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Date:   Wed, 29 May 2019 12:10:42 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Will Deacon <will.deacon@arm.com>
Cc:     Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk,
        mark.rutland@arm.com, mingo@redhat.com, bp@alien8.de,
        hpa@zytor.com, x86@kernel.org, kan.liang@linux.intel.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        ravi.bangoria@linux.vnet.ibm.com, mpe@ellerman.id.au
Subject: Re: [PATCH] perf: Fix oops when kthread execs user process
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On Wed, May 29, 2019 at 10:17:33AM +0100, Will Deacon wrote:
> On Tue, May 28, 2019 at 07:32:28PM +0200, Peter Zijlstra wrote:

> > 'funny' thing that, perf_sample_regs_user() seems to assume that
> > anything with current->mm is in fact a user task, and that assumption is
> > just plain wrong, consider use_mm().
> 
> Right, I suppose that was attempting to handle interrupt skid from the PMU
> overflow?

Nah, just a broken test to determine if there is userspace at all. It is
mostly right, just not completely :-)

> > So I'm thinking the right thing to do here is something like the below;
> > umh should get PF_KTHREAD cleared when it passes exec(). And this should
> > also fix the power splat I'm thinking.
> > 
> > ---
> > 
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index abbd4b3b96c2..9929404b6eb9 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5923,7 +5923,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
> >  	if (user_mode(regs)) {
> >  		regs_user->abi = perf_reg_abi(current);
> >  		regs_user->regs = regs;
> > -	} else if (current->mm) {
> > +	} else if (!(current->flags & PF_KTHREAD) && current->mm) {
> >  		perf_get_regs_user(regs_user, regs, regs_user_copy);
> 
> Makes sense, but under which circumstances would we have a NULL mm here?

Dunno; I'm paranoid, and also:

  mm/memcontrol.c:        if (in_interrupt() || !current->mm || (current->flags & PF_KTHREAD))
  mm/vmacache.c:  return current->mm == mm && !(current->flags & PF_KTHREAD);