[bug report] drm/prime: replace NULL with error value in drm_prime_pages_to_sg

[Oleksandr Andrushchenko]

On 06/15/2018 06:22 AM, YoungJun Cho wrote:
> Dear Dan,
>
> Your mail flashes back to my memory 5 years ago.
> Back then, I had cleaned up the exynos driver.
>
> And the replacement IS_ERR was one of items.
>
> IMHO it is still better to modify those two functions, 
> drm_gem_cma_prime_get_sg_table and xen_drm_front_gem_get_sg_table.
>
> Thank you.
> Best regards YJ
>
>
> On Thu, 14 Jun 2018, 23:42 Dan Carpenter, <dan.carpenter at oracle.com 
> <mailto:dan.carpenter at oracle.com>> wrote:
>
>     Hello YoungJun Cho,
>
>     The patch 7e3d88f9cce3: "drm/prime: replace NULL with error value in
>     drm_prime_pages_to_sg" from Jun 24, 2013, leads to the following
>     static checker warning:
>
>             drivers/gpu/drm/drm_prime.c:317 drm_gem_map_dma_buf()
>             warn: 'sgt' can also be NULL
>
>     drivers/gpu/drm/drm_prime.c
>        307          /*
>        308           * two mappings with different directions for the
>     same attachment are
>        309           * not allowed
>        310           */
>        311          if (WARN_ON(prime_attach->dir != DMA_NONE))
>        312                  return ERR_PTR(-EBUSY);
>        313
>        314          sgt = obj->dev->driver->gem_prime_get_sg_table(obj);
>
>     The problematic functions here are
>     drm_gem_cma_prime_get_sg_table() and
>     xen_drm_front_gem_get_sg_table().  My preference would be to update
>     those functions to return error pointers, but I'm not familiar
>     with the
>     code or able to test it.
>
I believe it is safe to change the return value to error pointer:
the code below (which is the only caller of .gem_prime_get_sg_table
callback) does not check for NULL anyways

Dan, do you want me to send the patch for Xen or you prefer to post both
Xen and CMA helpers fix yourself?
>
>     But this static checker test seems pretty good so I'm likely to
>     publish
>     it soon and then this sort of bug will normally be caught quickly.
>
>        315
>        316          if (!IS_ERR(sgt)) {
>        317                  if (!dma_map_sg_attrs(attach->dev,
>     sgt->sgl, sgt->nents, dir,
>        318 DMA_ATTR_SKIP_CPU_SYNC)) {
>        319                          sg_free_table(sgt);
>        320                          kfree(sgt);
>        321                          sgt = ERR_PTR(-ENOMEM);
>        322                  } else {
>        323                          prime_attach->sgt = sgt;
>        324                          prime_attach->dir = dir;
>        325                  }
>        326          }
>        327
>        328          return sgt;
>
>     regards,
>     dan carpenter
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