linux-kernel - Re: [PATCH] epoll: try to be a _bit

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Date: Mon, 6 May 2024 14:37:10 +0200
From: Daniel Vetter <daniel@...ll.ch>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <keescook@...omium.org>, Al Viro <viro@...iv.linux.org.uk>,
	axboe@...nel.dk, brauner@...nel.org, christian.koenig@....com,
	dri-devel@...ts.freedesktop.org, io-uring@...r.kernel.org,
	jack@...e.cz, laura@...bott.name, linaro-mm-sig@...ts.linaro.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-media@...r.kernel.org, minhquangbui99@...il.com,
	sumit.semwal@...aro.org,
	syzbot+045b454ab35fd82a35fb@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

On Fri, May 03, 2024 at 04:41:19PM -0700, Linus Torvalds wrote:
> On Fri, 3 May 2024 at 16:23, Kees Cook <keescook@...omium.org> wrote:
> >
> > static bool __must_check get_dma_buf_unless_doomed(struct dma_buf *dmabuf)
> > {
> >         return atomic_long_inc_not_zero(&dmabuf->file->f_count) != 0L;
> > }
> >
> > If we end up adding epi_fget(), we'll have 2 cases of using
> > "atomic_long_inc_not_zero" for f_count. Do we need some kind of blessed
> > helper to live in file.h or something, with appropriate comments?
> 
> I wonder if we could try to abstract this out a bit more.
> 
> These games with non-ref-counted file structures *feel* a bit like the
> games we play with non-ref-counted (aka "stashed") 'struct dentry'
> that got fairly recently cleaned up with path_from_stashed() when both
> nsfs and pidfs started doing the same thing.
> 
> I'm not loving the TTM use of this thing, but at least the locking and
> logic feels a lot more straightforward (ie the
> atomic_long_inc_not_zero() here is clealy under the 'prime->mutex'
> lock

The one the vmgfx isn't really needed (I think at least), because all
other drivers that use gem or ttm use the dma_buf export cache in
drm/drm_prime.c, which is protected by a bog standard mutex.

vmwgfx is unfortunately special in a lot of ways due to somewhat parallel
dev history. So there might be an uapi reason why the weak reference is
required. I suspect because vmwgfx is reinventing a lot of its own wheels
it can't play the same tricks as gem_prime.c, which hooks into a few core
drm cleanup/release functions.

tldr; drm really has no architectural need for a get_file_unless_doomed,
and I certainly don't want to spread it it further than the vmwgfx
historical special case that was added in 2013.
-Sima

> IOW, the tty use looks correct to me, and it has fairly simple locking
> and is just catching the the race between 'fput()' decrementing the
> refcount and and 'file->f_op->release()' doing the actual release.
> 
> You are right that it's similar to the epoll thing in that sense, it
> just looks a _lot_ more straightforward to me (and, unlike epoll,
> doesn't look actively buggy right now).
> 
> Could we abstract out this kind of "stashed file pointer" so that we'd
> have a *common* form for this? Not just the inc_not_zero part, but the
> locking rule too?
> 
>               Linus

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch