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

[Will Deacon <will.deacon@arm.com>]

On Wed, May 29, 2019 at 02:55:57PM +0200, Peter Zijlstra wrote:
> On Wed, May 29, 2019 at 11:20:22AM +0100, Will Deacon wrote:
> > Anyway, you can add my ack to your patch, but I bet we can remove that mm
> > check :D
> 
> I've ended up with the below. Ravi, can you test if that does indeed
> obsolete your PPC patch?
> 
> ---
> Subject: perf: Fix perf_sample_regs_user()
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Wed May 29 14:37:24 CEST 2019
> 
> perf_sample_regs_user() uses 'current->mm' to test for the presence of
> userspace, but this is insufficient, consider use_mm().
> 
> A better test is: '!(current->flags & PF_KTHREAD)', exec() clears
> PF_KTHREAD after it sets the new ->mm but before it drops to userspace
> for the first time.
> 
> Possibly obsoletes: bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs user process")
> 
> Reported-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
> Reported-by: Young Xiao <92siuyang@gmail.com>
> Cc: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Jiri Olsa <jolsa@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Stephane Eranian <eranian@google.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Acked-by: Will Deacon <will.deacon@arm.com>
> Fixes: 4018994f3d87 ("perf: Add ability to attach user level registers dump to sample")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/events/core.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct
>  	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?

Will