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Date:   Wed, 29 May 2019 11:11:36 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Will Deacon <will.deacon@arm.com>,
        Young Xiao <92siuyang@gmail.com>, linux@armlinux.org.uk,
        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 Tue, May 28, 2019 at 07:32:28PM +0200, Peter Zijlstra wrote:
> On Tue, May 28, 2019 at 04:32:24PM +0100, Will Deacon wrote:
> > On Tue, May 28, 2019 at 04:01:03PM +0200, Peter Zijlstra wrote:
> > > On Tue, May 28, 2019 at 08:31:29PM +0800, Young Xiao wrote:
> > > > When a kthread calls call_usermodehelper() the steps are:
> > > >   1. allocate current->mm
> > > >   2. load_elf_binary()
> > > >   3. populate current->thread.regs
> > > > 
> > > > While doing this, interrupts are not disabled. If there is a perf
> > > > interrupt in the middle of this process (i.e. step 1 has completed
> > > > but not yet reached to step 3) and if perf tries to read userspace
> > > > regs, kernel oops.
> > 
> > This seems to be because pt_regs(current) gives NULL for kthreads on Power.
> 
> '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().

Tagnentially, it looks like that assumption is made elsewhere, and could
do with a more general cleanup. IIUC, the following are suspect:

* kmemleak's scan_should_stop()
* x86's __kernel_fpu_begin()
* arm64's arch_dup_task_struct()

It's probably worth an is_thread(task) helper so that those can be
written in an obviously correct way.

> 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) {

Wouldn't !PF_KTHREAD imply current->mm anyhow?

Thanks,
Mark.