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    SubjectRe: Bad kfree of dma_parms in v5.7-rc5
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    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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    Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki.
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