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

[Sumit Garg]

On Wed, 24 Feb 2021 at 23:39, Doug Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> On Wed, Feb 24, 2021 at 12:17 AM Sumit Garg <sumit.garg@xxxxxxxxxx> 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.
>
> It might be worth it to mention that HW breakpoints aren't handled by
> this patch but it's probably not such a big deal.
>
>
> > Suggested-by: Doug Anderson <dianders@xxxxxxxxxxxx>
> > Signed-off-by: Sumit Garg <sumit.garg@xxxxxxxxxx>
> > ---
> >  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))
>
> A nit, but instead of 0 should this be passing "BREAK_INSTR_SIZE" ?
>
> Also: even if memory is about to get freed it still seems like it'd be
> wise to call this:
>
>   kgdb_arch_remove_breakpoint(&kgdb_break[i]);
>
> It looks like it shouldn't matter today but just in case an
> architecture decides to do something fancy in the future it might not
> hurt to tell it that the breakpoint is going away.
>
>
> Everything here is pretty nitty, though.  This looks good to me now.
>
> Reviewed-by: Douglas Anderson <dianders@xxxxxxxxxxxx>

Thanks Doug for your review.

-Sumit