From: "Christian König" <ckoenig.leichtzumerken@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Christian Brauner <brauner@kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>,
keescook@chromium.org, axboe@kernel.dk, christian.koenig@amd.com,
dri-devel@lists.freedesktop.org, io-uring@vger.kernel.org,
jack@suse.cz, laura@labbott.name, linaro-mm-sig@lists.linaro.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-media@vger.kernel.org, minhquangbui99@gmail.com,
sumit.semwal@linaro.org,
syzbot+045b454ab35fd82a35fb@syzkaller.appspotmail.com,
syzkaller-bugs@googlegroups.com
Subject: Re: [Linaro-mm-sig] Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes
Date: Tue, 7 May 2024 19:45:02 +0200 [thread overview]
Message-ID: <d68417df-1493-421a-8558-879abe36d6fa@gmail.com> (raw)
In-Reply-To: <CAHk-=wgh5S-7sCCqXBxGcXHZDhe4U8cuaXpVTjtXLej2si2f3g@mail.gmail.com>
Am 07.05.24 um 18:46 schrieb Linus Torvalds:
> 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.
>
> 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?
Well the generic approach yes, the DRM specific one maybe. IIRC we need
to be able to compare both DRM as well as DMA-buf file descriptors.
The basic problem userspace tries to solve is that drivers might get the
same fd through two different code paths.
For example application using OpenGL/Vulkan for rendering and VA-API for
video decoding/encoding at the same time.
Both APIs get a fd which identifies the device to use. It can be the
same, but it doesn't have to.
If it's the same device driver connection (or in kernel speak underlying
struct file) then you can optimize away importing and exporting of
buffers for example.
Additional to that it makes cgroup accounting much easier because you
don't count things twice because they are shared etc...
Regards,
Christian.
>
> Linus
next prev parent reply other threads:[~2024-05-07 17:45 UTC|newest]
Thread overview: 84+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <0000000000002d631f0615918f1e@google.com>
2024-05-03 11:54 ` [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove Bui Quang Minh
2024-05-03 18:26 ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Kees Cook
2024-05-03 18:49 ` Jens Axboe
2024-05-03 19:22 ` Kees Cook
2024-05-03 19:35 ` Jens Axboe
2024-05-03 19:59 ` Kees Cook
2024-05-03 20:28 ` Kees Cook
2024-05-03 21:11 ` Al Viro
2024-05-03 21:24 ` Linus Torvalds
2024-05-03 21:30 ` Al Viro
2024-05-06 17:46 ` Stefan Metzmacher
2024-05-06 18:17 ` Linus Torvalds
2024-05-08 8:47 ` David Laight
2024-05-03 21:36 ` Al Viro
2024-05-03 21:42 ` Linus Torvalds
2024-05-03 21:53 ` Al Viro
2024-05-06 12:23 ` Daniel Vetter
2024-05-04 9:59 ` Christian Brauner
2024-05-03 21:11 ` [PATCH] epoll: try to be a _bit_ better about file lifetimes Linus Torvalds
2024-05-03 21:24 ` Al Viro
2024-05-03 21:33 ` Linus Torvalds
2024-05-03 21:45 ` Al Viro
2024-05-03 21:52 ` Linus Torvalds
2024-05-03 22:01 ` Al Viro
2024-05-03 22:07 ` Al Viro
2024-05-03 23:16 ` Linus Torvalds
2024-05-03 23:39 ` Al Viro
2024-05-03 23:54 ` Linus Torvalds
2024-05-04 10:44 ` Christian Brauner
2024-05-03 22:46 ` Kees Cook
2024-05-03 23:03 ` Al Viro
2024-05-03 23:23 ` Kees Cook
2024-05-03 23:41 ` Linus Torvalds
2024-05-04 9:19 ` Christian Brauner
2024-05-06 12:37 ` Daniel Vetter
2024-05-04 9:37 ` Christian Brauner
2024-05-04 15:32 ` Linus Torvalds
2024-05-04 15:40 ` Linus Torvalds
2024-05-04 15:53 ` Linus Torvalds
2024-05-05 19:46 ` Al Viro
2024-05-05 20:03 ` Linus Torvalds
2024-05-05 20:30 ` Al Viro
2024-05-05 20:53 ` Linus Torvalds
2024-05-06 12:47 ` Daniel Vetter
2024-05-06 14:46 ` Christian Brauner
2024-05-07 10:58 ` Daniel Vetter
2024-05-06 16:15 ` Christian König
2024-05-05 10:50 ` Christian Brauner
2024-05-05 16:46 ` Linus Torvalds
2024-05-05 17:55 ` [PATCH v2] epoll: be " Linus Torvalds
2024-05-05 18:04 ` Jens Axboe
2024-05-05 20:01 ` David Laight
2024-05-05 20:16 ` Linus Torvalds
2024-05-05 20:12 ` [PATCH] epoll: try to be a _bit_ " Al Viro
2024-05-06 8:45 ` Christian Brauner
2024-05-06 9:26 ` Christian Brauner
2024-05-06 14:19 ` Christian Brauner
2024-05-07 21:02 ` David Laight
2024-05-04 18:20 ` Linus Torvalds
2024-05-06 14:29 ` [Linaro-mm-sig] " Christian König
2024-05-07 11:02 ` Daniel Vetter
2024-05-07 16:46 ` Linus Torvalds
2024-05-07 17:45 ` Christian König [this message]
2024-05-08 7:51 ` Michel Dänzer
2024-05-08 7:59 ` Christian König
2024-05-08 8:23 ` Christian Brauner
2024-05-08 9:10 ` Christian König
2024-05-07 18:04 ` Daniel Vetter
2024-05-07 19:07 ` Linus Torvalds
2024-05-08 5:55 ` Christian König
2024-05-08 8:32 ` Daniel Vetter
2024-05-08 10:16 ` Christian Brauner
2024-05-08 8:05 ` Christian Brauner
2024-05-08 16:19 ` Linus Torvalds
2024-05-08 17:14 ` Linus Torvalds
2024-05-09 11:38 ` Christian Brauner
2024-05-09 15:48 ` Linus Torvalds
2024-05-10 6:33 ` Christian Brauner
2024-05-08 10:08 ` Christian Brauner
2024-05-08 15:45 ` Daniel Vetter
2024-05-10 10:55 ` Christian Brauner
2024-05-11 18:25 ` David Laight
2024-05-05 17:31 ` Jens Axboe
2024-05-04 9:45 ` get_file() unsafe under epoll (was Re: [syzbot] [fs?] [io-uring?] general protection fault in __ep_remove) Christian Brauner
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