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

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

[Sumit Garg]

+ stable ML

On Thu, 25 Feb 2021 at 21:26, Daniel Thompson
<daniel.thompson@linaro.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@chromium.org>
> > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
>
> I saw Andrew has picked this one up. That's ok for me:
> Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
>
> I already enriched kgdbtest to cover this (and they pass) so I guess
> this is also:
> Tested-by: Daniel Thompson <daniel.thompson@linaro.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

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

[Greg KH]

On Fri, Feb 26, 2021 at 12:32:07PM +0530, Sumit Garg wrote:
> + stable ML
> 
> On Thu, 25 Feb 2021 at 21:26, Daniel Thompson
> <daniel.thompson@linaro.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@chromium.org>
> > > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> >
> > I saw Andrew has picked this one up. That's ok for me:
> > Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
> >
> > I already enriched kgdbtest to cover this (and they pass) so I guess
> > this is also:
> > Tested-by: Daniel Thompson <daniel.thompson@linaro.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.


<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>

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

[Sumit Garg]

On Fri, 26 Feb 2021 at 13:01, Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Fri, Feb 26, 2021 at 12:32:07PM +0530, Sumit Garg wrote:
> > + stable ML
> >
> > On Thu, 25 Feb 2021 at 21:26, Daniel Thompson
> > <daniel.thompson@linaro.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@chromium.org>
> > > > Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
> > >
> > > I saw Andrew has picked this one up. That's ok for me:
> > > Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
> > >
> > > I already enriched kgdbtest to cover this (and they pass) so I guess
> > > this is also:
> > > Tested-by: Daniel Thompson <daniel.thompson@linaro.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.
>
>
> <formletter>
>
> This is not the correct way to submit patches for inclusion in the
> stable kernel tree.  Please read:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly.
>
> </formletter>

Thanks for the pointer, let me wait for this patch to land in Linus’
tree and then will drop a mail to stable@vger.kernel.org.

-Sumit