Re: Bad kfree of dma_parms in v5.7-rc5

[Tomi Valkeinen <tomi.valkeinen@ti.com>]

On 20/05/2020 12:22, Marek Szyprowski wrote:
> Hi Tomi,
> 
> On 20.05.2020 11:18, Tomi Valkeinen wrote:
>> On 20/05/2020 12:13, Marek Szyprowski wrote:
>>> On 20.05.2020 11:00, Tomi Valkeinen wrote:
>>>> Commit 9495b7e92f716ab2bd6814fab5e97ab4a39adfdd ("driver core:
>>>> platform: Initialize dma_parms for platform devices") v5.7-rc5 causes
>>>> at least some v4l2 platform drivers to break when freeing resources.
>>>>
>>>> E.g. drivers/media/platform/ti-vpe/cal.c uses
>>>> vb2_dma_contig_set_max_seg_size() and
>>>> vb2_dma_contig_clear_max_seg_size() to manage the dma_params, and
>>>> similar pattern is seen in other drivers too.
>>>>
>>>> After 9495b7e92f716ab2, vb2_dma_contig_set_max_seg_size() will not
>>>> allocate anything, but vb2_dma_contig_clear_max_seg_size() will still
>>>> kfree the dma_params.
>>>>
>>>> I'm not sure what's the proper fix here. A flag somewhere to indicate
>>>> that vb2_dma_contig_set_max_seg_size() did allocate, and thus
>>>> vb2_dma_contig_clear_max_seg_size() must free?
>>>>
>>>> Or drop the kzalloc and kfree totally, if dma_params is now supposed
>>>> to always be there?
>>>
>>> Thanks for reporting this issue!
>>>
>>> Once the mentioned commit has been merged, the code should assume that
>>> the platform devices does have a struct dma_params allocated, so the
>>> proper fix is to alloc dma_params only if the bus is not a platform bus:
>>>
>>> if (!dev_is_platform(dev) && !dev->dma_parms) {
>>>        dev->dma_parms = kzalloc(sizeof(*dev->dma_parms), GFP_KERNEL);
>>>
>>> same check for the free path.
>>
>> There is also "amba: Initialize dma_parms for amba devices". And the
>> commit message says PCI devices do this too.
>>
>> Guessing this based on the device type doesn't sound like a good idea
>> to me.
> 
> Indeed. Then replace the allocation with a simple check for NULL
> dma_parms and return an error in such case. This should be enough for
> v5.8. Later we can simply get rid of those helpers and inline setting
> max segment size directly to the drivers.

Is that valid either? Then we assume that dma_parms is always set up by someone else. That's true 
for platform devices and apparently some other devices, but is it true for all devices now?

  Tomi

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