Re: FUSE passthrough: fd lifetime?

[Josef Bacik <josef@toxicpanda.com>]

On Tue, Aug 27, 2024 at 11:41:04AM +0200, Han-Wen Nienhuys wrote:
> Hi folks,
> 
> I am implementing passthrough support for go-fuse. It seems to be
> working now (code:
> https://review.gerrithub.io/c/hanwen/go-fuse/+/1199984 and
> predecessors), but I am unsure of the correct lifetimes for the file
> descriptors.
> 
> The passthrough_hp example seems to do:
> 
> Open:
>   backing_fd = ..
>   backing_id = ioctl(fuse_device_fd,
>      FUSE_DEV_IOC_BACKING_OPEN, backing_fd)
> 
> Release:
>   ioctl(fuse_device_fd,
>      FUSE_DEV_IOC_BACKING_CLOSE, backing_id)
>   close(backing_fd)
> 
> Is it necessary to keep backing_fd open while the backing file is
> used? In the case of go-fuse, the backing_fd is managed by client
> code, so I can't ensure it is kept open long enough. I can work around
> this by doing
> 
> Open:
>    new_backing_fd = ioctl(backing_fd, DUP_FD, 0)
>    backing_id = ioctl(fuse_device_fd,
>      FUSE_DEV_IOC_BACKING_OPEN, new_backing_fd)
> 
> Release:
>   ioctl(fuse_device_fd,
>      FUSE_DEV_IOC_BACKING_CLOSE, backing_id)
>   close(new_backing_fd)
> 
> but it would double the number of FDs the process uses.
> 
> I tried simply closing the backing FD right after obtaining the
> backing ID, and it seems to work. Is this permitted?

Yes that's permitted, we're holding a reference to the file in the kernel.  The
dup thing works too, but as you say that's a lot more fd's.  Thanks,

Josef