prior to initmem
> > pages being freed.
> >
> > Suggested-by: Doug Anderson <dianders@...omium.org>
> > Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
>
> I saw Andrew has picked this one up. That's ok for me:
> Acked-by: Daniel Thompson <daniel.thompson@...aro.org>
>
> I already enriched kgdbtest to cover this (and they pass) so I guess
> this is also:
> Tested-by: Daniel Thompson <daniel.thompson@...aro.org>
>

Thanks Daniel.

> BTW this is not Cc:ed to stable and I do wonder if it crosses the
> threshold to be considered a fix rather than a feature. Normally I
> consider adding safety rails for kgdb to be a new feature but, in this
> case, the problem would easily ensnare an inexperienced developer who is
> doing nothing more than debugging their own driver (assuming they
> correctly marked their probe function as .init) so I think this weighs
> in favour of being a fix.
>

Makes sense, Cc:ed stable.

-Sumit

>
> Daniel.
>
>
> > ---
> >  include/linux/kgdb.h      |  2 ++
> >  init/main.c               |  1 +
> >  kernel/debug/debug_core.c | 11 +++++++++++
> >  3 files changed, 14 insertions(+)
> >
> > diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> > index 57b8885708e5..3aa503ef06fc 100644
> > --- a/include/linux/kgdb.h
> > +++ b/include/linux/kgdb.h
> > @@ -361,9 +361,11 @@ extern atomic_t                  kgdb_active;
> >  extern bool dbg_is_early;
> >  extern void __init dbg_late_init(void);
> >  extern void kgdb_panic(const char *msg);
> > +extern void kgdb_free_init_mem(void);
> >  #else /* ! CONFIG_KGDB */
> >  #define in_dbg_master() (0)
> >  #define dbg_late_init()
> >  static inline void kgdb_panic(const char *msg) {}
> > +static inline void kgdb_free_init_mem(void) { }
> >  #endif /* ! CONFIG_KGDB */
> >  #endif /* _KGDB_H_ */
> > diff --git a/init/main.c b/init/main.c
> > index c68d784376ca..a446ca3d334e 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -1417,6 +1417,7 @@ static int __ref kernel_init(void *unused)
> >       async_synchronize_full();
> >       kprobe_free_init_mem();
> >       ftrace_free_init_mem();
> > +     kgdb_free_init_mem();
> >       free_initmem();
> >       mark_readonly();
> >
> > diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> > index 229dd119f430..319381e95d1d 100644
> > --- a/kernel/debug/debug_core.c
> > +++ b/kernel/debug/debug_core.c
> > @@ -465,6 +465,17 @@ int dbg_remove_all_break(void)
> >       return 0;
> >  }
> >
> > +void kgdb_free_init_mem(void)
> > +{
> > +     int i;
> > +
> > +     /* Clear init memory breakpoints. */
> > +     for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
> > +             if (init_section_contains((void *)kgdb_break[i].bpt_addr, 0))
> > +                     kgdb_break[i].state = BP_UNDEFINED;
> > +     }
> > +}
> > +
> >  #ifdef CONFIG_KGDB_KDB
> >  void kdb_dump_stack_on_cpu(int cpu)
> >  {
> > --
> > 2.25.1