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Date:   Tue, 28 May 2019 17:12:54 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Will Deacon <will.deacon@arm.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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 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.

I think you mean task_pt_regs(current) here.

> > > Fix it by setting abi to PERF_SAMPLE_REGS_ABI_NONE when userspace
> > > pt_regs are not set.
> > > 
> > > See commit bf05fc25f268 ("powerpc/perf: Fix oops when kthread execs
> > > user process") for details.
> > 
> > Why the hell do we set current->mm before it is complete? Note that
> > normally exec() builds the new mm before attaching it, see exec_mmap()
> > in flush_old_exec().
> 
> From the initial report [1], it doesn't look like the mm isn't initialised,
> but rather than we're dereferencing a NULL pt_regs pointer somehow for the
> current task (see previous comment). I don't see how that can happen on
> arm64, given that we put the pt_regs on the kernel stack which is allocated
> during fork.
> 
> Will
> 
> [1] https://git.kernel.org/linus/bf05fc25f268

One caveat is that for the idle threads, the initial SP overlaps the
task_pt_regs() area:

* __primary_switched starts SP at init_thread_union + THREAD_SIZE.

* __cpu_up() starts SP at task_stack_page(idle) + THREAD_SIZE.

... and in either case, sampling that would be bad.

For both arm, I believe similar holds true. AFAICT x86 seems to reserve
the regs area in its head_{64,32}.S, but I can't see what it does for
other threads.

Regardless, for arm, arm64, and x86, task_pt_regs(current) cannot be
NULL.

Thanks,
Mark.