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SubjectRe: [PATCH] drm/prime: fix a potential double put (release) bug
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Am 18.08.21 um 15:02 schrieb Wentao_Liang:
> In line 317 (#1), drm_gem_prime_import() is called, it will call
> drm_gem_prime_import_dev(). At the end of the function
> drm_gem_prime_import_dev() (line 956, #2), "dma_buf_put(dma_buf);" puts
> dma_buf->file and may cause it to be released. However, after
> drm_gem_prime_import() returning, the dma_buf may be put again by the
> same put function in lines 342, 351 and 358 (#3, #4, #5). Putting the
> dma_buf improperly more than once can lead to an incorrect dma_buf-
>> file put.
> We believe that the put of the dma_buf in the function
> drm_gem_prime_import() is unnecessary (#2). We can fix the above bug by
> removing the redundant "dma_buf_put(dma_buf);" in line 956.

Guys I'm getting tired of NAKing those incorrect reference count analysis.

The dma_buf_put() in the error handling of drm_gem_prime_import_dev()
function is balanced with the get_dma_buf() in the same function
directly above.

This is for the creating a GEM object for a DMA-buf imported from other
device use case and certainly correct.

The various dma_buf_put() in drm_gem_prime_fd_to_handle() is balanced
with the dma_buf_get(prime_fd) at the beginning of the function.

This is for extracting the DMA-buf from the file descriptor and keeping
a reference to it while we are busy importing it (e.g. to prevent a race
when somebody changes the fd at the same time).

As far as I can see this is correct as well.

Regards,
Christian.

>
> 314 if (dev->driver->gem_prime_import)
> 315 obj = dev->driver->gem_prime_import(dev, dma_buf);
> 316 else
> 317 obj = drm_gem_prime_import(dev, dma_buf);
> //#1 call to drm_gem_prime_import
> // ->drm_gem_prime_import_dev
> // ->dma_buf_put
> ...
>
> 336 ret = drm_prime_add_buf_handle(&file_priv->prime,
> 337 dma_buf, *handle);
>
> ...
>
> 342 dma_buf_put(dma_buf); //#3 put again
> 343
> 344 return 0;
> 345
> 346 fail:
>
> 351 dma_buf_put(dma_buf); //#4 put again
> 352 return ret;
>
> 356 out_put:
> 357 mutex_unlock(&file_priv->prime.lock);
> 358 dma_buf_put(dma_buf); //#5 put again
> 359 return ret;
> 360 }
>
> 905 struct drm_gem_object *drm_gem_prime_import_dev
> (struct drm_device *dev,
> 906 struct dma_buf *dma_buf,
> 907 struct device *attach_dev)
> 908 {
>
> ...
>
> 952 fail_unmap:
> 953 dma_buf_unmap_attachment(attach, sgt, DMA_BIDIRECTIONAL);
> 954 fail_detach:
> 955 dma_buf_detach(dma_buf, attach);
> 956 dma_buf_put(dma_buf); //#2 the first put of dma_buf
> // (unnecessary)
> 957
> 958 return ERR_PTR(ret);
> 959 }
>
> Signed-off-by: Wentao_Liang <Wentao_Liang_g@163.com>
> ---
> drivers/gpu/drm/drm_prime.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 2a54f86856af..cef03ad0d5cd 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -953,7 +953,6 @@ struct drm_gem_object *drm_gem_prime_import_dev(struct drm_device *dev,
> dma_buf_unmap_attachment(attach, sgt, DMA_BIDIRECTIONAL);
> fail_detach:
> dma_buf_detach(dma_buf, attach);
> - dma_buf_put(dma_buf);
>
> return ERR_PTR(ret);
> }

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