linux-kernel - Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough

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Date:   Mon, 1 Mar 2021 17:05:01 +0000
From:   Alessio Balsini <balsini@...roid.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Peng Tao <bergwolf@...il.com>,
        Alessio Balsini <balsini@...roid.com>,
        Akilesh Kailash <akailash@...gle.com>,
        Amir Goldstein <amir73il@...il.com>,
        Antonio SJ Musumeci <trapexit@...wn.link>,
        David Anderson <dvander@...gle.com>,
        Giuseppe Scrivano <gscrivan@...hat.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Martijn Coenen <maco@...roid.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Lawrence <paullawrence@...gle.com>,
        Stefano Duo <duostefano93@...il.com>,
        Zimuzo Ezeozue <zezeozue@...gle.com>, wuyan <wu-yan@....com>,
        fuse-devel <fuse-devel@...ts.sourceforge.net>,
        kernel-team <kernel-team@...roid.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for
 passthrough

On Fri, Feb 19, 2021 at 09:40:21AM +0100, Miklos Szeredi wrote:
> On Fri, Feb 19, 2021 at 8:05 AM Peng Tao <bergwolf@...il.com> wrote:
> >
> > On Wed, Feb 17, 2021 at 9:41 PM Miklos Szeredi <miklos@...redi.hu> wrote:
> 
> > > What I think would be useful is to have an explicit
> > > FUSE_DEV_IOC_PASSTHROUGH_CLOSE ioctl, that would need to be called
> > > once the fuse server no longer needs this ID.   If this turns out to
> > > be a performance problem, we could still add the auto-close behavior
> > > with an explicit FOPEN_PASSTHROUGH_AUTOCLOSE flag later.
> > Hi Miklos,
> >
> > W/o auto closing, what happens if user space daemon forgets to call
> > FUSE_DEV_IOC_PASSTHROUGH_CLOSE? Do we keep the ID alive somewhere?
> 
> Kernel would keep the ID open until explicit close or fuse connection
> is released.
> 
> There should be some limit on the max open files referenced through
> ID's, though.   E.g. inherit RLIMIT_NOFILE from mounting task.
> 
> Thanks,
> Miklos

I like the idea of FUSE_DEV_IOC_PASSTHROUGH_CLOSE to revoke the
passthrough access, that is something I was already working on. What I
had in mind was simply to break that 1:1 connection between fuse_file
and lower filp setting a specific fuse_file::passthrough::filp to NULL,
but this is slightly different from what you mentioned.

AFAIU you are suggesting to allocate one ID for each lower fs file
opened with passthrough within a connection, and maybe using idr_find at
every read/write/mmap operation to check if passthrough is enabled on
that file. Something similar to fuse2_map_get().
This way the fuse server can pass the same ID to one or more
fuse_file(s).
FUSE_DEV_IOC_PASSTHROUGH_CLOSE would idr_remove the ID, so idr_find
would fail, preventing the use of passthrough on that ID. CMIIW.

After FUSE_DEV_IOC_PASSTHROUGH_CLOSE(ID) it may happen that if some
fuse_file(s) storing that ID are still open and the same ID is reclaimed
in a new idr_alloc, this would lead to mismatching lower fs filp being
used by our fuse_file(s).  So also the ID stored in the fuse_file(s)
must be invalidated to prevent future uses of deallocated IDs.

Would it make sense to have a list of fuse_files using the same ID, that
must be traversed at FUSE_DEV_IOC_PASSTHROUGH_CLOSE time?
Negative values (maybe -ENOENT) might be used to mark IDs as invalid,
and tested before idr_find at read/write/mmap to avoid the idr_find
complexity in case passthrough is disabled for that file.

What do you think?

I agree with all the above comments to this patch, i.e., add
FOPEN_PASSTHROUGH, drop fuse_passthrough_out, header version+changelog,
that will be fixed in V13.

Thanks,
Alessio