Hi Tomasz, On Thu, Jun 08, 2023 at 07:24:29PM +0900, Tomasz Figa wrote: > On Wed, May 31, 2023 at 9:39 PM Laurent Pinchart wrote: > > On Wed, May 31, 2023 at 10:30:36AM +0200, Hans Verkuil wrote: > > > On 5/31/23 10:03, Laurent Pinchart wrote: > > > > On Wed, May 31, 2023 at 08:36:59AM +0200, Hans Verkuil wrote: > > > >> On 21/03/2023 11:28, Benjamin Gaignard wrote: > > > >>> Add module parameter "max_vb_buffer_per_queue" to be able to limit > > > >>> the number of vb2 buffers store in queue. > > > >>> > > > >>> Signed-off-by: Benjamin Gaignard <benjamin.gaignard@xxxxxxxxxxxxx> > > > >>> --- > > > >>> drivers/media/common/videobuf2/videobuf2-core.c | 15 +++------------ > > > >>> include/media/videobuf2-core.h | 11 +++++++++-- > > > >>> 2 files changed, 12 insertions(+), 14 deletions(-) > > > >>> > > > >>> diff --git a/drivers/media/common/videobuf2/videobuf2-core.c b/drivers/media/common/videobuf2/videobuf2-core.c > > > >>> index ae9d72f4d181..f4da917ccf3f 100644 > > > >>> --- a/drivers/media/common/videobuf2/videobuf2-core.c > > > >>> +++ b/drivers/media/common/videobuf2/videobuf2-core.c > > > >>> @@ -34,6 +34,8 @@ > > > >>> static int debug; > > > >>> module_param(debug, int, 0644); > > > >>> > > > >>> +module_param(max_vb_buffer_per_queue, ulong, 0644); > > > >> > > > >> There is no MODULE_PARM_DESC here? Please add. I see it is not there for > > > >> the debug param either, it should be added for that as well. > > > > > > > > Would this be the right time to consider resource accounting in V4L2 for > > > > buffers ? Having a module parameter doesn't sound very useful, an > > > > application could easily allocate more buffers by using buffer orphaning > > > > (allocating buffers, exporting them as dmabuf objects, and freeing them, > > > > which leaves the memory allocated). Repeating allocation cycles up to > > > > max_vb_buffer_per_queue will allow allocating an unbounded number of > > > > buffers, using all the available system memory. I'd rather not add a > > > > module argument that only gives the impression of some kind of safety > > > > without actually providing any value. > > Good point. It's even simpler, just keep opening new vim2m instances > and requesting max buffers :). > > > > > > > Does dmabuf itself provide some accounting mechanism? Just wondering. > > > > > > More specific to V4L2: I'm not so sure about this module parameter either. > > > It makes sense to have a check somewhere against ridiculous values (i.e. > > > allocating MAXINT buffers), but that can be a define as well. But otherwise > > > I am fine with allowing applications to allocate buffers until the memory > > > is full. > > > > > > The question is really: what is this parameter supposed to do? The only > > > thing it does is to sanitize unlikely inputs (e.g. allocating MAXINT buffers). > > > > > > I prefer that as a define, to be honest. > > > > > > I think it is perfectly fine for users to try to request more buffers than > > > memory allows. It will just fail in that case, not a problem. > > > > > > And if an application is doing silly things like buffer orphaning, then so > > > what? Is that any different than allocating memory and not freeing it? > > > Eventually it will run out of memory and crash, which is normal. > > > > Linux provides APIs to account for and limit usage of resources, > > including memory. A system administrator can prevent rogue processes > > from starving system resources. The memory consumed by vb2 buffer isn't > > taken into account, making V4L2 essentially unsafe for untrusted > > processes. > > I agree that proper accounting would be useful, although I wouldn't > really make this patch series depend on it, since it's not introducing > the loophole in the first place. No disagreement here, my concern was about introducing a workaround for the lack of proper memory accounting. I'd like to avoid the workaround, but it doesn't mean memory accounting has to be implement now. > We had some discussion about this in ChromeOS long ago and we thought > it would be really useful for killing browser tabs with big videos, > but otherwise using very little regular memory (e.g. via javascript). > > One challenge with accounting V4L2 allocations is how to count shared > DMA-bufs. If one process allocates a V4L2 buffer, exports it to > DMA-buf and then sends it to another process that keeps it alive, but > frees the V4L2 buffer (and even closes the DMA-buf fd), should that > memory be still accounted to it even though it doesn't hold a > reference to it anymore? I've thought about that too. It's an annoying problem, it should probably be discussed with memory management developers. > > Now, to be fair, there are many reasons why allowing access to v4L2 > > devices to untrusted applications is a bad idea, and memory consumption > > is likely not even the worst one. Still, is this something we want to > > fix, or do we want to consider V4L2 to be priviledged API only ? Right > > now we can't do so, but with many Linux systems moving towards pipewire, > > we could possibly have a system daemon isolating untrusted applications > > from the rest of the system. We may thus not need to fix this in the > > V4L2 API. -- Regards, Laurent Pinchart
