[1/1] gdb, breakpoint: add a destructor to the watchpoint struct

  Make sure to unlink the related breakpoint when the watchpoint instance
is deleted.  This prevents having a wp-related breakpoint that is
linked to a NULL watchpoint (e.g.  the watchpoint instance is being
deleted when the 'watch' command fails).  With the below scenario,
having such a left out breakpoint will lead to a GDB hang, and this
is due to an infinite loop when deleting all inferior breakpoints.

Scenario:
	(gdb) awatch <VAR>
	Target does not support this type of hardware watchpoint.
	(gdb) rwatch <VAR>
	Target does not support this type of hardware watchpoint.
	(gdb) <continue the program until the end>
	>> HANG <<

Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
---
 gdb/breakpoint.c | 15 +++++++++++++++
 gdb/breakpoint.h |  3 +++
 2 files changed, 18 insertions(+)
  

Mohamed Bouhaouel via Gdb-patches <gdb-patches@sourceware.org> writes:

> Make sure to unlink the related breakpoint when the watchpoint instance
> is deleted.  This prevents having a wp-related breakpoint that is
> linked to a NULL watchpoint (e.g.  the watchpoint instance is being
> deleted when the 'watch' command fails).  With the below scenario,
> having such a left out breakpoint will lead to a GDB hang, and this
> is due to an infinite loop when deleting all inferior breakpoints.
>
> Scenario:
> 	(gdb) awatch <VAR>
> 	Target does not support this type of hardware watchpoint.
> 	(gdb) rwatch <VAR>
> 	Target does not support this type of hardware watchpoint.
> 	(gdb) <continue the program until the end>
> 	>> HANG <<
>

Is it not possible to create a test case based on this scenario?

Thanks,
Andrew

> Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
> ---
>  gdb/breakpoint.c | 15 +++++++++++++++
>  gdb/breakpoint.h |  3 +++
>  2 files changed, 18 insertions(+)
>
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index 7228acfd8fe..b9ddc85c30c 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -9581,6 +9581,21 @@ break_range_command (const char *arg, int from_tty)
>    install_breakpoint (false, std::move (br), true);
>  }
>  
> +/* See breakpoint.h.  */
> +
> +watchpoint::~watchpoint ()
> +{
> +  /* Make sure to unlink the destroyed watchpoint from the related
> +     breakpoint ring.  */
> +
> +  breakpoint *bpt = this;
> +  breakpoint *related = bpt;
> +  while (related->related_breakpoint != bpt)
> +    related = related->related_breakpoint;
> +
> +  related->related_breakpoint = bpt->related_breakpoint;
> +}
> +
>  /*  Return non-zero if EXP is verified as constant.  Returned zero
>      means EXP is variable.  Also the constant detection may fail for
>      some constant expressions and in such case still falsely return
> diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
> index 7c5cf3f2bef..4f19e42dd83 100644
> --- a/gdb/breakpoint.h
> +++ b/gdb/breakpoint.h
> @@ -933,6 +933,9 @@ struct watchpoint : public breakpoint
>    void print_recreate (struct ui_file *fp) const override;
>    bool explains_signal (enum gdb_signal) override;
>  
> +  /* Destructor for WATCHPOINT.  */
> +  virtual ~watchpoint ();
> +
>    /* String form of exp to use for displaying to the user (malloc'd),
>       or NULL if none.  */
>    gdb::unique_xmalloc_ptr<char> exp_string;
> -- 
> 2.25.1
>
> Intel Deutschland GmbH
> Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
> Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
> Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva  
> Chairperson of the Supervisory Board: Nicole Lau
> Registered Office: Munich
> Commercial Register: Amtsgericht Muenchen HRB 186928
  

Thanks Andrew for your feedback.

> > Scenario:
> > 	(gdb) awatch <VAR>
> > 	Target does not support this type of hardware watchpoint.
> > 	(gdb) rwatch <VAR>
> > 	Target does not support this type of hardware watchpoint.
> > 	(gdb) <continue the program until the end>
> > 	>> HANG <<
> >
> 
> Is it not possible to create a test case based on this scenario?

It is difficult to produce such scenario due to a weird behavior [1],
which needs to be addressed as well.  As a result, deleting watchpoints
before resuming would hide the issue.


[1] When the watch command fails, it is not possible to resume
unless we explicitly delete the watchpoint.  More details can be
found here:
https://sourceware.org/pipermail/gdb-patches/2023-March/198371.html)

Kind regards,
Mohamed



> -----Original Message-----
> From: Andrew Burgess <aburgess@redhat.com>
> Sent: Tuesday, April 4, 2023 1:38 PM
> To: Bouhaouel, Mohamed <mohamed.bouhaouel@intel.com>; gdb-
> patches@sourceware.org
> Subject: Re: [PATCH 1/1] gdb, breakpoint: add a destructor to the watchpoint
> struct
> 
> Mohamed Bouhaouel via Gdb-patches <gdb-patches@sourceware.org>
> writes:
> 
> > Make sure to unlink the related breakpoint when the watchpoint instance
> > is deleted.  This prevents having a wp-related breakpoint that is
> > linked to a NULL watchpoint (e.g.  the watchpoint instance is being
> > deleted when the 'watch' command fails).  With the below scenario,
> > having such a left out breakpoint will lead to a GDB hang, and this
> > is due to an infinite loop when deleting all inferior breakpoints.
> >
> > Scenario:
> > 	(gdb) awatch <VAR>
> > 	Target does not support this type of hardware watchpoint.
> > 	(gdb) rwatch <VAR>
> > 	Target does not support this type of hardware watchpoint.
> > 	(gdb) <continue the program until the end>
> > 	>> HANG <<
> >
> 
> Is it not possible to create a test case based on this scenario?
> 
> Thanks,
> Andrew
> 
> > Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
> > ---
> >  gdb/breakpoint.c | 15 +++++++++++++++
> >  gdb/breakpoint.h |  3 +++
> >  2 files changed, 18 insertions(+)
> >
> > diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> > index 7228acfd8fe..b9ddc85c30c 100644
> > --- a/gdb/breakpoint.c
> > +++ b/gdb/breakpoint.c
> > @@ -9581,6 +9581,21 @@ break_range_command (const char *arg, int
> from_tty)
> >    install_breakpoint (false, std::move (br), true);
> >  }
> >
> > +/* See breakpoint.h.  */
> > +
> > +watchpoint::~watchpoint ()
> > +{
> > +  /* Make sure to unlink the destroyed watchpoint from the related
> > +     breakpoint ring.  */
> > +
> > +  breakpoint *bpt = this;
> > +  breakpoint *related = bpt;
> > +  while (related->related_breakpoint != bpt)
> > +    related = related->related_breakpoint;
> > +
> > +  related->related_breakpoint = bpt->related_breakpoint;
> > +}
> > +
> >  /*  Return non-zero if EXP is verified as constant.  Returned zero
> >      means EXP is variable.  Also the constant detection may fail for
> >      some constant expressions and in such case still falsely return
> > diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
> > index 7c5cf3f2bef..4f19e42dd83 100644
> > --- a/gdb/breakpoint.h
> > +++ b/gdb/breakpoint.h
> > @@ -933,6 +933,9 @@ struct watchpoint : public breakpoint
> >    void print_recreate (struct ui_file *fp) const override;
> >    bool explains_signal (enum gdb_signal) override;
> >
> > +  /* Destructor for WATCHPOINT.  */
> > +  virtual ~watchpoint ();
> > +
> >    /* String form of exp to use for displaying to the user (malloc'd),
> >       or NULL if none.  */
> >    gdb::unique_xmalloc_ptr<char> exp_string;
> > --
> > 2.25.1
> >
> > Intel Deutschland GmbH
> > Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
> > Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
> > Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva
> > Chairperson of the Supervisory Board: Nicole Lau
> > Registered Office: Munich
> > Commercial Register: Amtsgericht Muenchen HRB 186928

Intel Deutschland GmbH
Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva  
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928
  

> > > Scenario:
> > > 	(gdb) awatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) rwatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) <continue the program until the end>
> > > 	>> HANG <<
> > >
> >
> > Is it not possible to create a test case based on this scenario?
> 
> It is difficult to produce such scenario due to a weird behavior [1],
> which needs to be addressed as well.  As a result, deleting watchpoints
> before resuming would hide the issue.


I have been able to come up with a similar scenario that exposes the hang.
- set can-use-hw-watchpoints 0
- stop inside a scope with local variables.
- (gdb) awatch <LOCAL VAR>
  Can't set read/access watchpoint when hardware watchpoints are disabled.
- (gdb) rwatch <LOCAL VAR>
  Can't set read/access watchpoint when hardware watchpoints are disabled.
- (gdb) <continue the program until the end>
  >> HANG <<

I will add a regression test and submit a revised patch.

Regards,
Mohamed




> -----Original Message-----
> From: Bouhaouel, Mohamed
> Sent: Tuesday, April 4, 2023 4:39 PM
> To: Andrew Burgess <aburgess@redhat.com>; gdb-patches@sourceware.org
> Subject: RE: [PATCH 1/1] gdb, breakpoint: add a destructor to the watchpoint
> struct
> 
> 
> Thanks Andrew for your feedback.
> 
> > > Scenario:
> > > 	(gdb) awatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) rwatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) <continue the program until the end>
> > > 	>> HANG <<
> > >
> >
> > Is it not possible to create a test case based on this scenario?
> 
> It is difficult to produce such scenario due to a weird behavior [1],
> which needs to be addressed as well.  As a result, deleting watchpoints
> before resuming would hide the issue.
> 
> 
> [1] When the watch command fails, it is not possible to resume
> unless we explicitly delete the watchpoint.  More details can be
> found here:
> https://sourceware.org/pipermail/gdb-patches/2023-March/198371.html)
> 
> Kind regards,
> Mohamed
> 
> 
> 
> > -----Original Message-----
> > From: Andrew Burgess <aburgess@redhat.com>
> > Sent: Tuesday, April 4, 2023 1:38 PM
> > To: Bouhaouel, Mohamed <mohamed.bouhaouel@intel.com>; gdb-
> > patches@sourceware.org
> > Subject: Re: [PATCH 1/1] gdb, breakpoint: add a destructor to the watchpoint
> > struct
> >
> > Mohamed Bouhaouel via Gdb-patches <gdb-patches@sourceware.org>
> > writes:
> >
> > > Make sure to unlink the related breakpoint when the watchpoint instance
> > > is deleted.  This prevents having a wp-related breakpoint that is
> > > linked to a NULL watchpoint (e.g.  the watchpoint instance is being
> > > deleted when the 'watch' command fails).  With the below scenario,
> > > having such a left out breakpoint will lead to a GDB hang, and this
> > > is due to an infinite loop when deleting all inferior breakpoints.
> > >
> > > Scenario:
> > > 	(gdb) awatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) rwatch <VAR>
> > > 	Target does not support this type of hardware watchpoint.
> > > 	(gdb) <continue the program until the end>
> > > 	>> HANG <<
> > >
> >
> > Is it not possible to create a test case based on this scenario?
> >
> > Thanks,
> > Andrew
> >
> > > Signed-off-by: Mohamed Bouhaouel <mohamed.bouhaouel@intel.com>
> > > ---
> > >  gdb/breakpoint.c | 15 +++++++++++++++
> > >  gdb/breakpoint.h |  3 +++
> > >  2 files changed, 18 insertions(+)
> > >
> > > diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> > > index 7228acfd8fe..b9ddc85c30c 100644
> > > --- a/gdb/breakpoint.c
> > > +++ b/gdb/breakpoint.c
> > > @@ -9581,6 +9581,21 @@ break_range_command (const char *arg, int
> > from_tty)
> > >    install_breakpoint (false, std::move (br), true);
> > >  }
> > >
> > > +/* See breakpoint.h.  */
> > > +
> > > +watchpoint::~watchpoint ()
> > > +{
> > > +  /* Make sure to unlink the destroyed watchpoint from the related
> > > +     breakpoint ring.  */
> > > +
> > > +  breakpoint *bpt = this;
> > > +  breakpoint *related = bpt;
> > > +  while (related->related_breakpoint != bpt)
> > > +    related = related->related_breakpoint;
> > > +
> > > +  related->related_breakpoint = bpt->related_breakpoint;
> > > +}
> > > +
> > >  /*  Return non-zero if EXP is verified as constant.  Returned zero
> > >      means EXP is variable.  Also the constant detection may fail for
> > >      some constant expressions and in such case still falsely return
> > > diff --git a/gdb/breakpoint.h b/gdb/breakpoint.h
> > > index 7c5cf3f2bef..4f19e42dd83 100644
> > > --- a/gdb/breakpoint.h
> > > +++ b/gdb/breakpoint.h
> > > @@ -933,6 +933,9 @@ struct watchpoint : public breakpoint
> > >    void print_recreate (struct ui_file *fp) const override;
> > >    bool explains_signal (enum gdb_signal) override;
> > >
> > > +  /* Destructor for WATCHPOINT.  */
> > > +  virtual ~watchpoint ();
> > > +
> > >    /* String form of exp to use for displaying to the user (malloc'd),
> > >       or NULL if none.  */
> > >    gdb::unique_xmalloc_ptr<char> exp_string;
> > > --
> > > 2.25.1
> > >
> > > Intel Deutschland GmbH
> > > Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
> > > Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
> > > Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva
> > > Chairperson of the Supervisory Board: Nicole Lau
> > > Registered Office: Munich
> > > Commercial Register: Amtsgericht Muenchen HRB 186928

Intel Deutschland GmbH
Registered Address: Am Campeon 10, 85579 Neubiberg, Germany
Tel: +49 89 99 8853-0, www.intel.de <http://www.intel.de>
Managing Directors: Christin Eisenschmid, Sharon Heck, Tiffany Doon Silva  
Chairperson of the Supervisory Board: Nicole Lau
Registered Office: Munich
Commercial Register: Amtsgericht Muenchen HRB 186928
  

>>>>> Mohamed Bouhaouel via Gdb-patches <gdb-patches@sourceware.org> writes:

> +  /* Destructor for WATCHPOINT.  */
> +  virtual ~watchpoint ();

I don't think the 'virtual' is needed here.

Tom