Hi Jason & Daniel,
maybe I should explain once more where the problem with this approach is
and why I think we need to get that fixed before we can do something
like this here.
To summarize what this patch here does is that it copies the exclusive
fence and/or the shared fences into a sync_file. This alone is totally
unproblematic.
The problem is what this implies. When you need to copy the exclusive
fence to a sync_file then this means that the driver is at some point
ignoring the exclusive fence on a buffer object.
When you combine that with complex drivers which use TTM and buffer
moves underneath you can construct an information leak using this and
give userspace access to memory which is allocated to the driver, but
not yet initialized.
This way you can leak things like page tables, passwords, kernel data
etc... in large amounts to userspace and is an absolutely no-go for
security.
That's why I'm said we need to get this fixed before we upstream this
patch set here and especially the driver change which is using that.
Regards,
Christian.
Am 10.06.21 um 23:09 schrieb Jason Ekstrand:
Modern userspace APIs like Vulkan are built on an explicit
synchronization model. This doesn't always play nicely with the
implicit synchronization used in the kernel and assumed by X11 and
Wayland. The client -> compositor half of the synchronization isn't too
bad, at least on intel, because we can control whether or not i915
synchronizes on the buffer and whether or not it's considered written.
The harder part is the compositor -> client synchronization when we get
the buffer back from the compositor. We're required to be able to
provide the client with a VkSemaphore and VkFence representing the point
in time where the window system (compositor and/or display) finished
using the buffer. With current APIs, it's very hard to do this in such
a way that we don't get confused by the Vulkan driver's access of the
buffer. In particular, once we tell the kernel that we're rendering to
the buffer again, any CPU waits on the buffer or GPU dependencies will
wait on some of the client rendering and not just the compositor.
This new IOCTL solves this problem by allowing us to get a snapshot of
the implicit synchronization state of a given dma-buf in the form of a
sync file. It's effectively the same as a poll() or I915_GEM_WAIT only,
instead of CPU waiting directly, it encapsulates the wait operation, at
the current moment in time, in a sync_file so we can check/wait on it
later. As long as the Vulkan driver does the sync_file export from the
dma-buf before we re-introduce it for rendering, it will only contain
fences from the compositor or display. This allows to accurately turn
it into a VkFence or VkSemaphore without any over- synchronization.
This patch series actually contains two new ioctls. There is the export
one mentioned above as well as an RFC for an import ioctl which provides
the other half. The intention is to land the export ioctl since it seems
like there's no real disagreement on that one. The import ioctl, however,
has a lot of debate around it so it's intended to be RFC-only for now.
Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037
IGT tests: https://patchwork.freedesktop.org/series/90490/
v10 (Jason Ekstrand, Daniel Vetter):
- Add reviews/acks
- Add a patch to rename _rcu to _unlocked
- Split things better so import is clearly RFC status
v11 (Daniel Vetter):
- Add more CCs to try and get maintainers
- Add a patch to document DMA_BUF_IOCTL_SYNC
- Generally better docs
- Use separate structs for import/export (easier to document)
- Fix an issue in the import patch
v12 (Daniel Vetter):
- Better docs for DMA_BUF_IOCTL_SYNC
v12 (Christian König):
- Drop the rename patch in favor of Christian's series
- Add a comment to the commit message for the dma-buf sync_file export
ioctl saying why we made it an ioctl on dma-buf
Cc: Christian König <christian.koe...@amd.com>
Cc: Michel Dänzer <mic...@daenzer.net>
Cc: Dave Airlie <airl...@redhat.com>
Cc: Bas Nieuwenhuizen <b...@basnieuwenhuizen.nl>
Cc: Daniel Stone <dani...@collabora.com>
Cc: mesa-dev@lists.freedesktop.org
Cc: wayland-de...@lists.freedesktop.org
Test-with: 20210524205225.872316-1-ja...@jlekstrand.net
Christian König (1):
dma-buf: Add dma_fence_array_for_each (v2)
Jason Ekstrand (5):
dma-buf: Add dma_resv_get_singleton (v6)
dma-buf: Document DMA_BUF_IOCTL_SYNC (v2)
dma-buf: Add an API for exporting sync files (v12)
RFC: dma-buf: Add an extra fence to dma_resv_get_singleton_unlocked
RFC: dma-buf: Add an API for importing sync files (v7)
Documentation/driver-api/dma-buf.rst | 8 ++
drivers/dma-buf/dma-buf.c | 103 +++++++++++++++++++++++++
drivers/dma-buf/dma-fence-array.c | 27 +++++++
drivers/dma-buf/dma-resv.c | 110 +++++++++++++++++++++++++++
include/linux/dma-fence-array.h | 17 +++++
include/linux/dma-resv.h | 2 +
include/uapi/linux/dma-buf.h | 103 ++++++++++++++++++++++++-
7 files changed, 369 insertions(+), 1 deletion(-)
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