[PATCH v2] sysrq: Use panic() to force a crash

[PATCH v2] sysrq: Use panic() to force a crash

[Matthias Kaehlcke]

sysrq_handle_crash() currently forces a crash by dereferencing a
NULL pointer, which is undefined behavior in C. Just call panic()
instead, which is simpler and doesn't depend on compiler specific
handling of the undefined behavior.

Remove the comment on why the RCU lock needs to be released, it isn't
accurate anymore since the crash now isn't handled by the page fault
handler (for reference: the comment was added by commit 984cf355aeaa
("sysrq: Fix warning in sysrq generated crash.")). Releasing the lock
is still good practice though.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
---
 drivers/tty/sysrq.c | 13 +++----------
 1 file changed, 3 insertions(+), 10 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 06ed20dd01ba..d779a51499a0 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
 
 static void sysrq_handle_crash(int key)
 {
-	char *killer = NULL;
-
-	/* we need to release the RCU read lock here,
-	 * otherwise we get an annoying
-	 * 'BUG: sleeping function called from invalid context'
-	 * complaint from the kernel before the panic.
-	 */
+	/* release the RCU read lock before crashing */
 	rcu_read_unlock();
-	panic_on_oops = 1;	/* force panic */
-	wmb();
-	*killer = 1;
+
+	panic("sysrq triggered crash\n");
 }
 static struct sysrq_key_op sysrq_crash_op = {
 	.handler	= sysrq_handle_crash,
-- 
2.19.0.444.g18242da7ef-goog

Re: [PATCH v2] sysrq: Use panic() to force a crash

[Matthias Kaehlcke]

On Thu, Sep 20, 2018 at 10:12:53AM -0700, Matthias Kaehlcke wrote:
> sysrq_handle_crash() currently forces a crash by dereferencing a
> NULL pointer, which is undefined behavior in C. Just call panic()
> instead, which is simpler and doesn't depend on compiler specific
> handling of the undefined behavior.
> 
> Remove the comment on why the RCU lock needs to be released, it isn't
> accurate anymore since the crash now isn't handled by the page fault
> handler (for reference: the comment was added by commit 984cf355aeaa
> ("sysrq: Fix warning in sysrq generated crash.")). Releasing the lock
> is still good practice though.
> 
> Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
>  drivers/tty/sysrq.c | 13 +++----------
>  1 file changed, 3 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> index 06ed20dd01ba..d779a51499a0 100644
> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
>  
>  static void sysrq_handle_crash(int key)
>  {
> -	char *killer = NULL;
> -
> -	/* we need to release the RCU read lock here,
> -	 * otherwise we get an annoying
> -	 * 'BUG: sleeping function called from invalid context'
> -	 * complaint from the kernel before the panic.
> -	 */
> +	/* release the RCU read lock before crashing */
>  	rcu_read_unlock();
> -	panic_on_oops = 1;	/* force panic */
> -	wmb();
> -	*killer = 1;
> +
> +	panic("sysrq triggered crash\n");
>  }
>  static struct sysrq_key_op sysrq_crash_op = {
>  	.handler	= sysrq_handle_crash,

Does this need further changes or is it ready to land?

Re: [PATCH v2] sysrq: Use panic() to force a crash

[Greg Kroah-Hartman]

On Wed, Nov 14, 2018 at 02:57:18PM -0800, Matthias Kaehlcke wrote:
> On Thu, Sep 20, 2018 at 10:12:53AM -0700, Matthias Kaehlcke wrote:
> > sysrq_handle_crash() currently forces a crash by dereferencing a
> > NULL pointer, which is undefined behavior in C. Just call panic()
> > instead, which is simpler and doesn't depend on compiler specific
> > handling of the undefined behavior.
> > 
> > Remove the comment on why the RCU lock needs to be released, it isn't
> > accurate anymore since the crash now isn't handled by the page fault
> > handler (for reference: the comment was added by commit 984cf355aeaa
> > ("sysrq: Fix warning in sysrq generated crash.")). Releasing the lock
> > is still good practice though.
> > 
> > Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> >  drivers/tty/sysrq.c | 13 +++----------
> >  1 file changed, 3 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
> > index 06ed20dd01ba..d779a51499a0 100644
> > --- a/drivers/tty/sysrq.c
> > +++ b/drivers/tty/sysrq.c
> > @@ -134,17 +134,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
> >  
> >  static void sysrq_handle_crash(int key)
> >  {
> > -	char *killer = NULL;
> > -
> > -	/* we need to release the RCU read lock here,
> > -	 * otherwise we get an annoying
> > -	 * 'BUG: sleeping function called from invalid context'
> > -	 * complaint from the kernel before the panic.
> > -	 */
> > +	/* release the RCU read lock before crashing */
> >  	rcu_read_unlock();
> > -	panic_on_oops = 1;	/* force panic */
> > -	wmb();
> > -	*killer = 1;
> > +
> > +	panic("sysrq triggered crash\n");
> >  }
> >  static struct sysrq_key_op sysrq_crash_op = {
> >  	.handler	= sysrq_handle_crash,
> 
> Does this need further changes or is it ready to land?

Ugh, it somehow got dropped from my queue, sorry.  I'll add it back and
get to it next week, my fault.

greg k-h