Re: [PATCH 1/2] drivers/media/platform/soc_camera/pxa_camera.c: reposition free_irq to avoid access to invalid data

From: Robert Jarzmik
Date: Mon Jan 07 2013 - 15:08:50 EST

Guennadi Liakhovetski <g.liakhovetski@xxxxxx> writes:

> (adding Robert to CC)
> I don't think any data is freed by pxa_free_dma(), it only disables DMA on
> a certain channel. Theoretically there could be a different problem:
> pxa_free_dma() deactivates DMA, whereas pxa_dma_start_channels() activates
> it. But I think we're also protected against that: by the time
> pxa_camera_remove() is called, and operation on the interface has been
> stopped, client devices have been detached, pxa_camera_remove_device() has
> been called, which has also stopped the interface clock. And with clock
> stopped no interrupts can be generated. And the case of interrupt having
> been generated before clk_disabled() and only delivered to the driver so
> much later, that we're already unloading the module, seems really
> impossible to me. Robert, you agree?

Agreed that pxa_free_dma() doesn't free anything, that one is easy :)

And agreed too for the second part, with a slighly different explanation :
- pxa_camera_remove_device() has been called as you said
- inside this function, check comment
"/* disable capture, disable interrupts */"
=> this ensures no interrupt can be generated anymore

So after pxa_camera_remove_device() has been called, no interrupts can be

Yet as you said, it leaves the "almost impossible" scenario :
- a user begins a capture
- the user closes the capture device and unloads pxa-camera.ko:
the IRQ line is asserted but doesn't trigger yet the interrupt handler
(yes I know, improbable)
meanwhile, IRQs are disabled, DMA channels are stopped
=> yes I know, impossible, the interrupt handler must be run before, but
let's continue for love of discussion ...
rmmod pxa-camera
pxa_free_dma() * 3
----> here the IRQ handler kicks in !!!
=> pxa_camera_irq()
----> it hurts !

My call is that this is impossible because the switch_to() should run the IRQ
handler before pxa_camera_remove() is called.

So all this to say that I think we're safe, unless a heavy ion or a cosmic ray
strikes the PXA :)


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