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SubjectRe: [PATCH] drm: Create support for Write-Only property blob
Hi Jani, thanks for your review. I got all the user space
implementation ready to see it in context.

libdrm patch to wrap this functionality:
https://www.spinics.net/lists/dri-devel/msg347318.html

Chromium user space implementation making direct use of the new prop flag:
crrev.com/c/3655850
The first call made to such functionality is in
https://chromium-review.googlesource.com/c/chromium/src/+/3655850/2/ui/display/manager/content_protection_key_manager.cc#137
where the call stack flows to the libdrm wrapper call at
https://chromium-review.googlesource.com/c/chromium/src/+/3655850/2/ui/ozone/platform/drm/gpu/drm_display.cc#203

I also wrote an IGT test to verify the intended behavior:
https://patchwork.freedesktop.org/patch/487331/?series=104373&rev=1

Let me know if I would need to update the commit message with any of
the aforementioned context.

Thanks!
-Mark Yacoub

On Tue, May 17, 2022 at 3:53 AM Jani Nikula <jani.nikula@linux.intel.com> wrote:
>
> On Tue, 10 May 2022, Mark Yacoub <markyacoub@chromium.org> wrote:
> > [Why]
> > User space might need to inject data into the kernel without allowing it
> > to be read again by any user space.
> > An example of where this is particularly useful is secret keys fetched
> > by user space and injected into the kernel to enable content protection.
>
> I think we're going to need more than an example in the commit
> message. See Documentation/gpu/drm-uapi.rst.
>
> BR,
> Jani.
>
>
> >
> > [How]
> > Create a DRM_MODE_CREATE_BLOB_WRITE_ONLY flag used by user space to
> > create a blob and mark the blob as write only.
> > On reading back the blob, data will be not be copied if it's a write
> > only blob
> >
> > Signed-off-by: Mark Yacoub <markyacoub@chromium.org>
> >
> > ---
> > drivers/gpu/drm/drm_property.c | 3 ++-
> > include/drm/drm_property.h | 2 ++
> > include/uapi/drm/drm_mode.h | 6 ++++++
> > 3 files changed, 10 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c
> > index dfec479830e4..afedf7109d00 100644
> > --- a/drivers/gpu/drm/drm_property.c
> > +++ b/drivers/gpu/drm/drm_property.c
> > @@ -765,7 +765,7 @@ int drm_mode_getblob_ioctl(struct drm_device *dev,
> > if (!blob)
> > return -ENOENT;
> >
> > - if (out_resp->length == blob->length) {
> > + if (out_resp->length == blob->length && !blob->is_write_only) {
> > if (copy_to_user(u64_to_user_ptr(out_resp->data),
> > blob->data,
> > blob->length)) {
> > @@ -800,6 +800,7 @@ int drm_mode_createblob_ioctl(struct drm_device *dev,
> > ret = -EFAULT;
> > goto out_blob;
> > }
> > + blob->is_write_only = out_resp->flags & DRM_MODE_CREATE_BLOB_WRITE_ONLY;
> >
> > /* Dropping the lock between create_blob and our access here is safe
> > * as only the same file_priv can remove the blob; at this point, it is
> > diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
> > index 65bc9710a470..700782f021b9 100644
> > --- a/include/drm/drm_property.h
> > +++ b/include/drm/drm_property.h
> > @@ -205,6 +205,7 @@ struct drm_property {
> > * &drm_mode_config.property_blob_list.
> > * @head_file: entry on the per-file blob list in &drm_file.blobs list.
> > * @length: size of the blob in bytes, invariant over the lifetime of the object
> > + * @is_write_only: user space can't read the blob data.
> > * @data: actual data, embedded at the end of this structure
> > *
> > * Blobs are used to store bigger values than what fits directly into the 64
> > @@ -219,6 +220,7 @@ struct drm_property_blob {
> > struct list_head head_global;
> > struct list_head head_file;
> > size_t length;
> > + bool is_write_only;
> > void *data;
> > };
> >
> > diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> > index 0a0d56a6158e..de192d3813e9 100644
> > --- a/include/uapi/drm/drm_mode.h
> > +++ b/include/uapi/drm/drm_mode.h
> > @@ -1107,6 +1107,9 @@ struct drm_format_modifier {
> > __u64 modifier;
> > };
> >
> > +#define DRM_MODE_CREATE_BLOB_WRITE_ONLY \
> > + (1 << 0) /* data of the blob can't be read by user space */
> > +
> > /**
> > * struct drm_mode_create_blob - Create New blob property
> > *
> > @@ -1120,6 +1123,9 @@ struct drm_mode_create_blob {
> > __u32 length;
> > /** @blob_id: Return: new property ID. */
> > __u32 blob_id;
> > + /** Flags for special handling. */
> > + __u32 flags;
> > + __u32 pad;
> > };
> >
> > /**
>
> --
> Jani Nikula, Intel Open Source Graphics Center

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