Re: [PATCH 1/5] tracing: Prevent kernel oops with corrupted buffer

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 2010-01-26 at 14:32 -0800, Andrew Morton wrote:
> On Tue, 26 Jan 2010 17:09:24 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > From: Steven Rostedt <srostedt@redhat.com>
> > 
> > If the contents of the ftrace ring buffer gets corrupted and the trace
> > file is read, it could create a kernel oops (usualy just killing the user
> 
> "usually" ;)

I used "usually" since that is what happened every time I encountered
the issue. But I don't know 100% if it only oops the user task in every
instance.

> 
> > task thread). This is caused by the checking of the pid in the buffer.
> > If the pid is negative, it still references the cmdline cache array,
> > which could point to an invalid address.
> > 
> > The simple fix is to test for negative PIDs.
> > 
> > Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> > ---
> >  kernel/trace/trace.c |    5 +++++
> >  1 files changed, 5 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > index 0df1b0f..eac6875 100644
> > --- a/kernel/trace/trace.c
> > +++ b/kernel/trace/trace.c
> > @@ -951,6 +951,11 @@ void trace_find_cmdline(int pid, char comm[])
> >  		return;
> >  	}
> >  
> > +	if (WARN_ON_ONCE(pid < 0)) {
> > +		strcpy(comm, "<XXX>");
> > +		return;
> > +	}
> > +
> >  	if (pid > PID_MAX_DEFAULT) {
> >  		strcpy(comm, "<...>");
> >  		return;
> 
> But why is it WARN_ON_ONCE()?  That will only fix the problem a single
> time.  On the second occurrence, it will oops again.

Frederic correctly answered this.

Thanks,

-- Steve