On Tue, May 07, 2024 at 09:46:31AM -0700, Linus Torvalds wrote:
On Tue, 7 May 2024 at 04:03, Daniel Vetter daniel@ffwll.ch wrote:
It's really annoying that on some distros/builds we don't have that, and for gpu driver stack reasons we _really_ need to know whether a fd is the same as another, due to some messy uniqueness requirements on buffer objects various drivers have.
It's sad that such a simple thing would require two other horrid models (EPOLL or KCMP).
There'[s a reason that KCMP is a config option - *some* of that is horrible code - but the "compare file descriptors for equality" is not that reason.
Note that KCMP really is a broken mess. It's also a potential security hole, even for the simple things, because of how it ends up comparing kernel pointers (ie it doesn't just say "same file descriptor", it gives an ordering of them, so you can use KCMP to sort things in kernel space).
And yes, it orders them after obfuscating the pointer, but it's still not something I would consider sane as a baseline interface. It was designed for checkpoint-restore, it's the wrong thing to use for some "are these file descriptors the same".
The same argument goes for using EPOLL for that. Disgusting hack.
Just what are the requirements for the GPU stack? Is one of the file descriptors "trusted", IOW, you know what kind it is?
Because dammit, it's *so* easy to do. You could just add a core DRM ioctl for it. Literally just
struct fd f1 = fdget(fd1); struct fd f2 = fdget(fd2); int same; same = f1.file && f1.file == f2.file; fdput(fd1); fdput(fd2); return same;where the only question is if you also woudl want to deal with O_PATH fd's, in which case the "fdget()" would be "fdget_raw()".
Honestly, adding some DRM ioctl for this sounds hacky, but it sounds less hacky than relying on EPOLL or KCMP.
Well, in slightly more code (because it's part of the "import this dma-buf/dma-fence/whatever fd into a driver object" ioctl) this is what we do.
The issue is that there's generic userspace (like compositors) that sees these things fly by and would also like to know whether the other side they receive them from is doing nasty stuff/buggy/evil. And they don't have access to the device drm fd (since those are a handful of layers away behind the opengl/vulkan userspace drivers even if the compositor could get at them, and in some cases not even that).
So if we do this in drm we'd essentially have to create a special drm_compare_files chardev, put the ioctl there and then tell everyone to make that thing world-accessible.
Which is just too close to a real syscall that it's offensive, and hey kcmp does what we want already (but unfortunately also way more). So we rejected adding that to drm. But we did think about it.
I'd be perfectly ok with adding a generic "FISAME" VFS level ioctl too, if this is possibly a more common thing. and not just DRM wants it.
Would something like that work for you?
Yes.
Adding Simon and Pekka as two of the usual suspects for this kind of stuff. Also example code (the int return value is just so that callers know when kcmp isn't available, they all only care about equality):
https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/util/os_file.c#L239 -Sima
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch