Re: [PATCH 3/4] extable: Enable RCU if it is not watching in kernel_text_address()

[Steven Rostedt]

On Fri, 22 Sep 2017 15:44:06 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> On Fri, Sep 22, 2017 at 06:15:46PM -0400, Steven Rostedt wrote:
> > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > 
> > If kernel_text_address() is called when RCU is not watching, it can cause an
> > RCU bug because is_module_text_address() and the is_kprobe_*insn_slot()
> > functions require the use of RCU.
> > 
> > Only enable RCU if it is not currently watching before it calls
> > is_module_text_address(). The use of rcu_nmi_enter() is used to enable RCU
> > because kernel_text_address() can happen pretty much anywhere (like an NMI),
> > and even from within an NMI. It is called via save_stack_trace() that can be
> > called by any WARN() or tracing function, which can happen while RCU is not
> > watching (for example, going to or coming from idle, or during CPU take down
> > or bring up).
> > 
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx  
> 
> Do we need something calling out the dependency on the first two patches,
> or is that implied these days?

I believe the stable folks will simply apply patches marked for stable
in the order they are applied. This is a patch series, so I don't think
it will be an issue for them.

> 
> > Fixes: 0be964be0 ("module: Sanitize RCU usage and locking")
> > Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>  
> 
> Give or take Josh's feedback on the commit log and comment:

Yep.

> 
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Thanks!

-- Steve