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From: James Morse <james.morse@arm.com>
To: Will Deacon <will.deacon@arm.com>, Yang Shi <yang.shi@linaro.org>
Cc: catalin.marinas@arm.com, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linaro-kernel@lists.linaro.org
Subject: Re: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump
Date: Wed, 10 Feb 2016 11:52:31 +0000	[thread overview]
Message-ID: <56BB247F.6040202@arm.com> (raw)
In-Reply-To: <20160210102939.GD1052@arm.com>

On 10/02/16 10:29, Will Deacon wrote:
> On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
>> dump_backtrace may be called in kthread context, which is not bound to a single
>> cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug
>> report:
> 
> If we're preemptible here, it means that our irq_stack_ptr is potentially
> bogus. Whilst this isn't an issue for kthreads, it does feel like we
> could make this slightly more robust in the face of potential frame
> corruption. Maybe just zero the IRQ stack pointer if we're in preemptible
> context?

Switching between stacks is only valid if we are tracing ourselves while on the
irq_stack, we should probably prevent it for other tasks too.

Something like (untested):
---------------------
if (tsk == current && in_atomic())
	irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
else
	irq_stack_ptr = 0;
---------------------

This would work when we trace ourselves while on the irq_stack, but break*
tracing a running task on a remote cpu (khungtaskd doesn't do this).

The same fix would apply to unwind_frame(), we have 'tsk' in both functions.

Thoughts?


James


* If this were to ever happen, we would fail to switch to the original stack if
  tracing a remote irq stack, and print the wrong exception args. I'm not aware
  of any users of this, (panic() sends an IPI).

WARNING: multiple messages have this Message-ID (diff)
From: james.morse@arm.com (James Morse)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump
Date: Wed, 10 Feb 2016 11:52:31 +0000	[thread overview]
Message-ID: <56BB247F.6040202@arm.com> (raw)
In-Reply-To: <20160210102939.GD1052@arm.com>

On 10/02/16 10:29, Will Deacon wrote:
> On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
>> dump_backtrace may be called in kthread context, which is not bound to a single
>> cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug
>> report:
> 
> If we're preemptible here, it means that our irq_stack_ptr is potentially
> bogus. Whilst this isn't an issue for kthreads, it does feel like we
> could make this slightly more robust in the face of potential frame
> corruption. Maybe just zero the IRQ stack pointer if we're in preemptible
> context?

Switching between stacks is only valid if we are tracing ourselves while on the
irq_stack, we should probably prevent it for other tasks too.

Something like (untested):
---------------------
if (tsk == current && in_atomic())
	irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
else
	irq_stack_ptr = 0;
---------------------

This would work when we trace ourselves while on the irq_stack, but break*
tracing a running task on a remote cpu (khungtaskd doesn't do this).

The same fix would apply to unwind_frame(), we have 'tsk' in both functions.

Thoughts?


James


* If this were to ever happen, we would fail to switch to the original stack if
  tracing a remote irq stack, and print the wrong exception args. I'm not aware
  of any users of this, (panic() sends an IPI).

  reply	other threads:[~2016-02-10 11:53 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-02-09 21:26 [PATCH] arm64: use raw_smp_processor_id in stack backtrace dump Yang Shi
2016-02-09 21:26 ` Yang Shi
2016-02-10 10:29 ` Will Deacon
2016-02-10 10:29   ` Will Deacon
2016-02-10 11:52   ` James Morse [this message]
2016-02-10 11:52     ` James Morse
2016-02-10 12:10     ` Will Deacon
2016-02-10 12:10       ` Will Deacon
2016-02-10 18:12       ` Shi, Yang
2016-02-10 18:12         ` Shi, Yang
2016-02-11 10:41         ` James Morse
2016-02-11 10:41           ` James Morse
2016-02-11 17:36           ` Shi, Yang
2016-02-11 17:36             ` Shi, Yang

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