Re: [PATCH] epoll: try to be a _bit_ better about file lifetimes

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, May 03, 2024 at 03:46:25PM -0700, Kees Cook wrote:
> On Fri, May 03, 2024 at 02:52:38PM -0700, Linus Torvalds wrote:
> > That means that the file will be released - and it means that you have
> > violated all the refcounting rules for poll().
> 
> I feel like I've been looking at this too long. I think I see another
> problem here, but with dmabuf even when epoll is fixed:
> 
> dma_buf_poll()
> 	get_file(dmabuf->file)		/* f_count + 1 */
> 	dma_buf_poll_add_cb()
> 		dma_resv_for_each_fence ...
> 			dma_fence_add_callback(fence, ..., dma_buf_poll_cb)
> 
> dma_buf_poll_cb()
> 	...
>         fput(dmabuf->file);		/* f_count - 1 ... for each fence */
> 
> Isn't it possible to call dma_buf_poll_cb() (and therefore fput())
> multiple times if there is more than 1 fence? Perhaps I've missed a
> place where a single struct dma_resv will only ever signal 1 fence? But
> looking through dma_fence_signal_timestamp_locked(), I don't see
> anything about resv nor somehow looking into other fence cb_list
> contents...

At a guess,
                r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb);
		if (!r)
			return true;

prevents that - it returns 0 on success and -E... on error;
insertion into the list happens only when it's returning 0,
so...