Re: [PATCH] drm-buf: Add debug option

[John Stultz <john.stultz@linaro.org>]

On Wed, Jan 13, 2021 at 6:06 AM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> We have too many people abusing the struct page they can get at but
> really shouldn't in importers. Aside from that the backing page might
> simply not exist (for dynamic p2p mappings) looking at it and using it
> e.g. for mmap can also wreak the page handling of the exporter
> completely. Importers really must go through the proper interface like
> dma_buf_mmap for everything.
>
> Just an RFC to see whether this idea has some stickiness. default y
> for now to make sure intel-gfx-ci picks it up too.
>
> I'm semi-tempted to enforce this for dynamic importers since those
> really have no excuse at all to break the rules.
>
> Unfortuantely we can't store the right pointers somewhere safe to make
> sure we oops on something recognizable, so best is to just wrangle
> them a bit by flipping all the bits. At least on x86 kernel addresses
> have all their high bits sets and the struct page array is fairly low
> in the kernel mapping, so flipping all the bits gives us a very high
> pointer in userspace and hence excellent chances for an invalid
> dereference.
>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Sumit Semwal <sumit.semwal@linaro.org>
> Cc: "Christian König" <christian.koenig@amd.com>
> Cc: David Stevens <stevensd@chromium.org>
> Cc: linux-media@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> ---
>  drivers/dma-buf/Kconfig   |  8 +++++++
>  drivers/dma-buf/dma-buf.c | 49 +++++++++++++++++++++++++++++++++++----
>  2 files changed, 53 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/dma-buf/Kconfig b/drivers/dma-buf/Kconfig
> index 4f8224a6ac95..cddb549e5e59 100644
> --- a/drivers/dma-buf/Kconfig
> +++ b/drivers/dma-buf/Kconfig
> @@ -50,6 +50,14 @@ config DMABUF_MOVE_NOTIFY
>           This is marked experimental because we don't yet have a consistent
>           execution context and memory management between drivers.
>
> +config DMABUF_DEBUG
> +       bool "DMA-BUF debug checks"
> +       default y
> +       help
> +         This option enables additional checks for DMA-BUF importers and
> +         exporters. Specifically it validates that importers do not peek at the
> +         underlying struct page when they import a buffer.
> +
>  config DMABUF_SELFTESTS
>         tristate "Selftests for the dma-buf interfaces"
>         default n
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 1c9bd51db110..6e4725f7dfde 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -666,6 +666,30 @@ void dma_buf_put(struct dma_buf *dmabuf)
>  }
>  EXPORT_SYMBOL_GPL(dma_buf_put);
>
> +static struct sg_table * __map_dma_buf(struct dma_buf_attachment *attach,
> +                                      enum dma_data_direction direction)
> +{
> +       struct sg_table *sg_table;
> +
> +       sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> +
> +#if CONFIG_DMABUF_DEBUG


Hey Daniel,
  I just noticed a build warning in a tree I pulled this patch into.
You probably want to use #ifdef here, as if its not defined we see:
drivers/dma-buf/dma-buf.c:813:5: warning: "CONFIG_DMABUF_DEBUG" is not
defined, evaluates to 0 [-Wundef]

thanks
-john
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