Re: [PATCH] perf: Fix oops when kthread execs user process

[Peter Zijlstra]

On Wed, May 29, 2019 at 03:35:10PM +0100, Will Deacon wrote:
> On Wed, May 29, 2019 at 03:25:15PM +0200, Peter Zijlstra wrote:
> > On Wed, May 29, 2019 at 02:05:21PM +0100, Will Deacon wrote:
> > > On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote:
> > 
> > > >  	if (user_mode(regs)) {
> > > 
> > > Hmm, so it just occurred to me that Mark's observation is that the regs
> > > can be junk in some cases. In which case, should we be checking for
> > > kthreads first?
> > 
> > task_pt_regs() can return garbage, but @regs is the exception (or
> > perf_arch_fetch_caller_regs()) regs, and for those user_mode() had
> > better be correct.
> 
> So what should we report for the idle task?

If an interrupt hits the idle task, @regs would be !user_mode(regs),
we'll find current->flags & PF_KTHREAD (idle not having passed through
exec()) and therefore we'll take ABI_NONE for the user regs.

Or am I not getting it?