Re: Question about -rc and stable patches

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 3 Mar 2014 14:41:04 -0800
Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:

> On Mon, Mar 03, 2014 at 05:28:06PM -0500, Steven Rostedt wrote:
> > Hi Linus and Greg,
> > 
> > Recently I have been sent patches that fix a bug in the enabling of
> > ftrace function tracer, where if something goes wrong with the
> > enabling (or disabling), it can cause a crash of the computer.
> > 
> > Now, my question is, the bug requires something to go wrong at a point
> > where I've never seen it go wrong before. But if you inject a problem
> > (code that makes it fail on the 1000th iteration), sure enough, the
> > recovery is not correct and the machine crashes.
> 
> What exactly do you mean by "inject a problem"?  What kind of
> permissions does that require to do?

Modify kernel source ;-)  Basically, I added this patch to the kernel,
compiled it, enabled function tracing, and it crashed:

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 8cabf63..9f354e5 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -586,6 +586,9 @@ void ftrace_replace_code(int enable)
 	for_ftrace_rec_iter(iter) {
 		rec = ftrace_rec_iter_record(iter);
 
+		if (count == 1000)
+			ret = -EPERM;
+		else
 		ret = add_breakpoints(rec, enable);
 		if (ret)
 			goto remove_breakpoints;


Not something that can be done from normal operations.

-- Steve